Mark Schonbeck, Virginia Association for Biological Farming
Organic growers utilize a wide diversity of weed control tools that can be roughly grouped into the following categories:
Note that cover crops, diversified crop rotations, and optimum cash crop management are vital tools for preventive or cultural weed management. The tools discussed here are those used—in lieu of chemical herbicides—to deal with those weeds that emerge even in the best-managed organic vegetable fields.
Following is a brief description of the various weed control tools and how they are used to kill weeds or reduce weed competition with the crop.
Tillage done after harvest of one crop and prior to planting the next crop serves several functions, including incorporation of crop residues, cover crops, and soil amendments; removal of existing weed growth; and preparation of a seedbed for planting. Field preparation often consists of a primary tillage operation to break sod, loosen or invert soil, and kill existing vegetation; and a secondary tillage pass to form a seedbed of desired fineness. Because of the potential adverse impacts of tillage on soil quality, many farmers attempt to reduce tillage, either by making only a single pass before planting, restricting soil disturbance to only part of the field area (strip, ridge, or zone tillage), or minimizing the intensity of disturbance through shallow or noninversion tillage.
Tillage uproots weeds, severs or chops weed shoots and roots, buries weeds and weed seeds, or a combination of these, depending on the implement(s) used. Because different weed species and life stages are differently susceptible or tolerant to uprooting, chopping, and burial, the existing weed flora and weed pressure should be taken into consideration in choosing primary and secondary tillage implements, and in deciding whether, when, and how to reduce tillage.
Primary tillage tools include the moldboard (turn) plow, chisel plow, disk plows (such as heavy offset disk), rotary spaders, and rotary tillers operated near their maximum depth (6–8 inches). Secondary tillage tools include disk harrow (light disk), field cultivator, springtooth harrow, spiketooth harrow, and rotary tillers. The moldboard plow inverts the soil profile and thereby uproots and buries weeds without much chopping action. The chisel plow breaks sod with much less soil inversion and weed burial, whereas the sweep plow mainly severs weed shoots from roots but does not further fragment, uproot, or bury the weeds. Disks and rotary tillers chop weeds up and mix the fragments with soil to the tool’s working depth, which effects partial burial. They are often used as secondary tillage to chop up weeds left intact by moldboard or chisel plowing. Rotary and reciprocating spaders have become popular among organic vegetable growers, as they accomplish effective tillage with less hardpan formation or damage to soil structure than other implements, often chopping and incorporating weeds and cover crops and leaving a fairly good seedbed in a single pass.
Tillage is often necessary for removing established weeds, especially perennial weeds emerging from storage roots, rhizomes, or other underground vegetative propagules. Perennial weeds, especially wandering perennials that propagate through rhizomes and other underground structures, often require careful selection of tools and methods, depending on type and depth of the underground structures by which they persist and propagate. Chisel plows and field cultivators can bring the roots and rhizomes of some perennial weeds such as quack grass or Bermuda grass to the soil surface to dry out and die, but may be less effective against other weeds like Canada thistle or field bindweed that have deep root–rhizome networks.
Whereas most tillage practices effectively remove existing weed growth, their effect on weed seeds—whether still on the plant or in the soil’s weed seed bank—can vary from reducing to greatly enhancing subsequent weed emergence. Tillage can directly stimulate weed seeds to germinate by exposing the seeds to a brief flash of light or by scarifying (nicking or scratching) the seed coat. Tillage also moves seeds up or down in the soil profile, and changes soil conditions (loosening, aeration, mineralization of nitrogen, drying), which can promote either germination or dormancy. Weed seeds stimulated by tillage to germinate either emerge or die, depending on depth at which germination occurs, and whether current growing conditions are favorable. Generally, tillage hastens the decline in numbers of viable dormant weed seeds remaining in the soil. For more on the effects of tillage on weed seed populations, see Manage the Weed Seed Bank—Minimize "Deposits" and Maximize "Withdrawals", and Manipulating Weed Seed Banks to Promote their Decline.
At the garden scale, primary tillage is accomplished with shovel, spading fork, broadfork, or a heavy duty walk-behind rotary tiller or rotary plow operated at or near its maximum depth. Secondary tillage is done with rakes, four-prong cultivators, wheel hoes with claw attachments, or a rotary tiller run at a depth of 1–3 inches.
Cultivation is physical soil disturbance done primarily for the purpose of controlling weeds. Most cultivation is done between the time of crop planting and the point at which the crop closes canopy (thereby severely curtailing weed growth through competition) or becomes too large for cultivation operations.
A wide range of cultivation implements have been designed and developed for full field, interrow, near-row, and within-row cultivation (Bowman, 1997). These implements are mounted on one or more toolbars and pulled along crop rows or beds to effect weed removal. Tools have been designed specifically for various crops, planting patterns (single, double, or multiple rows; level field or raised beds), stages of crop and weed development, soil conditions, and amounts of surface residue. Implements are often combined to accomplish between-row and within- or near-row cultivation in a single pass. High precision in crop row spacing and in matching crop row spacing with implement spacing on the toolbar is essential. Various guidance systems have been developed to minimize crop damage during cultivation.
Full-field implements include the rotary hoe, spiketooth harrow, spring-tine harrow, and other weeding harrows. The rotary hoe consists of two ranks of wheels, each equipped with multiple (usually 16) spoon-like projections that throw small weeds out of the soil when the implement is pulled through the field at high speeds (7–12 mph). All of these implements are effective against small, recently-germinated weeds (white-thread stage to cotyledon stage), and are used either prior to emergence of large-seeded crops (sown below the 0.5–1 inch depth at which the tool works), or at certain stages of early crop establishment when the crop is rooted firmly enough to avoid uprooting, yet not so large as to sustain too much foliar damage by the tool. This full-field or “blind” cultivation removes within-row weeds that germinate just before or with the crop, thereby giving the crop an important head start.
Interrow cultivators consisting of various shovels or sweeps mounted on toolbars singly or in gangs (depending on crop row spacing) sever or uproot weeds up to six inches tall, and are perhaps the most widely-used cultivation tools. These implements dig down about two to four inches to take out weeds, and must be mounted at sufficient distance from crop rows to minimize root pruning damage to the crop. When the crop is young, cultivators are adjusted to avoid throwing soil into crop rows, either by providing shields or by using goosefoot shovels or narrow point shovels that minimize lateral movement of soil. Shield can consist of ground-driven wheels or rotary shields, or panels or “tents” over each row, affixed to the implement. Later in crop development, the cultivator can be adjusted to throw considerable soil into the crop row to bury small within-row weeds.
The rolling cultivator consists of gangs of rotating wheels with stout curved tines, or small disks. The two gangs within each interrow can be adjusted to work more or less aggressively, and to move soil either into or away from crop rows. Two of the more aggressive between-row cultivators, effective on fairly large weeds, include the rotary tilling cultivator or multivator (also useful for strip-till seedbed preparation), and the horizontal disk cultivator. These disturb the soil more intensively, and overuse can degrade soil structure.
Near-row cultivation tools are designed to work more shallowly in the area from about two to four inches from the crop row itself. Disk hillers or spyders (ground-driven toothed wheels) can be combined with between-row tools to remove weeds closer to the crop. Brush weeders (stiff polypropylene brushes mounted on horizontal or vertical axis, effective on weeds up to five inches) and basket weeders (two gangs of wire baskets rotating at different speeds to knock out small weeds up to two inches) work the entire interrow up to within a couple inches of the crop.
Precision within-row cultivation tools include torsion weeders, spring hoes, and ground-driven spinners and finger weeders (flexible rubber “fingers” mounted on a rotating hub) work into the crop row from either side to knock out small (up to one inch) weeds without damaging established crop plants. Tips of the torsion and spring hoes vibrate just below the soil surface on either side of the row, causing the soil to “boil up” and thereby uproot small within-row weeds.
Within-row and near-row cultivation requires high precision to avoid damaging crop stands. Many farmers use specially designed cultivation tractors with belly-mounted tools so the operator can easily see and adjust tool position relative to the crop. For rear-mounted cultivators, mechanical devices such as guide wheels that run along the sides of raised beds or ridges can keep the implement on course. Electronic guidance systems sense the position of the crop row and automatically adjust the position of the implement.
(adapted from Mohler, 2001, p. 170–173)
Market gardeners can choose from a wide range of hoes and other handheld weeding implements. The standard hoe, consisting of a fairly heavy-duty blade, is effective on a wide range of small to fairly large weeds, but the chopping motion can be tiring and tend to bring up more weed seeds to germinate. Other implements include the stirrup hoe (oscillating or hula hoe), the collinear hoe and various other lightweight, ergonomic hoes designed for very shallow cultivation of small weeds and ease of use. The short-handled Dutch hoe gets the gardener down on hands-and-knees but allows very close, precise cultivation similar to the torsion weeder and other within-row implements. The wheel hoe covers larger garden areas much more efficiently, and can be equipped with standard, stirrup, or sweep blades for different applications. The four-prong cultivator (“potato hook”) can be worked shallowly to uproot small weeds, or deeply to bring larger weed roots and rhizomes to the surface.
Farmers use rotary (bush-hog), sicklebar, or flail mowers to manage weeds in pastures, field margins, and sometimes in crop fields themselves. Whereas mowing simply removes top growth, leaving stubble of an inch to several inches in height, it can nevertheless have a significant impact on certain weeds. Some annual weeds are fairly mowing-susceptible and can be prevented from setting seed by one or two mowings, and even the growth and vegetative reproduction of perennial weeds can be restricted by timely or repeated mowing.
Three potential uses for mowing as part of integrated weed management in vegetable crops include between-row, over-the-top, and post-harvest mowing. When additional cultivation is not desired because of crop developmental stage or soil quality considerations, mowing weeds between rows of an established vegetable crop may be sufficient to prevent crop yield reductions and to reduce weed seed production. Examples might include mowing weeds just before cucurbit vegetables vine out, or between rows of snap beans, corn, or other vigorous crops a couple weeks before canopy closure. When weeds grow above the canopy of a low-growing crop like sweet potatoes or peanuts, some farmers mow just above the crop canopy to eliminate shading and seed set by the taller weeds. Finally, mowing rather than tillage after crop harvest can interrupt weed seed production without disturbing the soil or the habitat of ground beetles and other weed seed predators.
Market gardeners can use a lawn mower, weed whacker, sickle, scythe, or garden shears to cut weeds between rows, in margins, or after harvest. In addition, manual cutting tools can be used to remove weed “escapes” in larger fields to prevent weed seed production.
Many vegetable growers use propane-fueled flame weeders—backpack or tractor-mounted—to kill small weeds just prior to crop emergence (full field flaming), or between crop rows (using shields to protect crops from the heat). Organic growers often flame a stale seedbed to remove emerged weeds without additional soil disturbance, just before or just after planting the crop. Flame weeders equipped with a flame hood or shield concentrate the heat on the target weeds, and are therefore more energy efficient.
A few crops can tolerate within-row flaming at certain developmental stages, such as corn and onions that are several inches tall (growing points are protected within the plant structure), and cotton (whose stem is woody and resistant). The flame is directed toward the soil surface from either side of the row. The goal is not to “burn” the weeds, but to subject them to a brief exposure to intense heat, just sufficient to disrupt cell membranes and cause the weed to dehydrate and die in a few days. Flaming is most effective and energy-efficient on small weeds up to two inches tall (Diver, 2002).
Other modes of thermal weed control include the infrared heater, and hot water and steam weeders, all of which eliminate the potential fire hazards associated with flame weeders in dry conditions, especially in the presence of mulch or dry residues (Astatkie et al., 2007). The infrared weeder directs the propane flame at a ceramic or metal plate, which radiates the heat onto the weeds. It can be effective, but requires several times as much energy as direct flame. Hot water and steam units require hauling considerable volumes of water (e.g., 1,000 gal/ac) into the field, and may control weeds less effectively than flame or infrared.
Organic mulches such as hay, straw, tree leaves, and wood shavings keep light-responsive weed seeds in the dark, physically hinder emergence of weed seedlings, and can provide shelter for ground beetles and other weed seed consumers. They also conserve soil moisture for crop production, maintain good soil tilth, prevent surface crusting, feed soil life, and sometimes provide slow-release nutrients. About three or four inches of hay or straw mulch can greatly reduce the emergence of broadleaf weed seedlings. Organic mulches are less effective against grassy weeds, and usually do not significantly hinder the emergence of perennial weeds from rootstocks, tubers, rhizomes, or bulbs.
One limitation for organic mulch is that manual application of mulch materials may be too labor-intensive for multiacre plantings. Bale choppers have been developed to mechanize application of hay or straw between wide-spaced crop rows or beds. Another approach to mulching at the farm scale has been the production of in situ mulch in the form of high biomass cover crops. This entails no-till cover crop management and vegetable planting, which are most feasible where existing weed pressure is light to moderate, perennial weeds are scarce, and a transplanted or large-seeded vegetable will be grown in the cover crop mulch. Rollers, roll-crimpers, flail mowers, and undercutters are used to convert the mature cover crop into in situ mulch at the farm scale, leaving residues either chopped fine enough (flail mowing) or oriented parallel to future crops rows to permit mechanized planting. Gardeners can cut cover crops with a scythe, sickle, or weed whacker, and plant vegetables manually with a spade, trowel, or dibble.
Living mulches consisting of low-growing cover crops between cash crop rows are most workable for perennial fruit crops, especially tree fruits and grapes. Living mulches usually compete too strongly with vegetables, resulting in yield losses.
Black plastic film mulch effectively blocks emergence of most weeds, including perennials. They also eliminate the light stimulus for weed seed germination. However, these synthetic materials do not enhance soil quality, can interfere with infiltration of rainfall, and (unless a biodegradable material is used) require pickup and disposal at the end of the season. In addition, weeds often come up through planting holes, where they can be especially hard to control. Weed barriers (landscape fabric) last several seasons and can be especially useful for getting perennial crops established. Paper mulches used alone are less effective than plastic, but a paper mulch underneath hay or other organic mulch can enhance weed control over the organic mulch alone.
Clear plastic film mulch raises soil temperature much more than black plastic. During hot summer weather, it can effect soil solarization, another form of thermal weed control. Solarization kills emerging weeds, some soilborne crop pathogens and insect pests, and even some weed seeds and vegetative propagules of perennial weeds. However, if conditions are too cool or cloudy to support effective solarization, the clear plastic can simply accelerate weed growth under the plastic layer by creating near-optimum temperatures.
A limited number of products have been developed that organic growers can spray for weed control. Natural-product herbicides allowed for organic production, including acetic acid (concentrated vinegar), essential oils, and natural allelochemicals, are nonselective contact herbicides most useful for spot treatments of, for example, a localized infestation by a new weed species, or poison ivy on fencerows or near a farm stand. The Organic Materials Review Institute lists products that are allowed and those not allowed for use on organic farms, including herbicide products. A few bioherbicides based on specific fungal pathogens have been developed against specific weed species that have become especially problematic in particular regions. At this time, however, organic herbicides and bioherbicides play a minor role in the organic weed control toolbox. See the related article Can I Use This Input On My Organic Farm? for further information.
Several other biological agents that can potentially contribute to weed control in organic farming systems include allelopathic cover crops, weed seed consumers, soil microorganisms, insects, and farm animals including weeder geese. For more on the utilization of biological agents and processes in weed control, see the article, Utilize Biological Processes to Further Reduce Weed Pressure.
This article is part of a series on Twelve Steps Toward Ecological Weed Management in Organic Vegetables. See also Design the Cropping System and Select Tools for Effective Weed Control, and the following video clips:
This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.