Mark Schonbeck, Virginia Association for Biological Farming
Healthy soil, optimum nutrition, appropriate planting dates, and best cultural practices enhance the ability of most vegetables to deal with weed pressure. However, some crops are inherently weed-sensitive and require extra attention to protect them from weed competition even in the best of circumstances. Factors that make a crop especially vulnerable to weeds include slow early growth, a long establishment period, and cultural requirements that limit options for managing weeds with tillage and cultivation.
Perennial horticultural crops like asparagus, blueberry, raspberry, and other fruits require stringent weed control during the first year or two after planting in order to become established successfully. Asparagus and some perennial herbs and cut flowers do not compete well against weeds and may continue to need intensive weed management throughout their lifetimes. Invasive rhizomatous or tuber-forming perennial weeds pose especially severe problems in perennial crop production. Therefore, it is essential to minimize weed pressure through site selection and site preparation before these crops are planted.
Site preparation can include one or more management practices: tillage targeted at existing weeds, cover cropping, mulching, soil solarization, and manual spot-weeding. Many farmers grow and till in a series of highly competitive cover crops before setting out asparagus crowns, young berry bushes, or fruit trees. In addition, soil pH, nutrient levels, and physical condition should be carefully assessed and adjusted through appropriate amendments to optimize growing conditions for the desired crop. Good site preparation and pre-plant weed control can save the grower many days of backbreaking labor digging and pulling weeds out of the new planting, and can make the difference between success and failure.
Other weed-sensitive crops with shorter life cycles, such as strawberry, onion, leek, garlic, carrot, and parsnip, require essentially weed-free conditions for at least several months after planting. Growers often place these in their rotations following highly weed-competitive cover crops, “cleaning crops” that facilitate weed control and reductions in the weed seed bank, or both. Cleaning crops include vegetables like potatoes, corn, and broccoli that can be cultivated vigorously and hilled up early in their life cycle, and compete well against weeds later on. The wide spaces between rows of winter squash or sweet potatoes can be cultivated several times to flush out weeds before the vines spread out, at which point the crop canopy hinders further weed growth. For vegetables like carrots and onions, growers often employ fallow cultivation or stale seedbed for a few weeks before planting to further draw down the weed seed bank.
A high biomass cover crop that contains invasive perennial weeds or considerable amounts of other weeds can be managed in a two-step process to fight the weeds and incorporate the cover crop. Flail-mow the vegetation, then incorporate with a spading machine, heavy rototiller, moldboard plow, or heavy disk.
Although reducing tillage can be a valuable tactic against annual broadleaf weeds, organic no-till systems should be considered weed-sensitive, because post-plant weed control options are limited. Examples include tomatoes and peppers transplanted into roll-crimped winter rye–hairy vetch, fall brassicas planted into flail-mowed summer foxtail millet–soybean, and early spring vegetables planted into winter-killed cover crops. Whereas these minimum-tillage systems can enhance soil quality, reduce flushes of annual weeds, and give good vegetable yields where perennial weeds are scarce and overall weed pressure is light to moderate, they commonly fail when weed pressure is high. Nutsedges, quack grass, Bermuda grass, johnsongrass, Canada thistle, bindweeds, and even small clumps of fescue, timothy, and clover left from recently turned sod crops, will readily grow through the heaviest of cover crop mulches and compete severely with no-till planted vegetables. Heavy populations of annual weeds or a large seed bank of annual weeds that are likely to compete with the vegetable crop can also spell trouble. In all of these circumstances, bring the weeds under control first before attempting organic no-till or reduced-till practices.
In soil solarization, the farmer covers bare soil with a tight-fitting, non-porous, transparent plastic film to trap the sun’s heat in the upper few inches of soil. The plastic is left in place for several weeks during the summer to produce elevated temperatures (over 120°F) to kill weed seeds, vegetative propagules of wandering perennials, and soilborne plant pathogens. This technique requires both sufficient sun and heat and adequate soil moisture to work. Therefore, it is most successful in climates with reliably intense solar radiation and hot weather, such as California, the Southwest, and the High Plains, and on farms that have access to irrigation (Liebman and Mohler, 2001). Soil moisture is important because it helps conduct heat further down into the soil profile, and moist heat is more likely to kill weed seeds than dry heat at the same temperature. Weed seeds and deeply buried roots and rhizomes generally require higher temperatures and longer solarization than soil pathogens, insect pests, or vegetative weed propagules located in the top few inches.
Soil solarization normally follows immediately after tillage to produce a level, bare soil surface. When this practice is combined with allelopathic cover crops, the organic grower can achieve a “biofumigation” of the soil. Cover crops in the brassica family, or other strongly allelopathic crops like rye or sorghum–sudangrass are grown to a high biomass, mowed while still green and succulent, and tilled in as a green manure. The plastic is then laid as for solarization. The combined effects of heat and rapid cover crop decomposition can be quite effective on weed seeds, pests, and pathogens. Even if seeds are not killed outright, they may be stimulated to germinate, and the seedlings are then killed by a combination of allelopathy, microbial attack, and high temperature.
Disadvantages of these strategies include the cost of purchasing, laying, removing, and disposing of or recycling the plastic, and the fact that these processes can also kill beneficial soil organisms, leaving a biological vacuum. Prompt application of a high quality compost or compost tea right after the plastic is removed can help fill this vacuum with desirable organisms before opportunistic pathogens seize the niche. Soil solarization is most cost-effective for preparing relatively small areas of land for high value crops like asparagus and strawberries.
From Kuepper and Thomas (2001) of ATTRA:
Because asparagus is a perennial crop that will be in the ground for at least 10 to 15 years, attention to selection and preparation of the planting site is especially important. Asparagus performs best if the soil pH is within a range of 6.5 to 7.5. Phosphorous, potassium, and lime amendments—based on a soil test—should be incorporated prior to planting. If perennial weeds are a problem, a sequence of tillage and cover cropping—with smother crops like sorghum–sudan or buckwheat—at least a year in advance of planting will help control weeds. Green manure crops also improve soil structure and enhance soil fertility....
Weed control is the most serious challenge facing organic asparagus producers. Since asparagus is a perennial crop that increases in bed-width each year, cultivation for weeds ‘in the row’ during spear harvest, and following harvest during fern production, is not possible. Thus, elimination of perennial weeds such as bermudagrass [sic], quackgrass [sic], johnsongrass, and nutgrass [=nutsedges] prior to planting is especially critical. Annual weeds can be controlled through a combination of cultural, mechanical, and biological control techniques.
~ Kuepper and Thomas, 2001
From Abell et al. (2006) of Virginia Association for Biological Farming:
It is important that asparagus be planted in a location that makes weed control easy. Hence it is better not to plant asparagus next to a fence where the bed is easily invaded by grass and where there may be difficult access on one side. Since weed control is crucial to successful asparagus production it would be unwise to locate your bed in an area where you have had difficulties with a particularly pestiferous weed. Dig up any wild asparagus nearby, as it could harbor asparagus pests. At the garden scale, remove perennial weeds the year before planting. Pay particular attention to removing Bermuda Grass, Quackgrass [sic], Canada Thistle, Johnson Grass [sic], Nutsedge, Hedge Bindweed and Honeyvine Milkweed. From then until planting, keep weeds controlled by cultivating; covering the soil with a weed-smothering cover crop such as buckwheat during the frost-free season, or black plastic; or by using corn gluten as an organic pre-emergent herbicide. If your scale is too large for manual weed removal, grow and till in a series of heavy smothering cover crops....
[During early spring of the crop’s second and subsequent growing seasons:] First, root out any early weeds. Then fertilize with fish meal and greensand or a complete fertilizer or compost, spread over the whole bed, if you didn’t do this in the fall. Aim to supply 100-150 pounds of K per year as a maximum. Then cover, to a depth of at least four inches, with composted wood chips, horse bedding, sawdust, straw or old hay (although hay may include weed seeds). The purpose of this is for weed control and it should be spread out two feet on each side of the bed. It will smother most weeds and make summer weeding largely unnecessary, unless weed seeds are introduced. The asparagus spears will easily grow up through the mulch....
[During and after harvest each year:] Skim till the aisles between the asparagus rows to control weeds. Then a few days before your last harvest of the season, sow cowpeas, or soybeans. The timing of the sowing is aimed towards getting a good growth of the cover crop before the asparagus ferns close the canopy.
~ Abell et al., 2006
From Adam (2006) of ATTRA:
Weed management is the most significant challenge in onion production. Besides competing for water and nutrients, weeds can harbor destructive insects, can serve as alternate hosts of diseases, and can hamper hand-harvesting efficiency (in regions where crops are hand harvested). Weeds can seriously reduce yields....
Onions should be planted in fields relatively free of troublesome weeds such as nutsedge, field bindweed, and bermudagrass [sic], as well as clovers. Nonchemical controls are mostly employed pre-plant.
Rotational crops that employ tillage can reduce weed levels, as tillage of standing onion crops is not common due to risk of injury to bulbs. A method of stale seedbed weed management that does not involve chemicals consists of several light cultivations of the field before onions are planted to eliminate weeds germinating from the top soil layers. The most common and troublesome weeds are highly influenced by crop planting time, as well.
~ Adam, 2006
From Bachmann and Hinman (2008) of ATTRA:
Good weed control is essential in garlic production. Alliums are slow-growing, shallow-rooted crops that can suffer severe yield loss from weed competition. Planted in the fall and harvested in the mid-summer of the next year, garlic will be in the ground nine months. It is therefore vulnerable to competition from winter and summer annual weeds. Weed competition, even early in the growth of the crop, can reduce yields. In addition to reducing yield and quality, weeds also interfere with mechanical harvesting equipment.
If possible, advance field preparation with a weed-suppressing cover crop such as rye or sorghum–sudan grass can significantly reduce weeds as well as build up soil organic matter. Mulching new plantings has already been mentioned as a way to control weeds. If mulch is used, it should be thick enough to last until harvest. Avoid spent hay and inspect straw for weed seeds. If mulch is used, garlic will have to be dug by hand rather than machine harvested. Garlic can also follow any well-cultivated crop, like carrots.
~ Bachmann and Hinman, 2008
The weed management sections in ATTRA bulletins on organic production of strawberry, blueberry, and brambles all refer the reader to the following information provided for tree fruits (Kuepper et al., 2004):
The presence of certain weeds and forage species is of particular concern to the organic grower. Bermuda grass, Johnson grass [sic], quack grass, and several other pernicious species can be serious problems to fruit growers and are difficult to control with organic methods once an orchard is established....
It’s easier to manage weeds before an orchard is established. Cover crops (see ATTRA’s Cover Crops & Green Manures [Overview of Cover Crops & Green Manures, Sullivan, 2003]) produce a thick stand that will shade or choke out weeds. Combined with a well-planned sequence of tillage, cover cropping is an effective pre-plant weed suppression strategy that also contributes to soil fertility and stable humus. The basic strategy begins with plowing under or disking the existing vegetation, ripping or deep chiseling to loosen compaction, planting a cover crop to suppress weed growth, mowing down and tilling under the cover crop(s), and finally planting the fruit crop. Several cover crop and tillage sequences may be necessary before planting.
Specific cover crops and management strategies vary with location and purpose. The two cases below raise the kinds of questions you need to ask to choose an appropriate cover cropping system. The cover crops you choose for site preparation (before planting the orchard) may be entirely different from those you want once the orchard is established.
Bart Hall-Beyer, co-author of Ecological Fruit Production in the North (Beyer & Richard, 1983), provides one example of how cover crops can be used to suppress weeds in the growing season prior to fruit crop establishment. His program consists of fall plowing, to allow the sod to rot, then disking as soon as the soil is dry in the spring, followed by harrowing every 10 days for at least one month to kill germinating weeds. He next incorporates compost and mineral nutrients and seeds buckwheat as a smother crop. He then tills the buckwheat into the soil after it has started flowering but before seed-set. Hall recommends additional cultivations at 10-day intervals, followed with rye as a fall cover crop. The rye is incorporated the following spring and the fruit crop is planted.
In the Mid-South, Simon Billy and other researchers at the Kerr Center for Sustainable Agriculture in Poteau, Oklahoma, evaluated a number of cover crops for weed suppression on heavy soils. They converted pasture land to horticultural production, using rotations of cover crops and tillage. By this method, they virtually eradicated Bermuda grass from the fields in one to two years.
~ Kuepper et al., 2004
The Kerr Center found summer cover crops of sorghum–sudangrass, cowpea, sesbania, or crotolaria, followed by winter covers of rye and hairy vetch or oats and field peas, to be highly effective for preparing fields for orchard plantings. The first planting of cowpeas can be allowed to set seed to obtain a second cover crop at minimal cost, while the sorghum–sudangrass can be maintained by mowing for prolonged weed suppression (Kuepper et al., 2004).
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