Crop Weed Management Strategies

The details of any weed management approaches used will obviously vary from crop to crop and will incorporate elements of the cultural and direct control measures. In this section we have drawn together some of this information and provided pointers to organic weed management outlines for specific crops based on the results of research work and practical experience. These outlines are intended to give an overview of non-chemical weed control opportunities and developments in the crops covered. These include historical information and summaries of more recent research. Although many aspects of crop production that have some bearing on weed control may be covered here, the outlines are not meant to form a crop management protocol. Actual weeding practice in any crop is also likely to depend on crop factors (such as position in rotation, plant spacing and architecture), field factors (like soil type, weed seed bank history), as well as seasonal factors (like rainfall).

Amenity and Conservation Areas

Areas of amenity land are found on roadside verges, along railways, grassed areas on riverbanks, golf course roughs, in cemeteries, country and urban parks and on land associated with housing and landscaped areas around other building complexes. The planting may be formal or not depending on the location but visual appearance is often of some importance. Grasses make up the greatest proportion of the vegetation in amenity and conservation sites whether as a sports or playing surface or simply as ground cover. Whatever the vegetation some form of maintenance will be required and it is likely that the cost of doing so must be kept to a minimum.

While amenity plantings are not considered a crop where yield needs to be optimised, the desired plants do need to be protected from weed competition, at least during establishment. For instance, newly planted trees and shrubs can suffer severe stunting if left unweeded. However, there is increasing interest in using amenity areas to serve as refuges for wild plants and animals. Certain areas such as field boundaries, hedges and ditches are of particular importance for wild life conservation.

Apart from maintaining biodiversity, the objectives of management may include height control, maintenance of species composition, weed control to limit certain species, reduction of fire risk or some requirement specific to the location.

Field Vegetables

Alliums

Allium crops germinate and grow slowly, they generally have rather few leaves and for most of the growing season the soil is not fully covered. Weeds can therefore germinate over a long period. Allium crops are also very sensitive to weed competition, so weed control is of particular importance. Weeds are more of a problem in crops grown from seed than in transplanted or vegetatively propagated crops. In general a drilled onion crop will produce no yield if it is left unweeded. At crop harvest, weeds foul undercutting and lifting machinery and prevent onion bulbs drying in the windrow.

The basic strategy for weed control in Allium crops starts with the choice of the field. The structure of the soil is important, and so is the preceding crop. The preparation of the seedbed is also a very important part of any weed control strategy. The seedbed should be fine, crumbly and level. Allium crops should not be sown or planted too early, otherwise the crop will grow slowly, giving the weeds the advantage of a longer germination period. Because perennial weeds are very difficult to control in Allium crops, they have to be controlled in the preceding crop.

Crop establishment generally follows conventional primary and secondary cultivations. The main methods of weed control are mechanical and thermal. Mechanical control includes harrowing and hoeing, while thermal control involves flame weeding to control small seedling weeds. The success of these methods depends on timing, on weather and soil conditions, and on the composition and density of the weed population. Crop rotation is important for disease control but volunteer weeds can be a problem when following crops such as potatoes, cereals and oil-seed rape.

Artichokes

Jerusalem Artichokes: land should be cleared of perennial weeds in previous rotations. The crop is fast growing and able to smother annual weeds. The artichokes themselves are liable to become volunteer weeds in following crops if not all the tubers are harvested. The plant is capable of rapid spread by means of rhizomes and tubers although tubers and rhizomes that fail to shoot do not persist in the soil for more than a year.

Asparagus

It is vital that the land is free from perennial weeds before the crop is established. Cultivation is not possible within the row during spear production. Soil cultivation to build up and maintain the planting ridges allows for some weed control. Ridging may be done by hand but mechanisation is used on larger plantings. In early autumn the fern is cut down and cleared and the soil on the ridges and in the valley bottoms lightly cultivated to improve the tilth for ridging in late autumn. Shallow cultivations may be possible to control winter annual weeds that have emerged. In spring cultivations are kept to a minimum on the ridges to avoid spear damage. The in-row weeds may be controlled by hand-weeding, mulching or with the use of geese. Flame weeding has been used against annual broad-leaved weeds but little information is available on crop stage. A low-growing living mulch that will suppress weeds is another possibility but there may be some competition with the crop.

Beans

Beans such as field beans and runner beans can be an important component of both arable and vegetable rotations. Weed management will obviously depend on the specific situation in which crops are being grown and the ultimate market. Weeds are a particular problem in crops grown for processing. At crop harvest, easily uprooted weeds can contaminate the crop with soil and stones carried on their roots. Other weeds become entangled in the harvesting machinery. Berries, flower heads and seed capsules can contaminate the final product unless removed by flotation or by hand.

Field beans: Autumn-sown field beans are generally resilient to weed interference and in some field studies the maximum yield loss was only 33% although in studies of the time of weed emergence relative to the crop, the earlier the weeds emerged the greater the level of yield reduction.

The cultivations needed for seedbed preparation depend on the weeds present and the time of sowing. Disking may be required if there is a high weed population present. Later sowings demand a finer seedbed and may involve using spring-tines. There may also be time to include a stale seedbed approach. Broadcasting the seed or using narrow row spacing gives greater weed suppression but reduces the weed control options. Wide row spacing facilitates mechanical weed control. The number of passes depends on weed density and crop and weed growth stages. Weed control may be initially with spring-tine harrows then inter-row hoeing. Blind harrowing and inter-row cultivations can keep the weeds under control at early crop stages. Once the early weeds are controlled, field beans tend to smother out later emerging weeds.

Beets

Three cultural types of beets (Beta vulgaris) are commonly grown in the UK including sugar beet grown as a field crop, fodder beet grown as a field crop for its roots and foliage for animal consumption, and beetroot, red or table beet grown as a vegetable crop outdoors or under protected conditions for the edible roots. Weed management practice will depend on which beets are being grown.

Beetroot:the optimum time for weed removal is around 3-4 weeks after crop emergence. Once the weeds have been removed the crop has some capacity for recovery from a check to growth due to the weeds Land should be free of perennial weeds. Conventional primary and secondary cultivations are used to establish the crop. False or stale seedbeds will reduce weed numbers in the growing crop. As with sugar beet (see below), once the crop has emerged, regular inter-row cultivations with brush weeders, ridgers, steerage hoes, finger-tines etc will deal with weeds between the rows. There will be some effect on intra-row weeds but hand weeding may be required.

Fodder beet: may be drilled or broadcast. Punch planting makes use of the stale seedbed technique but minimises soil disturbance even further by dropping the seed into holes made by a dibber. This technique has been shown to reduce weed density by 30% compared with a normal drilled crop. Pre-emergence flaming has been shown to reduce weed numbers by 34 to 44 %. However, in relatively low value arable crops such as fodder beet that are grown on a large scale the cost of flame weeding may not be justified.

Sugar beet:Sugar beet usually follows a grass/legume leys or a cereal in the rotation and precedes a cereal or some other crop that will benefit from the residues of any manure application. Sufficient nitrogen from manure or compost application is important to ensure rapid the leaf development that will provide a dense leaf canopy and shade out the weeds. The primary and secondary cultivations required for seedbed preparation will have a considerable influence on the weeds. However, the nature and timing of these cultivations will vary with the previous crop, with soil type and with soil condition at the time of any operation. In general a level crumbly seedbed will give the crop the best start but growers may want to keep cultivations to a minimum. Weeds tend to emerge better and in greater numbers from a fine seedbed than a coarse one, but control measures are often more effective on a fine level seedbed.

It is important to achieve a good crop stand, as it is the dense leaf canopy that shades out emerged weeds and inhibits later flushes of seedling weeds. As the crop canopy does not close fully until mid-summer and tall growing weeds such as fat-hen and certain mayweeds may grow above the canopy before it closes. In the UK, the optimum weeding period is between 4 and 6 weeks after 50% crop emergence. In practice, weeding operations should commence at the 4-6 leaf stage and may cease at around the 10-12 leaf stage. Once the optimum weeding time has been reached yield may be depressed by 1.5% for each day the crop is left unweeded, although sugar beet has some ability to recover from an early check.

Brassicas

Brassicas can follow on directly after a ley or follow a cereal crop. The land should be ploughed and prepared early in the year to allow time for cultivation before planting that will kill any emerged weeds. Following crop establishment, weeds should be controlled without delay. Crop cultivars vary in the ability to suppress weeds because of differences in morphology. In vegetable brassicas the choice of cultivar may be limited by the need to schedule maturity to achieve continuity of production. The use of crop covers to advance maturity and protect vegetable brassicas from insect pests will hinder weed control and may enhance weed emergence and growth. Precise row spacing and careful alignment of cultivating tools facilitate mechanical weed control. Many vegetable crops, including brassicas, are grown on the bed system and cultivators and other implements should be matched to the bed widths. Weeding is repeated as necessary, and cultivations can become more thorough as the crop develops. When the crop plants are large enough not to be buried, tools can be arranged to move a 1-inch layer of soil towards and into the crop row. Where mechanical control is not possible, hand weeding may be required.

Broccoli: transplanted broccoli rapidly develops a broad, shading leaf canopy. Inter-row weed control with a row crop cultivator, spider gang tool or a brush hoe 15 to 25 days after planting has given good weed control. Flexi-tine harrows provide adequate within and between row weed control but damage poorly established crop plants and reduce yield. Mechanical weed control with round and flat flexi-tines, brush hoe, rolling cultivator or shovel cultivator, alone or in sequence with flexi-tine weeders just prior to weed emergence at 13-26 days after crop planting has been shown to control weeds. As with cabbage, the broccoli crop has some tolerance to flame weeding and treatment at 2 weeks after crop planting has been evaluated.

Brussels Sprouts: are generally a weed-suppressing crop and can be ridged up to improve weed control around the sprout plant. The wide spacing of the crop gives ample scope for mechanical weed control. Transplanted Brussels sprouts have an initial advantage over emerging weed seedlings, nevertheless, if left unweeded sprout yield is likely to be reduced by 13 to 24%. Close crop spacing (45 x 45 cm) increases weed suppression compared with wider spacing (61 x 61 cm) but within-crop competition then tends to reduce the yield of individual sprout plants. At closer row spacing, access for tractor steerage hoes becomes more difficult after early August. Early weed control is vital to avoid yield loss. As with cabbage, the Brussels sprout has some tolerance to flame weeding and treatment at 3 weeks after crop planting has been tried successfully.

Cabbage: different types of cabbage include spring, summer, autumn, and winter cabbage including winter white and Savoy. Crops are established following conventional primary and secondary cultivations. Cabbage may be direct-sown but is more likely to be grown from bare-root or module-raised transplants. Good weed control is essential to maximise crop uniformity, quality and yield. Weed control is helped by an appropriate rotation that will avoid the build up of large weed populations. Making the most of weed control opportunities in one crop can frequently ease weed control problems in the crop that follows. The key period of weed control is the first four weeks after transplanting. Weeds that emerge after this are less likely to compete with a well established crop. In the drilled crop, single weeding at 3 weeks after crop emergence can give yields similar to those of the weed-free crop. In transplanted cabbage a weeding a single but thorough weeding 3 to 8 weeks after planting has been sufficient to prevent yield loss.

The preparation of a false or stale seedbed in the direct-sown crop may be used to reduce weed numbers before drilling the crop. The use of a false seedbed to reduce weed numbers in the transplanted crop is also feasible. Secondary cultivations may be used to kill emerged and germinating weed seedlings but must be shallow enough to avoid stimulating a further flush of weed emergence. Flame weeding will also kill the emerged seedlings and without disturbing the soil. The period from drilling to crop emergence is usually too short to provide an opportunity to kill early emerging weeds before the crop appears. If conditions allow, delayed drilling of the crop may give weeds more time to emerge and be killed before crop emergence. Careful timing of flame weeding is needed to kill the maximum number of weeds without damaging the emerging crop seedlings.

The crop should be kept free of weeds by surface cultivations using a tractor-mounted cultivator between the rows and hand hoeing around the plants. Soil should be drawn up towards the plants. Weeds between crop rows can be removed mechanically using a steerage hoe or brush-weeder. Both these machines work well but the brush-weeder has more flexibility to work in wetter conditions. The main advantages of the steerage hoe are the work rate, which is about three times faster than the brush-weeder. The main disadvantage with mechanical weed control is that weeds within the crop row cannot be removed, but plant spacing can be adjusted to allow earlier ground cover within the row to smother weeds.

Hand-hoeing between the rows may be used during early crop establishment, but the crop rapidly forms a dense leaf canopy that helps to suppress further weed development. Hand-labour requirements for weeding have been estimated at 50 man hours/ha. Inter-row cultivations with brush-weeders, finger-tine weeders, harrows or tractor-drawn hoes will control weeds within the crop row. The number of passes needed will depend on the weed population. As the crop develops the leaves spread into the inter-row and care is needed to minimise crop damage. Flexi-tine weeders with either flat or round tines used 14 + 24 days after transplanting gave adequate weed control and damaged the cabbage only slightly. Similar results were given by flexitime cultivations at 10 + 20 days followed by an S-tine row crop cultivator at 30 days after planting. The tines caused only slight damage to the cabbage.

Transplanted cabbage has a relatively high tolerance to heat, enabling band-flaming to be used along the crop row. However, the level of tolerance within each individual crop will depend on the growth stage of the crop and the waxiness of the leaves. After crop establishment, thermal weed control may be tolerated by cabbage if shields are used and the burners are directed away from the crop row. The aim is to kill seedling weeds in the inter-row with minimal soil disturbance. At an early stage most broad-leaved weed seedlings will be killed but grass weeds have a basal growing point and therefore a higher tolerance of flame weeding. Perennial weeds are also unlikely to suffer any long-term damage. The tolerance of most weeds increases with age and growth stage.

Mulches laid directly onto the soil surface provide a physical barrier to weeds. Materials available for use as mulches include; black polythene, black non-woven polypropylene and paper. Black polythene is the cheapest but water penetration is poor and drip irrigation systems may need to be laid first. Machines are available that can lay the mulch and plant through in the same operation. Black polyethylene is generally left down for the duration of a crop but studies have been made where the sheeting has been laid on the seedbed for much shorter periods and then lifted before planting brassicas. The short term covering of the soil with black polyethylene reduces subsequent weed emergence giving the crop an advantage over the weeds. Straw mulches have been evaluated in trials, but there appears to be a detrimental effect on yield, probably due to a temporary shortage of nitrogen as the straw decomposes.

Cauliflower: should be placed early in the rotation to take advantage of higher fertility after the grass/clover leys. The seedbed should be weed free at the time of planting. One or two inter-row passes with a brush weeder, steerage hoe or finger weeder should be sufficient unless weed pressure or weather conditions result in poor control. Crop canopy closure should suppress later weed emergence and growth. As with cabbage, the cauliflower crop has some tolerance to flame weeding and treatment at 2 weeks after crop planting has been evaluated.

Kale: in transplanted kale, a single cultivation 10 days after planting was sufficient to prevent yield losses due to weeds. A further cultivation 10 days later reduced weed biomass but did not improve crop yield. Inter-seeding with the winter cover crops winter rye (Secale cereale), hairy vetch (Vicia villosa) or a mixture of the two after the final cultivation allowed the cover crop sufficient time to establish without sacrificing crop yield. Intercropping maize with kale for silage production can have advantages in terms of yield and weed suppression over the crops grown alone.

Swede: the crop is often sown late to avoid turnip fly and mildew, leaving time for cleaning the land before sowing. Inter-row cultivations can be used after crop emergence. The optimum time for weed removal is 6 weeks after sowing or 2-4 weeks after crop emergence. Weeds that emerged and competed with the swedes for up to 28 days after crop sowing had no effect if the weeds were removed and the crop then kept weeded. Swedes had some ability to recover after removal of weed competition.

Turnip: has been considered a cleaning crop in the rotation because regular inter-row cultivations can be carried out until canopy closure. Sowing time is generally mid to end of May but wet conditions may delay this until June. The crop is often sown late to avoid turnip fly and mildew, leaving time for cleaning the land before sowing. Inter-row cultivations can be used after crop emergence. By sowing turnips after a ley all the weeds that germinated could be destroyed by cultivations. The optimum time for weed removal is 2-4 weeks after crop emergence. In the past, the inter-rows were horse-hoed and the rows were hoed when the crop was singled. A second weeding was carried out later if required.

Carrots

In drilled carrots left weedy through to harvest yield losses can range from 0 to 100%. Even low numbers of weeds can have a serious effect and weed biomass gives a better measure of potential crop losses than weed density. They have little ability to recover after removal of weed competition. Keeping the crop weed-free for a period of up to 42 days after sowing is very effective in preventing yield loss. Hand weeding may be cost effective in organic carrots but growers may spend 100 to 300 man hours per ha on hand weeding. Stale seedbed cultivations, flame weeding or brush weeding and other inter-row cultivations can reduce the need for this and are more economic in the long term. On land with a low weed population, pre-emergence flaming followed by a single weeding at 3 to 5 weeks after crop emergence has been sufficient to prevent crop losses due to weeds. For post-emergence weed control, a steerage hoe was more successful than a flame or a brush weeder in the situation under which the trial was done. The weed flora was relatively tolerant of flaming and a hard soil cap made the brush weeder ineffective. Covers (applied for early emergence or carrot fly control) also increase weed growth up to four-fold and additional weeding may be needed under protected crops

Celery

Celery intercropped with leeks has been shown to aid weed suppression and increased the overall yield from the cropped area but reduced the quality of the leeks.

Lettuce

In drilled lettuce left weedy through to harvest yield losses can range from 0 to 100%. However, because of the effect on crop quality marketable yield is often zero.

Any site should be free of perennial weeds. The seedbed should have a fine tilth and if prepared early enough there will be an opportunity for a weed strike and final shallow cultivation to reduce the weed population before planting. The stale seedbed technique can be effective if there is sufficient moisture for weed germination. Flame weeding applied post weed emergence and before lettuce planting was most successful when weeds were small.

Precise row spacing and careful alignment of cultivating tools facilitate mechanical weed control. Closer spacing of lettuce may suppress weeds better but it will make mechanical or even hand weeding difficult should it become necessary. A minimum row spacing of 25 cm is needed to allow mechanical weeding. One or more passes with a brush weeder, steerage hoe or tine weeder may be required depending on the weed population. All can be equally effective but certain implements are better than others in particular conditions. A first pass may be needed 3-4 weeks after planting. Weeds should be dealt with while small. Hand-labour is not normally needed but has been estimated at 20-25 man hrs/ha. Tractor hoeing will take 5-6 hrs/ha.

Fleece covers are sometimes used for early production and for pest control. Weeds also benefit from the conditions under the covers, emerging in greater numbers and growing faster.

Parsnip

Parsnips are slow to emerge and slow to develop therefore early weed control is very important. As with carrots, the use of stale seedbeds and pre-emergence flaming followed later by crop ridging can aid weed control

Pea

The effect of weeds on the yield of peas depends on relative times of crop and weed emergence and differences in seasonal rainfall. Peas that emerge first generally suffer less competition. In vining peas, increasing crop density reduced the total fresh weight of the weed flora but at higher crop densities, within crop competition increases and can affect yield so this has to be balanced against the benefits of weed suppression. Even at high crop densities weeds can still reduce yield through competition while at low crop densities the weeds reduce tillering of the peas. Weeds also reduce vining throughput during crop harvest.

Cultivar choice and seeding rate are important aspects of weed control in organic pea production. Seeding rate should be as high as economically possible, 120 plants per m-2 being seen as the minimum. Vigorous cultivars that grow fast and have a high biomass accumulation are needed to achieve the greatest competitive ability. Larger-seeded cultivars tend to be more competitive than the smaller petit pois types. Zelda and Ambassador are good quality high yielding varieties.

The semi-leafless nature of many modern cultivars allows greater light penetration through the crop to the weeds. In tests of the competitive ability of pea cultivars it was shown that vigorous types had a higher competitive ability. Smaller cultivars and semi-leafless peas appeared less competitive.

Chain harrows can be used pre-emergence and even post-emergence of the crop if compensated for by higher seed rates. In vining peas a single, relatively late inter-row hoeing when peas had 4-9 nodes and weeds were 5-10 cm tall, generally controlled weeds better than an earlier treatment. A sequence of an early followed by a later cultivation did not given any better weed control. Additional treatment with a torsion weeder gave only slightly improved weed control.

Radish

In favourable conditions, radish emerges earlier and grows faster than naturally occurring weeds. The crop is able to mature before the onset of weed competition. In addition, few weeds have reached the flowering stage before crop harvest so seed shed is not a problem. There is therefore no need for a carefully timed weeding if crop yield is the only concern.

Squashes

Marrows and Courgettes: there is usually the opportunity to prepare a stale seedbed and then use inter-row cultivations and hand-weeding to deal with subsequent weeds. Marrows and courgettes may also be planted into a black polyethylene mulch. The system is expensive due to the costs of materials and labour. The plastic mulch could be combined with the use of crop covers for earlier harvesting.

Sweet Corn

Weed competition trials with rows 25, 51 or 89 cm apart (with plants 30 cm apart in the row) demonstrated that in unweeded plots there was somewhat less yield loss in the narrower crop rows. Keeping the crop weed-free early, for 2, 3, 4 or 5 weeks was better than leaving the crop weedy for 5 weeks and weeding from then onwards.

Trials on inter-row cultivation with a row crop cultivator, a spider gang tool or a brush hoe did not provide adequate weed control in sweet corn, nor did flexi-tine harrowing. Cultivations must be shallow to avoid root damage. Flame weeding has given short-term weed control but could not maintain control of germinating weeds through the season. Others suggest corn can be flame weeded when it reaches a height of 4 inches and can be flamed until canopy closure. If flamed earlier than 4 inches tall the crop should not be treated again until it reaches a height of 6-8 inches. The crop row is flamed across with burners mounted in pairs, but staggered to avoid overlap, and set at an angle of 30 to 60 degrees from the horizontal. In addition to directed burners the use of leaf protectors reduces crop injury.

Swede and Turnips

(see Brassicas)

Fruit

Soft Fruit Crops

Weeds hamper picking and make ripening uneven. Newly planted crops are particularly sensitive to weed competition.It is vital that soft fruit crops are given the best growing conditions possible to give them an advantage over weeds. Soil preparation should be thorough to allow crop roots to penetrate as deeply as possible. Sub-soiling to break up the soil pan is an important preparation. Before crop planting the soil should be moist enough to sustain the crop. Bare roots should not be allowed to dry out or the crop will suffer a check to growth that will reduce yield.

Strawberries: as in other row crops, strawberry normally has a limited ability to cover the ground to suppress weeds. This is most evident in the year of transplanting when annual weeds may be problematic. In established strawberry fields, the cover of strawberry plants is normally good enough to suppress the annual weeds to some extent in the row. Perennial weeds may be problematic even within the rows. Between the rows both annual and perennial weeds have no competition from the crop. Annual weeds germinating in spring-planted strawberries had no effect on crop growth if removed by late May. Delaying weed removal inhibits stolon growth and leaf production but crop plants are able to survive the competition. Allowing weeds to remain until July, August or November in the planting year, reduces crop yield in the following year by 34, 54 and 67% respectively. It is important that the main spring flush of weeds is controlled. Under some circumstances weeds germinating in June may also adversely affect crop growth.

The land where strawberries are planted should as far as possible be free from perennial weeds. A traditional fallow based on cultivation can be an effective pre-planting treatment against shallow-rooted perennial weeds, such as common couch (Elymus repens), but to a lesser extent against deep-rooted broadleaved weeds such as creeping thistle (Cirsium arvense) and field bindweed (Convolvulus arvensis) . A bastard fallow may be used to clean up land containing some perennial weeds but strawberries should not be grown where creeping thistle is a serious problem. The planting bed can be prepared in advance using the stale seedbed technique allowing weeds to be flamed or rotovated before crop planting. Mechanical weed control is possible between the rows where the strawberries are planted in single rows without polythene mulch. A range of cultivators is used including brush weeders, finger weeders and rotary or rolling cultivators. Within the rows, straw can help to suppress weeds. Straw is also used between the beds. Perennial weeds may need to be removed by hand.

Where strawberries are planted into polyethylene mulch, the sheeting can be laid up to 4 weeks in advance of planting to encourage weed seedlings to emerge and die under the covering. Sheeting of 38 microns thickness is used for 1-year crops and 50 microns thickness for 3-year crops. Woven plastic mulch (Mypex) will last 9-10 years, much longer than black polyethylene, but is expensive. Straw mulch, 15 cm deep can be laid in the pathways to keep down the weeds. Straw can also be used as an alternative to plastic mulch.

Rhubarb: covers the ground early and for much of the season which helps to suppress annual weeds but the land must be clear of perennial weeds. After planting on clean land the rhubarb should be mulched heavily thereafter. In one set of trials with rhubarb a straw mulch 15 cm thick controlled weeds better and was more cost effective than herbicide or hand-weeding treatments. The straw-mulched plots produced larger plants and higher yields in field trials over 6 years. A layer of straw applied before January protects young growth from frost and brings the crop on earlier. In Yorkshire, wool shoddy is still applied as a nitrogen source to rhubarb roots being grown for the 2 years prior to forcing. Weeds known as the wool aliens have been introduced in this way over many years. The country of origin of the weeds depends on the source of the wool.

Bush Fruits: plantations, once established can remain productive for many years. New plantations should be laid out on land that has not grown fruit crops previously. Bush fruits can follow arable crops or grassland. The land may require subsoiling to remove compaction or shallow cultivation to break up the vegetation cover prior to ploughing. Soil inversion will bury established weeds and encourage further seed germination. If the soil is left for a while the seedling weeds can be killed by flaming or shallow cultivation. Perennial weeds may require removal by hand but land infested with perennial weeds should be avoided as far as possible. Black polyethylene entire or woven sheeting is widely used to suppress weeds within the crop row. A range of other materials is also used including straw, bark chippings, old carpet, grass clipping and composted organic materials. Some organic materials are likely to lock up nitrogen as they decompose. Loose materials need to be applied as a substantial layer, at least 3 inches deep to prevent weed seedling emergence. Where polyethylene mulch is going to be used, a trickle irrigation system may need to be put in place before the plastic sheeting is laid if irrigation is likely to be required.

Top Fruit Crops

Most orchards now have grassed alleys but recently established crops are particularly sensitive to competition. Even the grass strips can reduce extension growth by 20 to 60% if too close to the crop row. Repeated inter-row cultivations may be needed initially supplement with hand-hoeing. In widely spaced crops the area between rows could be ploughed or rotary cultivated. Cultivations will bury the emerged weeds but may stimulated further flushes of emergence and spread perennial weeds. Once the crop is established, soil disturbance may damage the surface roots.

Apple: Weed control by mulching with entire or woven polythene sheeting is possible but is difficult to lay around established trees. Once laid, the mulch needs to be cleared off periodically to prevent weeds establishing on top of it. In a comparison of mulches of asphalt paper, bark, plastic, barley- and rape-straw, with flaming and mechanical weed control, covering with straw gave the highest yield. Yield in plots covered with bark was low because common couch grew through the 10 cm layer of mulch. Flame weeding did not control annual meadow-grass.

Miscellaneous Crops

Grassland

Grassland or clover/grass leys are an important part of the organic farming system in the UK. Up to 70% of the farmed area comprises of mixed grasses and legume leys. Grass may be managed as a short, medium or long-term crop and this may determine the composition of the desirable sward species and the nature of the associated weeds. A survey of 502 farms in England and Wales where grass was the major crop (and at least half was permanent grass) found that 50% of farmers thought thistles, chiefly creeping thistle, (Cirsium arvense) were a problem while 40% considered docks (Rumex spp.) to be a problem. Thistles were mentioned more by beef farmers while docks were highlighted by dairy farmers. Docks appeared to be associated with low potassium and high phosphate while the opposite was true for thistles.

Oil and Fibre Crops

Oil Seed Rape: in organic crops broad-leaved weeds are considered to be less of a problem than wild oats and blackgrass. In a survey of conventional winter oilseed rape in central southern England in 1985 cleavers was the most frequent weed being found in 57% of fields whilst common poppy, prickly sowthistle (Sonchus asper) and scentless mayweed (Tripleurospermum inodorum) were also common (21, 18 and 14% of fields respectively). Volunteer barley can have a severe effect in the autumn on the growth of winter oilseed rape and, in Canada, it has been shown that increasing the seed rate improves crop yield when volunteer barley is a problem but there is no advantage when the crop is weed-free.

Linseed/ Flax: the crop is not competitive and cannot compete against common chickweed, charlock, wild oats and other quick growing annuals. Fields should be free of perennial weeds. Experiments to determine the competitive effects of weeds on spring-sown linseed found wild oat (Avena fatua) and knotgrass (Polygonum aviculare) to be highly competitive. Chickweed (Stellaria media) and fat-hen (Chenopodium album) were less damaging. Chickweed formed a low mat that did not affect the crop growth while the fat-hen did not become as vigorous as it does in many crops. In winter linseed, chickweed and cleavers could be left in the crop until March-April without seriously jeopardising yields. Flax cultivars differ in their susceptibility to the allelochemicals produced by wild oats and some other weeds.

Hemp: there is time before drilling in late April to prepare a stale seedbed. Once the crop is established the foliage is dense and suppresses weeds, minimising further weed control needs. Weed free crop material is essential for processing.

Potato crops

Weeds reduce crop yield by an average of around 36% but losses can be anything from 14 to 80%. Weeds are not always a problem in potato but control may be considered necessary to safeguard crop quality and yield. Perennial broad-leaved weeds including creeping thistle, field bindweed, the docks, and the perennial grass weeds, common couch and black bent are particular problems in potatoes. Among the annual weeds, taller species such as fat-hen are the most problematic. In the UK a single weeding between 2 and 8 weeks after crop planting has been shown to be sufficient to prevent a significant yield loss.

In potato cultivars, greater competitive ability is associated with early emergence, rapid early growth and the ability to develop a dense leaf canopy. Chitting the tubers aids early establishment of potato crops. Cultivars may differ both in growth rate and habit which may affect the time to canopy closure. Some cultivars are known to produce taller and leafier foliage than others. However, mechanical weed control can damage lush foliage especially later in the season. Increasing planting density may improve weed suppression but it also increases establishment costs and may encourage blight due to the greater density of foliage.

The normal cultural practice is to ridge shortly after planting and let the ridges settle. Weed control is then applied ten days after planting using chain harrows, ridgers or purpose built weeders. Harrowing down the ridges between planting and emergence, hastens crop emergence as well as controlling weeds. The number of passes depends on weed density. Where a second harrowing is needed this may be carried out at or shortly before crop emergence. Thermal weed control can also be used to control seedling weeds prior to crop emergence.

Inter-row cultivations between the ridges and re-ridging are carried out as needed post crop emergence. Cultivations are best done when weeds are small and unlikely to re-establish. Rolling cultivators used by some growers have tines that weed between the rows and rolling star-shaped tines to cultivate the sides of the ridges. The ridges are then rebuilt by ridging bodies that follow the tines. In addition to controlling weeds, earthing up the ridges covers the developing tubers and reduces the incidence of tuber blight. Post planting cultivations should be kept to a minimum to limit water loss and crop injury. Crop damage can result in an average yield loss of 7% but losses due to weeds can be considerably higher.

There has been some interest in the use of living mulches for weed control. Cover crops such as vetch, oats, barley and red clover have been evaluated for their potential ability to suppress early-season weeds in potatoes where the cover-crops were inter-seeded at ridging or 3, 4 or 5 weeks after crop planting. Weed control with red clover was consistently poor. The cereals and vetches gave good weed suppression but reduced crop yield. The application of green manure material from mustard and oilseed rape can result in weed suppression due to the release of allelochemicals as the mulch decomposes. In the USA, an autumn-sown rapeseed green manure crop incorporated in spring prior to potato planting reduced weed density by 73 and 85% in different years, and reduced weed biomass by 50 and 96% in those years. Mulches of paper, plastic and other materials have given good control of weeds but are economic only in high value early potato crops. Where early potatoes are grown under floating covers, weed control is difficult until the covers are removed.