Weed control
Encyclopedia
Weed control is the botanical component of pest control
, using physical and chemical methods to stop weeds from reaching a mature stage of growth when they could be harmful to domesticated plant
s and livestock
. In order to reduce weed growth, many "weed control" strategies have been developed in order to contain the growth and spread of weeds.
The most basic is ploughing which cuts the roots of annual weeds. Today, chemical weed killers known as herbicide
s are widely used.
Weeds tend to thrive at the expense of the more refined edible or ornamental crops. They provide competition for space, nutrient
s, water and light, although how seriously they will affect a crop depends on a number of factors. Some crops have greater resistance than others- smaller, slower growing seedlings are more likely to be overwhelmed than those that are larger and more vigorous. Onions are one of the crops most susceptible to competition, for they are slow to germinate and produce slender, upright stems. Quick growing, broad leafed weeds therefore have a distinct advantage, and if not removed, the crop is likely to be lost. Broad beans however produce large seedlings, and will suffer far less profound effects of weed competition other than during periods of water shortage at the crucial time when the pods are filling out. Transplanted crops raised in sterile seed or potting compost
will have a head start over germinating weed seeds.
Weeds also differ in their competitive abilities, and can vary according to conditions and the time of year. Tall growing vigorous weeds such as fat hen
(Chenopodium album) can have the most pronounced effects on adjacent crops, although seedlings of fat hen that appear in late summer will only produce small plants. Chickweed (Stellaria media), a low growing plant, can happily co-exist with a tall crop during the summer, but plants that have overwintered will grow rapidly in early spring and may swamp crops such as onions or spring greens.
The presence of weeds does not necessarily mean that they are competing with a crop, especially during the early stages of growth when each plant can find the resources it requires without interfering with the others. However, as the seedlings’ size increases, their root systems will spread as they each
begin to require greater amounts of water and nutrients. Estimates suggest that weed and crop can co-exist harmoniously for around three weeks, therefore it is important that weeds be removed early on in order to prevent competition occurring. Weed competition can have quite dramatic effects on crop growth. Harold A Roberts cites research carried out with onions wherein "Weeds were carefully removed from separate plots at different times during the growth of the crop and the plots were then kept clean. It was found that after competition had started, the final yield of bulbs was being reduced at a rate equivalent to almost 4% per day. So that by delaying weeding for another fortnight, the yield was cut to less than half that produced on ground kept clean all the time." (The Complete Know And Grow Vegetables, Bleasdale, Salter and others, OUP 1991). He goes on to record that "by early June, the weight of weeds per unit area was twenty times that of the crop, and the weeds had already taken from the soil about half of the nitrogen and a third of the potash which had been applied".
Perennial
weeds with bulbils, such as lesser celandine
and oxalis
, or with persistent underground stems such as couch grass (Agropyron repens) or creeping buttercup
(Ranunculus repens) are able to store reserves of food, and are thus able to grow faster and with more vigour than their annual counterparts. There is also evidence that the roots of some perennials such as couch grass exude allelopathic chemicals which inhibit the growth of other nearby plants.
Weeds can also host pests and diseases that can spread to cultivated crops. Charlock and Shepherd's purse
may carry clubroot
, eelworm can be harboured by chickweed, fat hen and shepherd's purse, while the cucumber mosaic virus
, which can devastate the cucurbit family
, is carried by a range of different weeds including chickweed and groundsel.
However, at times the role of weeds in this respect can be over-rated. As far as insect
pests are concerned, often the species that live on weeds are not the same as those that attack vegetable crops; "Tests with the common cruciferous weeds such as shepherds purse have shown that they do not act as hosts for the larvae of the cabbage root fly
. One exception was found to be the wild radish
, but this is not usually a weed of established vegetable gardens" (Roberts, The Complete Know And Grow Vegetables). However pests such as cutworm
s may first attack weeds then move on to cultivated crops.
While charlock, a common weed in southeastern USA
, may be considered a weed by row crop growers, it is highly valued by beekeepers, who seek out places where it blooms all winter, thus providing pollen for honeybees and other pollinators. Its bloom is resistant to all but a very hard freeze, and even that will only kill it back briefly. By feeding an array of pollinators during a seasonal dearth, it can redound to the farmer's advantage. Many weeds are likewise highly beneficial to pollinators.
In the case of using the black plastic sheet, the greenhouse effect
is used to kill the plants beneath the sheet. A 5-10 cm layer of wood chip mulch
on the ground will also prevent most weeds from sprouting. Also, gravel can be spread over the ground as an inorganic mulch. In agriculture, irrigation is sometimes used as a weed control measure such as in the case of paddy fields. Many people find that although the black plastic sheeting is extremely effective at preventing the weeds in areas where it covers, but in actual use it is difficult to achieve full coverage.
Knowing how weeds reproduce, spread and survive adverse conditions can help in developing effective control and management strategies. Weeds have a range of techniques that enable them to thrive;
Annual
and biennial
weeds such as chickweed
, annual meadow grass, shepherd's purse
, groundsel, fat hen
, cleaver
, speedwell
and hairy bittercress propagate
themselves by seed
ing. Many produce huge numbers of seed several times a season, some all year round. Groundsel can produce 1000 seed, and can continue right through a mild winter, whilst Scentless Mayweed produces over 30,000 seeds per plant. Not all of these will germinate at once, but over several seasons, lying dormant in the soil sometimes for years until exposed to light. Poppy
seed can survive 80-100 years, dock 50 or more. There can be many thousands of seeds in a square foot or square metre of ground, thus and soil disturbance will produce a flush of fresh weed seedlings.
See also Bradley Method of Bush Regeneration, which uses ecological processes to do much of the work.
’, which involves cultivating the soil, then leaving it for a week or so.
When the initial flush of weeds has germinated, the grower will lightly hoe
off before the desired crop is planted. However, even a freshly cleared bed will be susceptible to airborne seed from elsewhere, as well as seed brought in by passing animals which can carry them on their fur, or from freshly imported manure
. The organic solution to the problem of spreading annual weeds lies in regular, properly timed weeding, preferably just before flowering (fortuitously, this is also the time at which they will be of the most value in composting).
This technique is also quite often used by farmers who let weeds germinate then return the soil before crop sowing.
Perennial
weeds also propagate by seeding; the airborne seed of the dandelion and the rose-bay willow herb
are parachuted far and wide. But they also have an additional range of vegetative means of spreading that gives them their pernicious reputation. Dandelion and dock put down deep tap roots, which, although they do not spread underground, are able to regrow from any remaining piece left in the ground. Removal of the complete tap root is the only sure remedy.
The most persistent of the perennials are those that spread by underground creeping rhizome
s that can regrow from the tiniest fragment. These include couch grass, bindweed
, ground elder, nettle
s, rosebay willow herb, Japanese knotweed
, horsetail
and bracken
, as well as creeping thistle, whose tap roots can put out lateral roots. Other perennials put out runners that spread along the soil
surface. As they
creep along they set down roots, enabling them to colonise bare ground with great rapidity. These include creeping buttercup and ground ivy. Yet another group of perennials propagate by stolon
s- stems that arch back into the ground to reroot. Most familiar of these is the bramble
.
All of the above weeds can be very difficult to eradicate- thick black plastic mulch
es can be effective to a degree, although will probably need to be left in place for at least two seasons. In addition, hoeing off weed leaves and stems as soon as they appear can eventually weaken and kill the plants, although this will require persistence in the case of plants such as bindweed. Nettle infestations can be tackled by cutting back at least three times a year, repeated over a three year period. Bramble can be dealt with in a similar way. Some plants are said to produce root exudates that suppress herb
aceous weeds. Tagetes minuata is claimed to be effective against couch and ground elder, whilst a border of comfrey
is also said to act as a barrier against the invasion of some weeds including couch.
.
However weed control can also be achieved by the use of herbicides. Selective herbicides kill certain targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often based on plant hormone
s. Herbicides are generally classified as follows;
In agriculture
large scale and systematic weeding is usually required, often by machines, such as liquid
herbicide
sprayer
s, or even by helicopter
(such as in the USA), to eliminate the massive amount of weeds present on farming lands.
However there are a number of techniques that the organic farmer
can employ such as mulch
ing and carefully timed cutting of weeds before they are able to set seed.
(See also section below on UK legislation regarding the control of certain weeds)
is used as a farming technique to clean soil completely from weeds. Several research results confirm the high effectivness of humid heat against weedes and its seeds.
, furze
and Lantana
s. The first may be destroyed by frequent ploughing and harrowing, in the same manner as other perennial weeds are. Another method of destroying broom is by pasturing the field where it grows with grazers.
The best method of extirpating furze is to set fire to it in frosty weather, for frost has the effect of withering and making them burn readily. The stumps must then be cut over with a hatchet, and when the ground is well softened by rain it may be ploughed up, and the roots taken out by a harrow adapted to that purpose. If the field is soon laid down to grass
, they will again spring up; in this case, pasturing with grazers is an effectual remedy. The thorn, or bramble, can only be extirpated by ploughing up the ground and collecting the roots.
In June, weeds are in their most succulent state, and in this condition, after they have lain a few hours to wither, cattle will eat almost every species. There is scarcely a hedge, border, or a nook, but what at that season is valuable.
s and other rural
settings rather than the allotment
or garden
-scale grower.
There are five ‘injurious’ (that is, likely to be harmful to agricultural production) weeds covered by the provisions of the Weeds Act. These are:
The Department for Environment, Food and Rural Affairs
(DEFRA) provides guidance for the treatment removal of these weeds from infested land. Much of this is oriented towards the use of herbicide
s, the majority of which may not be acceptable to the organic producer
(apart from non-synthetic substances like sulphur
, which in some circumstances are accepted within Soil Association
standards) but in most cases there are manual techniques that can be used such as digging out the roots, mulch
ing out or carefully timed cutting before seeds are able to spread.
Primary responsibility for weeds control rests with the occupier of the land on which the weeds are growing, therefore it is important to be alert to potential weed problems and to take prompt action. However, it should be remembered that most common farmland weeds are not "injurious" within the meaning of the Weeds Act, and many such plant species have conservation and environmental value. When dealing with complaints under the Weeds Act, DEFRA has a duty in law to try to achieve a reasonable balance among different interests. These include agriculture, countryside conservation
and the public in general. Constructive discussion about problems caused by weeds can often result in effective solutions and avoid the need for DEFRA to take official action. In addition to those weeds covered by the 1959 act, under section 14 of the Wildlife and Countryside Act 1981
, it can be an offence to plant or grow certain specified plants in the wild (see Schedule 9 to the Wildlife and Countryside Act 1981), including Giant Hogweed
and Japanese Knotweed
. Problems involving these plants can be referred to the local authority for the area where those weeds are growing as some local authorities have bye-laws controlling these plants. There is no statutory
requirement for landowners to remove these plants from their property.
Pest control
Pest control refers to the regulation or management of a species defined as a pest, usually because it is perceived to be detrimental to a person's health, the ecology or the economy.-History:...
, using physical and chemical methods to stop weeds from reaching a mature stage of growth when they could be harmful to domesticated plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s and livestock
Livestock
Livestock refers to one or more domesticated animals raised in an agricultural setting to produce commodities such as food, fiber and labor. The term "livestock" as used in this article does not include poultry or farmed fish; however the inclusion of these, especially poultry, within the meaning...
. In order to reduce weed growth, many "weed control" strategies have been developed in order to contain the growth and spread of weeds.
The most basic is ploughing which cuts the roots of annual weeds. Today, chemical weed killers known as herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
s are widely used.
The effects of weeds on other plants
Weeds can compete with productive crops or pasture, or convert productive land into unusable scrub. Weeds are also often poisonous, distasteful, produce burrs, thorns or other damaging body parts or otherwise interfere with the use and management of desirable plants by contaminating harvests or excluding livestock.Weeds tend to thrive at the expense of the more refined edible or ornamental crops. They provide competition for space, nutrient
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...
s, water and light, although how seriously they will affect a crop depends on a number of factors. Some crops have greater resistance than others- smaller, slower growing seedlings are more likely to be overwhelmed than those that are larger and more vigorous. Onions are one of the crops most susceptible to competition, for they are slow to germinate and produce slender, upright stems. Quick growing, broad leafed weeds therefore have a distinct advantage, and if not removed, the crop is likely to be lost. Broad beans however produce large seedlings, and will suffer far less profound effects of weed competition other than during periods of water shortage at the crucial time when the pods are filling out. Transplanted crops raised in sterile seed or potting compost
Compost
Compost is organic matter that has been decomposed and recycled as a fertilizer and soil amendment. Compost is a key ingredient in organic farming. At its most essential, the process of composting requires simply piling up waste outdoors and waiting for the materials to break down from anywhere...
will have a head start over germinating weed seeds.
Weeds also differ in their competitive abilities, and can vary according to conditions and the time of year. Tall growing vigorous weeds such as fat hen
Chenopodium album
Chenopodium album is a fast-growing weedy annual plant in the genus Chenopodium.Though cultivated in some regions, the plant is elsewhere considered a weed...
(Chenopodium album) can have the most pronounced effects on adjacent crops, although seedlings of fat hen that appear in late summer will only produce small plants. Chickweed (Stellaria media), a low growing plant, can happily co-exist with a tall crop during the summer, but plants that have overwintered will grow rapidly in early spring and may swamp crops such as onions or spring greens.
The presence of weeds does not necessarily mean that they are competing with a crop, especially during the early stages of growth when each plant can find the resources it requires without interfering with the others. However, as the seedlings’ size increases, their root systems will spread as they each
begin to require greater amounts of water and nutrients. Estimates suggest that weed and crop can co-exist harmoniously for around three weeks, therefore it is important that weeds be removed early on in order to prevent competition occurring. Weed competition can have quite dramatic effects on crop growth. Harold A Roberts cites research carried out with onions wherein "Weeds were carefully removed from separate plots at different times during the growth of the crop and the plots were then kept clean. It was found that after competition had started, the final yield of bulbs was being reduced at a rate equivalent to almost 4% per day. So that by delaying weeding for another fortnight, the yield was cut to less than half that produced on ground kept clean all the time." (The Complete Know And Grow Vegetables, Bleasdale, Salter and others, OUP 1991). He goes on to record that "by early June, the weight of weeds per unit area was twenty times that of the crop, and the weeds had already taken from the soil about half of the nitrogen and a third of the potash which had been applied".
Perennial
Perennial plant
A perennial plant or simply perennial is a plant that lives for more than two years. The term is often used to differentiate a plant from shorter lived annuals and biennials. The term is sometimes misused by commercial gardeners or horticulturalists to describe only herbaceous perennials...
weeds with bulbils, such as lesser celandine
Lesser celandine
Lesser celandine is a low-growing, hairless perennial plant, with fleshy dark green, heart-shaped leaves. The plant is found throughout Europe and west Asia and is now introduced in North America. It prefers bare, damp ground and in the UK it is often a persistent garden weed...
and oxalis
Oxalis
Oxalis is by far the largest genus in the wood-sorrel family Oxalidaceae: of the approximately 900 known species in the Oxalidaceae, 800 belong here...
, or with persistent underground stems such as couch grass (Agropyron repens) or creeping buttercup
Creeping buttercup
Ranunculus repens is a flowering plant in the buttercup family, native to Europe, Asia and northwestern Africa. It is also called Creeping Crowfoot and Sitfast.-Growth:...
(Ranunculus repens) are able to store reserves of food, and are thus able to grow faster and with more vigour than their annual counterparts. There is also evidence that the roots of some perennials such as couch grass exude allelopathic chemicals which inhibit the growth of other nearby plants.
Weeds can also host pests and diseases that can spread to cultivated crops. Charlock and Shepherd's purse
Shepherd's Purse
Capsella bursa-pastoris, known by its common name shepherd's-purse because of its triangular, purse-like pods, is a small annual and ruderal species, and a member of the Brassicaceae or mustard family...
may carry clubroot
Clubroot
Clubroot is a common disease of cabbages, radishes, turnips and other plants belonging to the family Cruciferae . It is caused by Plasmodiophora brassicae, which was once considered a slime mold but is now put in the group Phytomyxea. It has as many as nine races. Gall formation or distortion takes...
, eelworm can be harboured by chickweed, fat hen and shepherd's purse, while the cucumber mosaic virus
Cucumber mosaic virus
Cucumber mosaic virus is a plant pathogenic virus in the family Bromoviridae.It is the type member of the plant virus genus, Cucumovirus. This virus has a worldwide distribution and a very wide host range. In fact it has the reputation of having the widest host range of any known plant virus...
, which can devastate the cucurbit family
Cucurbitaceae
The plant family Cucurbitaceae consists of various squashes, melons, and gourds, including crops such as cucumber, pumpkins, luffas, and watermelons...
, is carried by a range of different weeds including chickweed and groundsel.
However, at times the role of weeds in this respect can be over-rated. As far as insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
pests are concerned, often the species that live on weeds are not the same as those that attack vegetable crops; "Tests with the common cruciferous weeds such as shepherds purse have shown that they do not act as hosts for the larvae of the cabbage root fly
Cabbage root fly
Delia radicum, known variously as the cabbage fly, cabbage root fly, root fly or turnip fly, is a pest of crops. The larvae of the cabbage root fly are sometimes known as the cabbage maggot or root maggot...
. One exception was found to be the wild radish
Wild radish
Wild Radish or Jointed Charlock, Raphanus raphanistrum, is a flowering plant in the family Brassicaceae. It is sometimes claimed to be the ancestor of the edible radish, Raphanus sativus...
, but this is not usually a weed of established vegetable gardens" (Roberts, The Complete Know And Grow Vegetables). However pests such as cutworm
Cutworm
Cutworms are not worms, biologically speaking, but caterpillars; they are moth larvae that hide under litter or soil during the day, coming out in the dark to feed on plants...
s may first attack weeds then move on to cultivated crops.
While charlock, a common weed in southeastern USA
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, may be considered a weed by row crop growers, it is highly valued by beekeepers, who seek out places where it blooms all winter, thus providing pollen for honeybees and other pollinators. Its bloom is resistant to all but a very hard freeze, and even that will only kill it back briefly. By feeding an array of pollinators during a seasonal dearth, it can redound to the farmer's advantage. Many weeds are likewise highly beneficial to pollinators.
Methods
In domestic gardens, methods of weed control include covering an area of ground with several layers of wet newspaper or one black plastic sheet for several weeks. In the case of using wet newspaper, the multiple layers prevent light from reaching all plants beneath, which kills them. Saturating the newspaper with water daily speeds the decomposition of the dead plants. Any weed seeds that start to sprout because of the water will also be deprived of sunlight, be killed, and decompose. After several weeks, all germinating weed seeds present in the ground should be dead. Then the newspaper can be removed and the ground can be planted. The decomposed plants will help fertilise the plants or seeds planted later.In the case of using the black plastic sheet, the greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...
is used to kill the plants beneath the sheet. A 5-10 cm layer of wood chip mulch
Mulch
In agriculture and gardening, is a protective cover placed over the soil to retain moisture, reduce erosion, provide nutrients, and suppress weed growth and seed germination. Mulching in gardens and landscaping mimics the leaf cover that is found on forest floors....
on the ground will also prevent most weeds from sprouting. Also, gravel can be spread over the ground as an inorganic mulch. In agriculture, irrigation is sometimes used as a weed control measure such as in the case of paddy fields. Many people find that although the black plastic sheeting is extremely effective at preventing the weeds in areas where it covers, but in actual use it is difficult to achieve full coverage.
Knowing how weeds reproduce, spread and survive adverse conditions can help in developing effective control and management strategies. Weeds have a range of techniques that enable them to thrive;
Annual
Annual plant
An annual plant is a plant that usually germinates, flowers, and dies in a year or season. True annuals will only live longer than a year if they are prevented from setting seed...
and biennial
Biennial plant
A biennial plant is a flowering plant that takes two years to complete its biological lifecycle. In the first year the plant grows leaves, stems, and roots , then it enters a period of dormancy over the colder months. Usually the stem remains very short and the leaves are low to the ground, forming...
weeds such as chickweed
Chickweed
Chickweed, a common name, can refer to:* Cerastium - Mouse-ear Chickweed* Holosteum - Jagged Chickweed* Moenchia - Upright Chickweed* Paronychia - Chickweed* Stellaria pro parte - Chickweed...
, annual meadow grass, shepherd's purse
Shepherd's Purse
Capsella bursa-pastoris, known by its common name shepherd's-purse because of its triangular, purse-like pods, is a small annual and ruderal species, and a member of the Brassicaceae or mustard family...
, groundsel, fat hen
Chenopodium album
Chenopodium album is a fast-growing weedy annual plant in the genus Chenopodium.Though cultivated in some regions, the plant is elsewhere considered a weed...
, cleaver
Galium aparine
Galium aparine is a herbaceous annual plant of the family Rubiaceae. It is native to North America and Eurasia. It has several common names, including Cleavers, Clivers, Goosegrass, Stickywilly, Stickyjack, Stickyweed, Stickyleaf, Catchweed, Robin-run-the-hedge and Coachweed.-Growth:The long stems...
, speedwell
Veronica (plant)
Veronica is the largest genus in the flowering plant family Plantaginaceae, with about 500 species; it was formerly classified in the family Scrophulariaceae...
and hairy bittercress propagate
Plant propagation
Plant propagation is the process of creating new plants from a variety of sources: seeds, cuttings, bulbs and other plant parts. Plant propagation can also refer to the artificial or natural dispersal of plants.-Sexual propagation :...
themselves by seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
ing. Many produce huge numbers of seed several times a season, some all year round. Groundsel can produce 1000 seed, and can continue right through a mild winter, whilst Scentless Mayweed produces over 30,000 seeds per plant. Not all of these will germinate at once, but over several seasons, lying dormant in the soil sometimes for years until exposed to light. Poppy
Poppy
A poppy is one of a group of a flowering plants in the poppy family, many of which are grown in gardens for their colorful flowers. Poppies are sometimes used for symbolic reasons, such as in remembrance of soldiers who have died during wartime....
seed can survive 80-100 years, dock 50 or more. There can be many thousands of seeds in a square foot or square metre of ground, thus and soil disturbance will produce a flush of fresh weed seedlings.
See also Bradley Method of Bush Regeneration, which uses ecological processes to do much of the work.
"Stale seed bed" technique
One technique employed by growers is the ‘stale seed bedStale seed bed
A false or stale seed bed is a useful organic weed control technique which involves creating a seedbed some weeks before seed is due to be sown...
’, which involves cultivating the soil, then leaving it for a week or so.
When the initial flush of weeds has germinated, the grower will lightly hoe
Hoe (tool)
A hoe is an ancient and versatile agricultural tool used to move small amounts of soil. Common goals include weed control by agitating the surface of the soil around plants, piling soil around the base of plants , creating narrow furrows and shallow trenches for planting seeds and bulbs, to chop...
off before the desired crop is planted. However, even a freshly cleared bed will be susceptible to airborne seed from elsewhere, as well as seed brought in by passing animals which can carry them on their fur, or from freshly imported manure
Manure
Manure is organic matter used as organic fertilizer in agriculture. Manures contribute to the fertility of the soil by adding organic matter and nutrients, such as nitrogen, that are trapped by bacteria in the soil...
. The organic solution to the problem of spreading annual weeds lies in regular, properly timed weeding, preferably just before flowering (fortuitously, this is also the time at which they will be of the most value in composting).
This technique is also quite often used by farmers who let weeds germinate then return the soil before crop sowing.
Perennial
Perennial plant
A perennial plant or simply perennial is a plant that lives for more than two years. The term is often used to differentiate a plant from shorter lived annuals and biennials. The term is sometimes misused by commercial gardeners or horticulturalists to describe only herbaceous perennials...
weeds also propagate by seeding; the airborne seed of the dandelion and the rose-bay willow herb
Epilobium
Epilobium is a genus in the family Onagraceae, containing about 160-200 species of flowering plants with a worldwide distribution. They are generally abundant in the subarctic, temperate and subantarctic regions, whereas in the subtropics and tropics they are restricted to the cool montane biomes,...
are parachuted far and wide. But they also have an additional range of vegetative means of spreading that gives them their pernicious reputation. Dandelion and dock put down deep tap roots, which, although they do not spread underground, are able to regrow from any remaining piece left in the ground. Removal of the complete tap root is the only sure remedy.
The most persistent of the perennials are those that spread by underground creeping rhizome
Rhizome
In botany and dendrology, a rhizome is a characteristically horizontal stem of a plant that is usually found underground, often sending out roots and shoots from its nodes...
s that can regrow from the tiniest fragment. These include couch grass, bindweed
Bindweed
Bindweed may refer to:* Convolvulaceae , a family including about 60 genera and more than 1,650 species** Calystegia , a genus of about 25 species of flowering plants...
, ground elder, nettle
Nettle
Nettles constitute between 24 and 39 species of flowering plants of the genus Urtica in the family Urticaceae, with a cosmopolitan though mainly temperate distribution. They are mostly herbaceous perennial plants, but some are annual and a few are shrubby...
s, rosebay willow herb, Japanese knotweed
Japanese knotweed
Japanese Knotweed is a large, herbaceous perennial plant, native to eastern Asia in Japan, China and Korea...
, horsetail
Horsetail
Equisetum is the only living genus in the Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds.Equisetum is a "living fossil", as it is the only living genus of the entire class Equisetopsida, which for over one hundred million years was much more diverse and...
and bracken
Bracken
Bracken are several species of large, coarse ferns of the genus Pteridium. Ferns are vascular plants that have alternating generations, large plants that produce spores and small plants that produce sex cells . Brackens are in the family Dennstaedtiaceae, which are noted for their large, highly...
, as well as creeping thistle, whose tap roots can put out lateral roots. Other perennials put out runners that spread along the soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
surface. As they
creep along they set down roots, enabling them to colonise bare ground with great rapidity. These include creeping buttercup and ground ivy. Yet another group of perennials propagate by stolon
Stolon
In biology, stolons are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external skeletons.-In botany:...
s- stems that arch back into the ground to reroot. Most familiar of these is the bramble
Bramble
Brambles are thorny plants of the genus Rubus, in the rose family . Bramble fruit is the fruit of any such plant, including the blackberry and raspberry. The word comes from Germanic *bram-bezi, whence also German Brombeere , Dutch Braam and French framboise...
.
All of the above weeds can be very difficult to eradicate- thick black plastic mulch
Plastic mulch
Plastic mulch is a product used, in a similar fashion to mulch, to suppress weeds and conserve water in crop production and landscaping. Certain plastic mulches also act as a barrier to keep methyl bromide, both a powerful fumigant and ozone depleter, in the soil. Crops grow through slits or...
es can be effective to a degree, although will probably need to be left in place for at least two seasons. In addition, hoeing off weed leaves and stems as soon as they appear can eventually weaken and kill the plants, although this will require persistence in the case of plants such as bindweed. Nettle infestations can be tackled by cutting back at least three times a year, repeated over a three year period. Bramble can be dealt with in a similar way. Some plants are said to produce root exudates that suppress herb
Herb
Except in botanical usage, an herb is "any plant with leaves, seeds, or flowers used for flavoring, food, medicine, or perfume" or "a part of such a plant as used in cooking"...
aceous weeds. Tagetes minuata is claimed to be effective against couch and ground elder, whilst a border of comfrey
Comfrey
Comfrey is an important herb in organic gardening. It is used as a fertilizer and also has many purported medicinal uses...
is also said to act as a barrier against the invasion of some weeds including couch.
Use of herbicides
The above described methods of weed control avoid using chemicals. They are often used by farmers. However, these methods may damage a fragile soil by restructuring it, hence are not always used. They are those preferred by the organic gardener or organic farmerOrganic farming
Organic farming is the form of agriculture that relies on techniques such as crop rotation, green manure, compost and biological pest control to maintain soil productivity and control pests on a farm...
.
However weed control can also be achieved by the use of herbicides. Selective herbicides kill certain targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often based on plant hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
s. Herbicides are generally classified as follows;
- Contact herbicides destroy only that plant tissue in contact with the chemical spray. Generally, these are the fastest acting herbicides. They are ineffective on perennial plants that are able to re-grow from roots or tubers.
- Systemic herbicides are foliar-applied and are translocated through the plant and destroy a greater amount of the plant tissue. Modern herbicides such as glyphosate are designed to leave no harmful residue in the soil.
- Soil-borne herbicides are applied to the soil and are taken up by the roots of the target plant.
- Pre-emergentPreemergent herbicidesPreemergent herbicides prevent the germination of seeds by inhibiting a key enzyme. In some areas of the world, preemergent herbicides are used to prevent crabgrass from appearing in summer lawns.-Brands:-Crabgrass:...
herbicides are applied to the soil and prevent germination or early growth of weed seeds.
In agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
large scale and systematic weeding is usually required, often by machines, such as liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
sprayer
Sprayer
A sprayer is a device used to spray a liquid.In agriculture, a sprayer is a piece of equipment that spray nozzles to apply herbicides, pesticides, and fertilizers to agricultural crops...
s, or even by helicopter
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
(such as in the USA), to eliminate the massive amount of weeds present on farming lands.
However there are a number of techniques that the organic farmer
Organic farming
Organic farming is the form of agriculture that relies on techniques such as crop rotation, green manure, compost and biological pest control to maintain soil productivity and control pests on a farm...
can employ such as mulch
Mulch
In agriculture and gardening, is a protective cover placed over the soil to retain moisture, reduce erosion, provide nutrients, and suppress weed growth and seed germination. Mulching in gardens and landscaping mimics the leaf cover that is found on forest floors....
ing and carefully timed cutting of weeds before they are able to set seed.
(See also section below on UK legislation regarding the control of certain weeds)
Organic methods
Typically a combination of methods are used in organic situations.- Drip irrigationDrip irrigationDrip irrigation, also known as trickle irrigation or microirrigation or localized irrigation , is an irrigation method which saves water and fertilizer by allowing water to drip slowly to the roots of plants, either onto the soil surface or directly onto the root zone, through a network of valves,...
: Rubber hoses and other methods are used to bring water directly to the roots of the desired plants. This limits weed access to water. - Manually pulling weeds: Labourers are used to pull weeds at various points in the growing process.
- Mechanically tilling around plants: Tractors are used to carefully till weeds around the crop plants at various points in the growing process. Besides tilling, other mechanical weed control methods also exist
- Ploughing: Ploughing includes tilling of soil, intercultural ploughing and summer ploughing. Ploughing through tilling of soil uproots the weeds which causes them to die. In summer ploughing is done during deep summers. Summer ploughing also helps in killing pests.
- Crop rotation: Rotating crops with ones that kill weeds by choking them out, such as hempHempHemp is mostly used as a name for low tetrahydrocannabinol strains of the plant Cannabis sativa, of fiber and/or oilseed varieties. In modern times, hemp has been used for industrial purposes including paper, textiles, biodegradable plastics, construction, health food and fuel with modest...
, Mucuna pruriensMucuna pruriensMucuna pruriens is a tropical legume known as velvet bean or cowitch and by other common names , found in Africa, India and the Caribbean. The plant is infamous for its extreme itchiness produced on contact, particularly with the young foliage and the seed pods...
, and other crops, can be a very effective method of weed control. It is a way to avoid the use of herbicides, and to gain the benefits of crop rotation. - Weed mat: A weed mat is an artificial mulchMulchIn agriculture and gardening, is a protective cover placed over the soil to retain moisture, reduce erosion, provide nutrients, and suppress weed growth and seed germination. Mulching in gardens and landscaping mimics the leaf cover that is found on forest floors....
, fibrous cloth material, bark or newspaper laid on top of the soil preventing weeds from growing to the surface.
Thermal methods
There are several thermal methods known to control weed. Weed burners heat up soil quickly and destroy superficial parts of the plants. Weed seeds are often heat resistant and even react with an increase of growth on dry heat. Since the 19th century soil steam sterilizationSoil steam sterilization
Soil steam sterilization is a farming technique that sterilizes soil with steam in open fields or greenhouses. Pests of plant cultures such as weeds, bacteria, fungi and viruses are killed through induced hot steam which causes their cell structure to physically degenerate. Biologically, the...
is used as a farming technique to clean soil completely from weeds. Several research results confirm the high effectivness of humid heat against weedes and its seeds.
Wood
Besides those kinds of weeds which are of an herbaceous nature, there are others which are woody, and grow to a very considerable size; such as broomBroom (shrub)
Brooms are a group of evergreen, semi-evergreen, and deciduous shrubs in the subfamily Faboideae of the legume family Fabaceae, mainly in the three genera Chamaecytisus, Cytisus and Genista, but also in many other small genera . All genera in this group are from the tribe Genisteae...
, furze
Gorse
Gorse, furze, furse or whin is a genus of about 20 plant species of thorny evergreen shrubs in the subfamily Faboideae of the pea family Fabaceae, native to western Europe and northwest Africa, with the majority of species in Iberia.Gorse is closely related to the brooms, and like them, has green...
and Lantana
Lantana
Lantana is a genus of about 150 species of perennial flowering plants in the verbena family, Verbenaceae. They are native to tropical regions of the Americas and Africa but exist as an introduced species in numerous areas, especially in the Australian-Pacific region. The genus includes both...
s. The first may be destroyed by frequent ploughing and harrowing, in the same manner as other perennial weeds are. Another method of destroying broom is by pasturing the field where it grows with grazers.
The best method of extirpating furze is to set fire to it in frosty weather, for frost has the effect of withering and making them burn readily. The stumps must then be cut over with a hatchet, and when the ground is well softened by rain it may be ploughed up, and the roots taken out by a harrow adapted to that purpose. If the field is soon laid down to grass
Pasture
Pasture is land used for grazing. Pasture lands in the narrow sense are enclosed tracts of farmland, grazed by domesticated livestock, such as horses, cattle, sheep or swine. The vegetation of tended pasture, forage, consists mainly of grasses, with an interspersion of legumes and other forbs...
, they will again spring up; in this case, pasturing with grazers is an effectual remedy. The thorn, or bramble, can only be extirpated by ploughing up the ground and collecting the roots.
In June, weeds are in their most succulent state, and in this condition, after they have lain a few hours to wither, cattle will eat almost every species. There is scarcely a hedge, border, or a nook, but what at that season is valuable.
UK legislation concerning weeds
The Weeds Act, 1959 is described as "Preventing the spread of harmful or injurious weeds", and is mainly relevant to farmerFarmer
A farmer is a person engaged in agriculture, who raises living organisms for food or raw materials, generally including livestock husbandry and growing crops, such as produce and grain...
s and other rural
Rural
Rural areas or the country or countryside are areas that are not urbanized, though when large areas are described, country towns and smaller cities will be included. They have a low population density, and typically much of the land is devoted to agriculture...
settings rather than the allotment
Allotment (gardening)
An allotment garden, often called simply an allotment, is a plot of land made available for individual, non-professional gardening. Such plots are formed by subdividing a piece of land into a few or up to several hundreds of land parcels that are assigned to individuals or families...
or garden
Garden
A garden is a planned space, usually outdoors, set aside for the display, cultivation, and enjoyment of plants and other forms of nature. The garden can incorporate both natural and man-made materials. The most common form today is known as a residential garden, but the term garden has...
-scale grower.
There are five ‘injurious’ (that is, likely to be harmful to agricultural production) weeds covered by the provisions of the Weeds Act. These are:
- Spear thistle (Cirsium vulgare)
- Creeping or field thistle (Cirsium arvense)
- Curled DockCurled DockCurled Dock , also known as Curly Dock, Yellow Dock, Sour Dock, Narrow Dock, sometimes as "narrow-leaved dock" , and ambiguously as "garden patience", is a perennial flowering plant in the family Polygonaceae, native to Europe and Western Asia.The mature plant is a reddish brown colour, and...
(Rumex crispus) - Broad leaved dockRumex obtusifoliusRumex obtusifolius, commonly known as Broad-leaved Dock, Bitter Dock, Bluntleaf Dock, Dock Leaf or Butter Dock, is a perennial weed, native to Europe but can now be found in the United States and many other countries around the world....
(Rumex obtusifolius) - Common ragwort (Senecio jacobaea) (n.b., this weed is poisonous to livestockLivestockLivestock refers to one or more domesticated animals raised in an agricultural setting to produce commodities such as food, fiber and labor. The term "livestock" as used in this article does not include poultry or farmed fish; however the inclusion of these, especially poultry, within the meaning...
. Livestock should not be allowed to graze where ragwort has grown until it is eradicated, and any traces have disintegrated. Ragwort should not be allowed to be harvested in hay or silage for feed).
The Department for Environment, Food and Rural Affairs
Department for Environment, Food and Rural Affairs
The Department for Environment, Food and Rural Affairs is the government department responsible for environmental protection, food production and standards, agriculture, fisheries and rural communities in the United Kingdom...
(DEFRA) provides guidance for the treatment removal of these weeds from infested land. Much of this is oriented towards the use of herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
s, the majority of which may not be acceptable to the organic producer
Organic farming
Organic farming is the form of agriculture that relies on techniques such as crop rotation, green manure, compost and biological pest control to maintain soil productivity and control pests on a farm...
(apart from non-synthetic substances like sulphur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, which in some circumstances are accepted within Soil Association
Soil Association
The Soil Association is a charity based in the United Kingdom. Founded in 1946, it has over 27,000 members today. Its activities include campaign work on issues including opposition to intensive farming, support for local purchasing and public education on nutrition; as well the certification of...
standards) but in most cases there are manual techniques that can be used such as digging out the roots, mulch
Mulch
In agriculture and gardening, is a protective cover placed over the soil to retain moisture, reduce erosion, provide nutrients, and suppress weed growth and seed germination. Mulching in gardens and landscaping mimics the leaf cover that is found on forest floors....
ing out or carefully timed cutting before seeds are able to spread.
Primary responsibility for weeds control rests with the occupier of the land on which the weeds are growing, therefore it is important to be alert to potential weed problems and to take prompt action. However, it should be remembered that most common farmland weeds are not "injurious" within the meaning of the Weeds Act, and many such plant species have conservation and environmental value. When dealing with complaints under the Weeds Act, DEFRA has a duty in law to try to achieve a reasonable balance among different interests. These include agriculture, countryside conservation
Conservation ethic
Conservation is an ethic of resource use, allocation, and protection. Its primary focus is upon maintaining the health of the natural world: its, fisheries, habitats, and biological diversity. Secondary focus is on materials conservation and energy conservation, which are seen as important to...
and the public in general. Constructive discussion about problems caused by weeds can often result in effective solutions and avoid the need for DEFRA to take official action. In addition to those weeds covered by the 1959 act, under section 14 of the Wildlife and Countryside Act 1981
Wildlife and Countryside Act 1981
The Wildlife and Countryside Act 1981 is an Act of Parliament in the United Kingdom and was implemented to comply with the Directive 2009/147/EC on the conservation of wild birds...
, it can be an offence to plant or grow certain specified plants in the wild (see Schedule 9 to the Wildlife and Countryside Act 1981), including Giant Hogweed
Giant Hogweed
Heracleum mantegazzianum, commonly known as giant hogweed, cartwheel-flower, wild parsnip, wild rhubarb, giant cow parsnip, or giant cow parsley, is a plant in the family Apiaceae. It typically grows to heights of , sometimes reaching...
and Japanese Knotweed
Japanese knotweed
Japanese Knotweed is a large, herbaceous perennial plant, native to eastern Asia in Japan, China and Korea...
. Problems involving these plants can be referred to the local authority for the area where those weeds are growing as some local authorities have bye-laws controlling these plants. There is no statutory
requirement for landowners to remove these plants from their property.
See also
- Biological pest controlBiological pest controlBiological control of pests in agriculture is a method of controlling pests that relies on predation, parasitism, herbivory, or other natural mechanisms...
- Invasive speciesInvasive species"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....
- MulchMulchIn agriculture and gardening, is a protective cover placed over the soil to retain moisture, reduce erosion, provide nutrients, and suppress weed growth and seed germination. Mulching in gardens and landscaping mimics the leaf cover that is found on forest floors....
- Noxious weed
- Pesticide applicationPesticide applicationPesticide application refers to the practical way in which pesticides, are delivered to their biological targets...
External links
- Montana Weed Control Association
- Section "Wood" originated from the 1881 Household CyclopediaHousehold CyclopediaThe Household Cyclopedia was an American guide to housekeeping published in 1881.- External links :**...
.