Direct weed controls

Although cultural methods provide the basis for weed management in organic crop rotations it is likely that some form of direct action will be needed against weeds to prevent crop loss at some time. Before taking any action it is important to take an overview and assess whether the weeds present are likely to develop to such an extent that they will cause an immediate loss of crop or will store up potential future problems (e.g. by shedding seed and adding to the soil seed bank so exacerbating future weed problems). If the weed burden is judged to have the potential to cause damage the cost of this should be offset against the likely costs of any immediate or future direct control measures so that direct weeding is only underaken when it is economically beneficial to do so.

  • Mechanical weed control provides an overview of the range of options and implements available for direct mechanical weed control in the field.
  • Manual weed control is still an important component of many weed management programmes and this section provides an overview of techniques available for use on farm.
  • Thermal weed control is becoming more popular and is described in this section.
  • Mulching provides a physical barrier to weed development and is often used in horticultural crops to control weeds.
  • Biological weed control aims to get insects, pathogens or even other plants to do the work of weed management for the farmer.
  • Allelopathy can be regarded as a component of biological control in which plants are used to reduce the vigour and development of other plants.

Mechanical Weed Control

Mechanical weed control may involve weeding the whole crop, or it may be limited to selective inter-row or intra-row weeding. Machines can be used to kill weeds by burying, cutting or uprooting. Tools without a cutting action are only effective on small weeds. Inter-row implements have been designed that control weeds within the crop row by directing soil along the row to cover small weeds. With slow germinating crops inter-row weeding may have to be delayed until the crop seedlings emerge. In some situations it may be possible to include a few seeds of a fast germinating crop in the seed mixture to give an early indication of the position of the crop row.

Mechanical weeders range from basic hand tools to sophisticated tractor driven or self-propelled devices. These may include cultivating tools such as hoes, harrows, tines and brush weeders, cutting tools like mowers and strimmers, as well as implements like thistle-bars that may do both. Two wheeled pedestrian or walking tractors are a smaller alternative that can power a similar range of implements. Custom-made basket or cage-wheeled weeders, with gangs of rolling wire cylinders, offer another way to deal with seedling weeds in a friable soil. The choice of implement, and the timing and frequency of its use may depend on the crop and on the weed population. Some implements, such as fixed harrows, are thought more suitable for arable crops, while others like inter-row brush weeders may be considered to be more effective for horticultural use.

The weather and soil conditions under which the operation is carried out will have a major influence on its efficacy. Soil type, surface structure and moisture content affect the choice and efficacy of mechanical weed control implements. The options may be more limited on heavy or stony soils, most implements work better on light, stone-free soils. Mechanical weeding is less effective when soils are wet during or after weeding operations. Some implements like brushweeders are able to work at a higher soil moisture content than others. Conversely, brushweeders do not work well on dry soils where the surface has capped because the brushes cannot penetrate the crust.

The optimum timing for mechanical weed control is influenced by the competitive ability of the crop. A single inter-row cultivation at any time may provide excellent weed control in a crop like transplanted broccoli that rapidly develops a broad, shading leaf canopy but may be poorer in crops like sweet corn (Zea mays) where early growth is slow, or in green beans (Phaseolus vulgaris) where the growing season is relatively long. In the UK, the optimum timings for mechanical weed control have been defined for onions and for carrots grown in both organic and conventional systems. In organic winter cereals studies have found that , found that corn poppy (Papaver rhoeas) was more effectively controlled in the autumn whilst chickweed (S. media) was controlled best in spring when using a spring-tine weeder.

Weed morphology and stage of growth will also influence the selection and efficacy of weeding implement. In experiments to determine the type of physical damage that gave the most effective control of a range of seedling weeds it has been found that burial to 1 cm depth is the most effective treatment, closely followed by cutting at the soil surface. Plants need to be buried totally to be killed but plant size, angle and growth habit influence the depth of covering required. Some advisors suggest that if weeds have emerged you are already late with your weeding operation, and that the best time to kill them is at the white thread stage. At high weed densities, even with the most effective mechanical weeders, sufficient weeds are likely to survive control measures and profoundly reduce crop yield in cereals and direct control needs to be linked with long term preventative measures to maintain the weed population at a manageable level.

With most mechanical weeding implements, operator skill, experience and knowledge are critical to success. Drawbacks to mechanical weed control include low work rates, delays due to wet conditions, and the subsequent risk of weed control failure as weeds become larger. A review of the merits of six different mechanical weeding mechanisms in controlling inter-row weeds at different growth stages and at different tractor speeds indicated that weed control was not necessarily better at earlier weed stages and weeding too early often missed late germinating weeds. Increasing forward speed did not improve the performance of all the implements equally.

There may be some disadvantages to the greater use of mechanical weed control. The additional cultivations associated with mechanical weeding could harm soil structure and possibly encourage soil erosion. The increased mineralisation of soil nitrogen due to cultivation may be seen by some growers as a problem and by others as an advantage (although this is likely to be limited in effect). There is concern about the impact of mechanical weeding on ground nesting birds and management practices may some alteration to minimise disruption at critical times although evidence is at times contradictory. Any soil cultivation will also contribute to the movement of weed seeds in the soil. Studies of the horizontal movement of freshly shed seeds have shown that a sequence of cultivations could move seeds over 2 m horizontally.

Manual Weed Control

Manual methods of weed control are still widely used in organic systems. Hand weeding is most useful on annual weeds and some perennial weeds. There are times when hand roguing individual plants or patches of plants is the most effective way of preventing them spreading and multiplying. It is widely used for dealing with the removal of difficult-to-control species such as docks, thistles and ragwort.

Manual methods of weed control are also widely used in intensive horticultural crops where it is important to perform a good first weed to prevent weed competition. Hand weeding can often follow after a mechanical inter-row weeding operation in order to thoroughly remove weeds in the crop row. It is a practical method of removing weeds within rows and hills where a cultivating implement cannot be used. It obviously requires more labour than other direct weed control methods and therefore costs are likely to be higher so it is only employed by growers with high value crops like vegetables.

It is generally more efficient for groups or gangs of workers to hoe or hand weed crops as a team, whether directly pulling the weeds or using some type of hoe. Hand rogueing or pulling is a widely used technique for patches of weeds or removing. There are many modern hoe designs that are more comfortable to use than traditional designs and these should be investigated where large areas are being covered. Some designs include the stirrup hoe, the diamond hoe and the collinear hoe as well as wheeled push hoes. Other tools have been designed to tackle specific weeds such as docks, thistles or ragwort.

In more mechanised systems teams of workers lie on a flat bed weeder pulled by a tractor or on other specially designed machines. The speed of the machines can be adjusted to accommodate the level of weeds in the crop.

Thermal Weed Control

One of the earliest forms of thermal weed control, stubble burning, is now banned because of the smoke and other hazards it created. However, this traditional form of thermal weed control was effective in reducing the number of viable weed seeds returned to the soil after cereal harvest. Soil surface temperatures under the burning straw reached in excess of 200oC for 10 -30 seconds and reduced the viability of freshly shed wild oat (Avena fatua) and blackgrass (Alopecurus myosuroides) seed by up to 30% and 80% respectively. Current thermal weed control methods use a variety of thermal weeders to generate the heat needed to kill weed seeds and weed seedlings.

Flaming equipment has been developed in several countries including Germany, Holland, Sweden and Denmark, and a range of tractor and smaller hand operated burners is available in the UK. The main fuel used in the burners is liquefied petroleum gas (LPG) usually propane.

Mulching

Covering or mulching the soil surface can reduce weed problems by preventing weed seed germination or by suppressing the growth of emerging seedlings. Mulches are generally ineffective against established perennial weeds. A mulch may take many forms: a living plant ground cover, loose particles of organic or inorganic matter spread over soil, and sheets of artificial or natural materials laid on the soil surface. Residues from preceding crops may be used to form a mulch but this is discussed in more detail in the use of cover crops to suppress weeds. With mulches consisting of organic materials, crop stand and vigour, particularly of direct-seeded small-seeded crops, may be reduced by chemicals released from the decomposing residues.

It is most practical to use mulches in well-spaced crops, particularly transplants. Plastic sheeting and straw mulches have long been used in soft fruit such as strawberries. In perennial crops and some other situations mulches may be intended to remain effective for many years. Mypex, a black, woven, polypropylene mulch, is expected to last for up to three crops (9-10 years). These mulches may be expensive but labour costs are reduced in the long term. Other uses for mulches include as an alternative to cultivation to clear vegetation before cropping by leaving them in place for 12 to 18 months. In freshly prepared seedbeds, short term mulching can be used to manipulate or reduce weed seedling emergence, by for instance, laying black plastic on the seedbed 2 to 8 weeks and then lifting it before planting brassicas or other crops.

The high cost of mulching makes it economic only for high value horticultural crops unless there is another reason for its use. In addition to weed control, mulches may be used: to prevent soil erosion, reduce pest problems, to aid moisture and to prevent nitrate loss. In strawberries, rain splash dispersal of disease spores like those of black spot (Colletotrichum acutatum) is reduced by straw mulch. Mulches can also moderate soil temperatures. Organic mulches in particular reduce heat loss from the soil in cold conditions and help to prevent frost heave. In hot weather the mulch slows down the warming of soil.

Types of mulches include:

Living mulches: consist of a dense stand of low growing plants established prior to or after the crop. The undersowing of cereals with clover and grass could be seen as forming a living mulch. It has been argued that annual weeds would provide a natural ground cover if managed properly. Living mulches are sometimes referred to as cover crops, but they grow at least part of the time simultaneously with the crop. Cover crops are generally killed off prior to crop establishment.

Often, the primary purpose of a living mulch is that of improving soil structure, aiding nutrition or avoiding pest attack, and weed suppression may be just an added benefit. Maintaining vegetation cover is important for preventing soil erosion, nitrate leaching and weed emergence in slowly developing crops like maize. An investigation of the influence of different mulch species on weed density and diversity indicated that weed numbers were reduced and maize yield was not affected where growth of the mulch was reduced by cutting or flaming treatments. When the growth of a living mulch is not restricted, or when soil moisture is inadequate, even a relatively vigorous crop like potato may suffer competition and loss of yield. Studies have been made of the use of living mulches to suppress weed emergence in horticultural crops but there are many different factors to take into account and it is difficult to get the balance between crop and mulch right. Living mulches are well suited to use in perennial crops such as fruit where self-reseeding is an advantage. However, even in established apple and apricot orchards a living mulch growing along the planted row may depress crop growth. In amenity situations, ground covering plants are established to form a dense canopy and suppress weed germination and growth.

Particle mulches: may be organic or inorganic. Loose materials like straw, bark and composted municipal green waste provide effective weed control but the depth of mulch needed to suppress weed emergence is likely to make transport costs prohibitive unless the material is produced on the farm. It has been shown that a 3 cm layer of compost was needed to prevent the emergence of annual weeds and weed control usually improves as the thickness of the organic mulch increases. Weed seeds in the mulch itself can be a problem if the composting process has not been fully effective or there is contamination by wind blown seeds. In straw mulches, volunteer cereal seedlings are a particular problem due to shed cereal grains and even whole ears remaining in the straw after crop harvest. With particle mulches like straw that consist of light materials there is the possibility of them being blown around by the wind. Organic mulches like straw with a high carbon to nitrogen ratio may deplete the soil of nitrogen as they decompose. Mulch improves water filtration into the soil and prevents the compaction and erosion that heavy rainfall can cause.

Before applying a particle mulch weeds should be removed, dry soil should be moistened, and compacted soil loosened. Old mulch should be removed or incorporated to prevent a build-up. Most mulch is applied 7.5 to 10 cm deep. Coarser textured materials require thicker layers. On sandy soils, the mulch layer needs to be deeper than on heavy or wet soils. The mulch should be raked periodically and topped up if necessary. Machinery has been developed for applying/spreading particle mulches. Bark blowers are widely available. Self-feeding straw blowers can handle 1-2 bales per minute and can cover an acre per hour. Flail-beater chains break the straw into 5-10 cm lengths. For green waste there are all-in-one collectors, shredders and spreaders.

Sheeted mulches: a layer of material such as plastic, paper of woven fabric covers the soil surface. Black polyethylene mulches are widely used for weed control in organic and conventional systems in the UK and elsewhere. Clear mulches are better than black for warming the soil but do not control the weeds. Plastic mulches have been developed that selectively filter out the photosynthetically active radiation (PAR) but let through infra red light to warm the soil. Infra red transmitting (IRT) mulches have been shown to be effective in controlling weeds. Various colours of woven and solid film plastics have been tested in the field. White and green coverings had little effect on the weeds, brown, black, blue, and white on black (double colour) films prevented weeds emerging. There are indications that mulching films, like white on black, with a higher rate of light reflectance are beneficial to the crop. Light reflectance may also affect the behaviour of certain insects, and plastic mulches in a greater array of colours are likely to become available. The woven and non-woven polypropylene films or geotextiles (like Mypex) are sometimes referred to as weed barriers and landscape fabrics. They are more durable than polyethylene films permitting multi-year use and are permeable to water. There are advantages both in reduced laying and disposal costs compared with single season materials.

Sheeted materials are relatively expensive and are usually laid by machine. Machinery has been developed that will raise the soil into beds and lay the plastic mulch, securing it at the edges. Beds can be prepared in advance of crop planting. Heavy duty plastic is used for long term crops such as perennial herbs. Woven polypropylene fabrics allow water to penetrate and are less likely to scorch crops when temperatures are high. Non-woven black fabric mulch may not be sufficiently opaque to prevent weed growth completely. After cropping, lifting and disposal may be a problem with plastic and other durable mulches and this adds to the overall costs. Even the degradable plastics may break into fragments that litter the soil. Sheeting made from paper and other natural fibres have the advantage of breaking down naturally, and can be incorporated into the soil after use. Paper mulches have compared favourably with black polyethylene in trials with transplanted lettuce, Chinese cabbage and calabrese in the UK although tearing and wind blowing can be a problem.

Biodegradable film mulches have been developed that adjust their degradation according to the soil and weather conditions. The polymers are strong enough to withstand mechanical laying.

Biological Weed Control

Biological weed control involves the release of organisms that attack plants to control weeds. The aim of biological control is to shift the balance of competition between the weed and the crop in favour of the crop and against the weed. The biological control agent, normally a fungus or insect, may not necessarily kill the target weed but should, at the least, reduce its vigour and competitive ability. From a practical point of view the organism or agent should prevent the weed setting seed or producing other reproductive parts. There is considerable potential for encouraging the use of native biological control agents against weeds and substantial research effort has been put into biological control in general. However, the application of biological weed control in agricultural systems in Europe has proved difficult and their are no well documented successes.

In practice, there are three basic types of biological control:

Classical (or innoculative) biological control involves the release of exotic natural enemies to control exotic weeds and has been successful against weeds like thistles in the US and Australia where weevils (native to Europe) have been introduced onto the thistles. It has been suggested that some introduced weeds like hogweed (Heracleum mantegazzium), Himalayan balsam (Impatiens glandulifera) and the Japanese knotweeds (Reynoutria spp.) would be ideal candidates for classical biological control but so far it has only been attempted with bracken (Pteridium aquilinum) where attempts to use two South African moths as potential biological control agents were not successful.

The introduction of a classical biological control agent may not be deliberate. A rust, Puccinia lagenophorae, of Australian origin, which attacks a range of Senecio species, was unknown in Europe before 1960 but has since become established in France and the UK on groundsel where it reduces the viability of groundsel plants on which it can be regularly found. It is unknown how this pathogen reached Europe or how it established.

Inundative control involves the mass production and release of native natural enemies against native weeds, for example rust fungus is often used against weeds. Work in this area has concentrated on fungal pathogens of plants as they can potentially be applied as sprays in the same way as conventional herbicides (hence their name myco- or bio-herbicides). Studies on bioherbicides have concentrated mainly on foliar treatments using fungi. Commercial products have been developed (mainly in the US) but success has been limited Soil micro-organisms are often overlooked but are also important as plant pathogens. Several are being investigated as potential biological control agents particularly for control of grass weeds such as downy brome, wild oat and green foxtail.

Although much of the work on biological control agents has concentrated on the growing weed plant, there is considerable potential for using micro-organisms to manipulate or deplete the soil weed seedbank. The persistence of weed seeds in the soil is the key to their success in continuing to emerge despite repeated control measures over many years. Greater predation or an increase in natural decay would reduce the soil seedbank and hence future weed populations. However, there are as of yet no practical or commercial applications available.

Conservation control is an indirect method, which manipulates the habitat around the weeds with the aim of encourging those organisms that attack the weed. This is a long term strategy that requires a detailed knowledge of the ecology of the crop weed habitat, the target weeds and the control agents. It has received little attention to date. One recent example is the upsurge of interest in looking at encouraging the dock beetle on dock plants by creating conditions that favour the beetle.

Livestock can also be considered as biological control agents which can give a broad spectrum control of weeds in various situations. These are discussed in the section on livestock.

The assessment of the potential risks involved in introducing biological control agents remains a difficult and (sometimes) contentious issue as any predictions of how biological control may affect the interaction between species, and influence the life cycle of non-target species is extremely complicated. Even if there were no risk to non-target species, there could still be a conflict of interests because some may perceive a particular plant as a weed while others see it as a desirable wild flower, or even a potential crop. For this reason it seems difficult to imagine that of the shelf biological control of weeds is a realistic prospect in the short to medium term.

Download our Review of biological Weed Control, 2007

Allelopathy

Allelopathy can be regarded as a component of biological control in which plants are used to reduce the vigour and development of other plants. Allelopathy refers to the direct or indirect chemical effects of one plant on the germination, growth, or development of neighbouring plants. This can be through the release of allelochemicals while the plant is growing or from plant residues as it rots down. These chemicals can be released from around the germinating seed, in exudates from plant roots, from leachates in the aerial part of the plant and in volatile emissions from the growing plant. Both crops and weeds are capable of producing these compounds and in this case the desired effect is the impaired germination, reduced growth and poor development of weeds.

Potentially allelopathy could be used in various ways:

  • to manipulate the crop-weed balance by increasing the toxicity of the crop plants to weeds thereby reducing weed germination in the direct area of the crop, which is the most difficult area to control physically
  • as cover crops to suppress weed germination and development over a whole field in part of a rotation
  • as mulched residues or incorporated residues which could prevent weed germination and allow transplanted crops to be grown, producing a residual weed control effect

Many crops have been reported as showing allelopathic properties at one time or another and farmers report that some crops such as oats seem to clean fields of weeds better than others. The current list includes: wheat, barley, oats, cereal rye, brassicas, red clover, yellow sweet clover, trefoil, vetch, buckwheat, lucerne, rice, sorghum.

However, several weed species have also been reported to show allelopathic properties. They include couch grass, creeping thistle and chickweed. Where they occur together they may have a synergistic negative effect on crops.

Allelopathic effects might also depend on a number of other factors that might be important in any given situation:

  • Varieties: there can be a great deal of difference in the strength of allelopathic effects between different crop varieties
  • Specificity: there is a significant degree of specificity in allelopathic effects. Thus, a crop which is strongly allelopathic against one weed may show little or no effect against another
  • Autotoxicity: allelopathic chemicals may not only suppress the growth of other plant species, they can also suppress the germination or growth of seeds and plants of the same species. Lucerne is particularly well known for this and has been well researched. The toxic effect of wheat straw on following wheat crops is also well known
  • Crop on crop effects: residues from allelopathic crops can hinder germination and growth of following crops as well as weeds. A sufficient gap must be left before the following crop is sown. Larger seeded crops are effected less and transplants are not affected
  • Environmental factors: several factors impact on the strength of the allelopathic effect. These include pests and disease and especially soil fertility. Low fertility increases the production of allelochemicals. After incorporation the alleopathic effect declines fastest in warm wet conditions and slowest in cold wet conditions