Vegetable and Berry Specialist
University of Vermont Extension
157 Old Guilford Rd. #4
Brattleboro VT 05301-3669
Many organic vegetable growers consider weeds to be their primary pest problem. Integrating a variety of weed control techniques is the best way to achieve effective organic weed control.
Weed management techniques are aimed at preventing weeds before they appear, or at suppressing weeds once they are present.
Weed prevention techniques include:
· Rotation of crops, fields and tillage tools
· Composting animal manures to kill weed seeds
· Cleaning farm implements between use in different fields
· Controlling weeds in hedgerows, alleys, ditches, etc. before they set seed
· Growing allelopathic cover crops
· Mulching with plastics or organic residues
· Applying pre-emergent organic herbicide (corn gluten meal)
Weed suppression techniques include:
· Growing smother crops, intercrops and/or aggressive cash crops
· Hand-hoeing and hand-pulling
· Mechanical cultivation
· Applying post-emergent organic herbicide (soaps, acetic acid, etc.)
Weed prevention is essential to organic weed management. Otherwise the need for cultivation can be excessive, eating into time and profits, and potentially damaging soil structure. Hand weeding is obviously costly and must be kept to a minimum. Weed prevention practices should begin in the years prior to planting a crop, with cover crops in a well-designed rotation. In the cropping year, clean fallowing and stale seedbeds may prepare for crop planting. Then, weed-crop competition can be managed through various combinations of cultivation, mulching, intercropping, mowing, and concentrating resources near the crop.
Crop rotation subjects weeds to an ever-changing habitat, reducing the opportunity for certain species to proliferate. Rotation strategies for weed control include row versus sod crops, frequently versus rarely cultivated crops, deep versus shallow tillage crops, early- versus late-season crops, and fallow versus cash-cropped periods.
Smother cropping is cover cropping with competitive species in an attempt to starve weeds of light, nutrients, moisture and space. Smothering can weaken perennial weeds by depleting their carbohydrate reserves, and can lessen annual weed pressure by slowing growth and reducing seed production. Fast-growing, high-biomass species make good smother crops because they can get a jump on, and over, weeds.
Warm-season cover crops such as buckwheat, Japanese millet and sorghum-Sudangrass are good summer smother crops in hot conditions, but they should not be planted until soils are thoroughly warm. Cool season crops like oats, field pea, and ryegrass are candidates for smother cropping in the early spring and fall. High seeding rates, adequate moisture and fertility, and good soil-seed contact by drilling or otherwise covering seed are important to establishing a thick smother crop.
Stale seedbeds takes advantage of the fact that most weeds have small seeds that germinate from the top inch or two of the soil, usually within a couple of weeks of preparing soil for planting. By letting these weeds germinate and then killing them without disturbing the soil and bringing up new weed seeds, subsequent weed pressure can be greatly reduced. In some cases, growers using stale seedbeds actually encourage weed germination with irrigation or row covers to overcome dry or cool conditions that slow weed growth.
Weeds can be killed with flaming, non-selective ‘organic’ herbicides (potassium salts of fatty acids, acetic acid, etc.) or extremely shallow scraping, after which it is critical to minimize soil movement when planting or transplanting. Early-season stale seedbeds are often ineffective, as most broadleaf weeds germinate in warm soils. Many growers create a stale seedbed only in the crop row using a hand-held flamer, while cultivating between the rows. This works well with crops that are slow to establish, like carrots and onions.
Composting animal manure helps reduce the number of weed seeds added to the soil. In composting where temperatures reach approximately 140 degrees F many weed seeds can be killed. Maintaining the proper C:N ratio, moisture and aeration, and turning piles inside-out several times is important to ensure that most or all of the compost does indeed get hot. Turning also keeps weeds that sprout on the pile surface from going to seed.
Mulching the soil provides a physical barrier to weed growth. Organic residues such as straw, leaves, etc. can suppress weeds for many weeks if put on thickly, but they keep soil temperatures cooler which may slow the growth of warm season crops. A thin layer of organic residues can be worse then none at all, since weed prevention will be poor but cultivation may be hampered.
Black plastic mulch completely blocks light from weeds and is a very effective weed suppressor. Clear plastic mulches warm the soil to a greater extent than black plastic, but allow light to penetrate thereby encouraging weed growth. Selective wavelength mulches, such as IRT, behave in between black and clear plastic, allowing limited weed growth. If weed pressure is high and canopy growth is slow then there may be a lot of weeds under clear or IRT plastic. However, under conditions of strong sunlight, solarization may kill weeds that grow under clear plastic if it is left undisturbed for several weeks prior to making planting holes in it. Paper mulches have been used with limited success since they tend to deteriorate during the season. Throwing soil up on the edges as they decompose has helped keep paper mulches in place for some growers. Organic growers need to avoid paper mulches treated with prohibited materials.
The edges of plastic or paper mulch pose a special challenge because weeds often grow well there but the mulch is easily ripped by close cultivation. To avoid hand-weeding, some growers have developed innovative cultivation tools to deal with this unique zone. These tools usually undercut then replace the mulch, or cultivate extremely close it. Both approaches require straight runs of mulch with uniformly buried edges. The planting hole may require hand weeding with slow-growing crops. Bulb planters, propane torches and other devices have been used to create small, uniform holes for planting into.
Interseeding is a form of inter-cropping that can add to soil organic matter, reduce soil erosion, improve field trafficability and also suppress weeds. To avoid competition with the cash crop it is advisable to interseed moderately competitive cover crops like low-growing clovers and/or ryegrasses into relatively vigorous cash crops like corn, potatoes or squash; to leave a bare planting strip for the cash crop that is kept free of intercrops as well as weeds; to sow the cover crop after the cash crop is well-established, usually at last cultivation; and to provide irrigation.
Mowing (or weed-whacking) vegetation between the rows of a cash crop can control weeds, and or certain intercrops. It also enhances the environment for harvesting and pick-your-own sales. In most cases, between-row mowing is only practical on a small scale, or where rows are wide enough to accommodate tractor-pulled mowers. Side-discharge mowers damage crops by blowing debris on them.
Placement of resources can be used to favor crops over weeds. By banding and sidedressing fertilizer, and in some cases by using drip irrigation, weeds can be denied nutrients or water needed for good growth. A word of caution is that many plants send roots laterally to obtain resources, so locating water and fertilizer only in the row may be more useful in limiting between-row weed pressure only if the rows are far apart.
Crop establishment techniques that encourage the rapid early growth of vegetables and discourage rapid early growth of weeds can minimize weed control costs. Transplants give crops an obvious jump over weeds compared to direct seeding. Carefully placed starter fertilizer feeds crops not weeds.
Cultivation equipment for weed control varies in aggressiveness and is usually suited to killing weeds either before or after the crop emerges. Tools designed for use after a crop emerges provide either between-row weed control or in-row weed control, or in some cases, both. Cultivation implements are designed to dislodge, cut and/or bury plants. Matching the tool to the weeds, crops and soil conditions is key. Juggling the uncertainties of weather and other management demands on the farm is the challenge to using a cultivation tool at the right time.
Pre-emergence cultivation often involves shallow tillage of the soil with rotovators, various harrows or field cultivators. If performed repeatedly, this approach is called a “clean fallow” that can occur before or in-between plantings. The objective is to kill annual weeds, reduce the soil weed seed bank and remove perennial weed growth. While rather harsh on soil structure and organic matter, this technique can be justified if used in combination with a good cover crop rotation and/or addition of soil amendments to maintain soil structure.
Disk harrows are often used for clean fallowing, but they may not be the best choice with a perennial weed problem like quackgrass, since rhizomes tend to be chopped up and spread them throughout a field. Field cultivators are used to create a seedbed and incorporate residues and soil amendments, as well as for weed control. Equipped with rows of S-tines and sweeps or shovel with lifting action, they can be used to dig up and lift quackgrass rhizomes to the surface of the soil where they will dry out and die in hot dry weather. A variety of field cultivators are on the market, they vary in tine shape, flexibility, action, and spacing, as well as options like rollers, cultipackers, crop shields, leveling bars, and gauge wheels.
Blind cultivation relies on tools that work the entire surface of the soil ‘over the top’ of a recently seeded or recently emerged crop. The technique combines in-row and between-row cultivation, often to control weeds that have germinated but not yet emerged, the so-called ‘white-thread’ stage of growth. This disturbs and dries out small, vulnerable weeds before they start to size up alongside the crop. Such fast, shallow cultivation works best on weeds that have been up for week or less. Large-seeded crops sown deeply and young vigorous crops are able to tolerate such cultivation, while very small annual weeds cannot.
Flex-tine weeders can be used for blind cultivation on a number of vegetable crops. The weight of these units may be borne by the numerous thin metal tines that wiggle and dislodge weeds, or by gauge wheels attached to the frame. Gauge wheels help control tine depth and avoid gouging soil on uneven fields. With some units, the 3-point hitch can be used to adjust downward pressure on the tines. Tine weeders width ranges from narrow, bed-covering sizes to very wide units useful in large fields. Lely and Einbock are common brands.
A more aggressive blind cultivator is the rotary hoe. It has many thin spider wheels each with 16 or 18 tips, or spoons, which dislodge very small weeds. The spiders move independently and bear the weight of the unit, although gauge wheels are available. Rotary hoes come in widths of 6 feet and up. They are most often used for weed control in corn and beans and for breaking up the surface of soils prone to crusting. Rocks will jam in the spiders of a rotary hoe, keeping them from turning properly. Plastic mulch pieces in the field will also collect on spider wheels and require removal. Dull spoons reduce the effectiveness of rotary hoeing.
High clearance tractors facilitate post-emergence mechanical weed control since crops up to several feet tall can pass under the tractor, and tools can be “belly-mounted” in view of the operator, increasing the precision of cultivation. Off-set high clearance tractors are even better for cultivating because the driver's seat is off to the side of the tractor body, further enhancing vision of the row.
Cultivating tractors are small, low to the ground, easy to guide and used only for precision cultivation of young or low-growing crops. The cultivation tools and the crop row(s) being cultivated are easily viewed by the operator. The Allis Chalmers "G" tractor, no longer made, is the classic of this type. Other brands, Hyspan, Saulkville, and Friday, are currently available.
Basket weeders are metal cages that roll on top of and scuff the soil surface without moving soil sideways into the crop rows. This action makes them ideal for crops like lettuce that have to be kept free of soil and are not suited to hilling. Buddingh basket weeders are custom built for two to eight row beds. Angled baskets are available to work the sides of raised beds. Basket widths range from 2 to 14 inches depending on the space between rows. For wider widths, and for inner row widths that change as crops grow, overlapping baskets are available that "telescope" or expand in and out to adjust for the width.
Commonly used at speeds of 4 to 8 mph, straight rows and an experienced operator are helpful to avoid crop damage. The front row of baskets turn at ground speed and a chain drives the rear row of baskets a little bit faster; these kick up soil, and dislodge weeds that survive the first baskets. This tool is almost always belly-mounted to assist with close cultivation. The baskets handle some small stones but work best in fine soils free of clods and residues.
Finger weeders cultivate around the stems of crop plants that are sturdy enough to handle some contact. Rubber-coated metal fingers provide some in-row weeding. These are connected to a lower set of metal fingers work deeper in the ground and drive the unit at ground speed. These units can be used at just a few miles per hour since they are in such close proximity to the plants. They require belly-mounting, and are ideal for a G-type tractor. Wet clayey soils can stick to fingers and require frequent removal.
Brush-hoe weeders are European tools for close cultivation in narrow rows, not very common here, and expensive. Shields protect plants from bristle wheels that rotate independently between the rows, "sweeping" weeds out of the soil. An operator sits behind the rotating wheels and steers the unit to assure precision.
Sweeps, shovels and knives attach to the shanks (vertical pieces of metal that attach to the toolbar). Depending on their shape and orientation, these tools move soil in different ways as they are pulled between crop rows. People are not consistent in how they name these tools, so there can be some confusion. The shanks can be clamped to different places on the toolbar(s) to achieve an arrangement that provides the desired in-row coverage and extent of soil movement. Toolbars can be rear-mounted, belly-mounted (underneath a high-clearance tractor), or with special attachments, front-mounted.
Sweeps are wing-shaped, come in various sizes and angles, and are used to dig up larger weeds between crop rows while throwing soil into the row. Big sweeps, or duckfeet, are used to cultivate wheel tracks. Half-sweeps have a wing on just one side, so the wingless-side can cut closer to the crop rows or plastic edges. Shovels are narrower than sweeps, throw less soil, and sometimes have 2 points that are reversible. Knives are like angled shovels that are used to cut more horizontally and closer to a crop.
Wiggle-hoes have shanks with half sweeps attached that can be hand-steered around plants in a row by an operator seated on the back of the tractor-pulled unit. Close cultivation is possible, but extra labor is required for the operator. Slow tractor speed and wide crop spacing must be used to allow shanks to be moved in and out of the row. The Reigi weeder has PTO-driven rotating hubs with stiff tines that can be steered in and out of the row by an operator seated behind the tractor.
Row-crop cultivators consist of toolbars on a tractor-pulled frame with various shanks and cultivation implements attached so as to leave space for the crop rows to pass. These are much like field cultivators except for the spacing of the shanks and the absence of implements that completely cover the soil such as rolling baskets. Shields may be mounted on either side of the crop rows to protect them from soil and rock movement during cultivation.
Spyders, spring-hoes and torsion weeders (made by Bezzerides) are used alone or in combination for close between-row cultivation. The spyder wheel has staggered curved teeth and is ground-driven on a ball-bearing hub. A pair of 12-inch spyders can be angled at 45 degrees toward or away from the row to either pull soil away or throw it back. Aggressive and rapid cultivation of a variety of row crops is possible, even on stony soils. Torsion weeders are square stock metal bars that can be mounted to follow the spyders, leveling the soil and flexing around the plants to clean up spots missed by the spyders. Spring hoes are flat blades 16 inches long that are a bit more aggressive than the torsion weeders, oscillating just below the soil surface
Rolling Cultivators consist of gangs of heavy slicer tines that aggressively dig up weeds and pulverize soil between rows. Individual gang width ranges from 10 to 16 inches depending on number of slicer tines, and units are available for one to 12 rows. Gangs can be angled to hill up or throw soil into the row. Used with fertilizer attachments, sidedressing is possible while cultivating. Rocks may jam in tines, and action may be unduly aggressive for sandy soils.
Hilling disks are used to aggressively throw soil into the rows of crops such as potatoes, leeks, sweet corn and other crops that tolerate or benefit from being buried. Properly timed, this results in excellent in-row weed control.
Flame weeding is the killing of weeds with intense, directed heat, usually with a propane burner. Flame weeding is used primarily to control small weeds in a stale seedbed (without disturbing the soil). Because weeds tend to emerge in ‘flushes’ stimulated by tillage, the initial emergence of weeds represents a major portion of the weed pressure in a given field, provided subsequent tillage that brings new seeds to the surface is delayed or avoided. Prior to flaming, the soil is prepared for planting in the normal fashion, then weeds are allowed, even encouraged, to emerge, so they can be killed with flame. After planting the crop, but just before it emerges, another flaming may be applied to kill weeds that have emerged in the interim. With slow-to-germinate crops, this final flame weeding is most critical to success.
Backpack or hand-held flamers are the simplest, least expensive and safest method of flame weeding. A small canister is carried by hand or in a backpack, while a single burner at the end of a wand is aimed at the area to be flamed. The burner size, walking speed, and flame adjustment determine how much heat is applied to an area. This technique is popular for small plantings of crops that will later be close-cultivated between the rows but will not tolerate soil being thrown into the row. For example: carrot, lettuce, radish, spinach, herbs, etc.
Tractor-mounted flame weeders have been custom-built by growers to flame multiple rows or wide beds. The components include a tank (or several), valves, gas lines, regulators, pilot lights, and emergency shut-off. Gas may flow directly to individual burners, or it may be distributed through a manifold first. Burners are specific for propane in the gas or the liquid phase and are available with different BTU ratings. Liquid burners can avoid ‘ice up’ of gas lines. Burners may be arranged in a row to flame the entire width of a bed, including wheel, or they may be aimed at or between the rows. They may be fixed to the unit, or adjustable. Having individual burner shut-off valves and angle adjustments provides flexibility in how a flame weeder can be used.
Tractor speed when flaming is just a few mph. The larger the weeds or the heavier the dew, the slower tractor speed needs to be. Flaming is not intended to burn the weeds, but to provide a short exposure to intense heat which ‘blanches’ the weeds, and they collapse and die within minutes or hours. Exceptions to this are grasses, with below-ground growing points, and some succulent weeds like purslane, which can take the heat. These weeds require hotter temperatures and/or subsequent cultivation to control.
The propane containers used on tractor-mounted flamers must be ‘motor fuel’ tanks, which are rather expensive, but intended for mobile use, unlike stationary propane canisters. The design of the system and the selection of valves and controls should be done in consultation with a propane professional. All tractor mounts, canister straps, and lines should be carefully examined before using the flamer. Besides the potential for explosion, concerns include: fires started in dry grass or hedgerows, liability insurance and regulations.
Effective organic weed control on vegetable farms is possible through the use of cultural practices, cultivation and flaming. Organic weed control practices can be economically viable when utilized as part of a whole-farm management system that includes rigorous use of cover crops, crop rotation, and sanitation.
Altieri, M. and M. Leibman, 1988. Weed-Crop Ecology: Principles in Weed
Management. Breton Publishers, Boca Raton FL
Colquhoun, J. and R. Bellinder. 1997. New Cultivation Tools for Mechanical Weed Control in Vegetables. IPM Fact Sheet, Cornell Cooperative Extension, Ithaca NY
Bowman, G. (ed.) 1997. Steel in the Field: A Farmer’s Guide to Weed Management Tools. Northeast SARE Program, UVM, Burlington VT
Grubinger, V.P. and M.J. Else. 1996. Vegetable Farmers and their Weed Control Equipment (video) Center for Sustainable Agriculture, UVM, Burlington VT
SOME SOURCES OF CULTIVATION EQUIPMENT
P.O. Box 651
Grand Haven, MI 49417
Bezzerides Bros., Inc
P.O. Box 211
Orosi, CA 93647
BDi Machinery Sales Co.
430 E. Main St.
Macunie, PA 18062
Buddingh Weeder Co.
7015 Hammond Ave.
Dutton, MI 49316
119 Bridle Rd.
Antrim, NH 03440
HWE Agricultural Technology (Einbock)
Embrun, ON K0A 1W0
Market Farm Implement
257 Fawn Hollow Rd.
Friedens, PA 15541
P.O. Box 1060
Wilson, NC 27894
Univerco (Reigi weeder)
713 route 219
Napierville Quebec/Canada JOJ 1LO
800-663-8423 / 450-245-7152
P.O. Box 357
Kalida, OH 45853
Wasco Hardfacing Co.
P.O. Box 2476
Fresno, CA 93745
Mention of brand name equipment, suppliers, and prices is for information purposes only; no guarantee or endorsement is intended nor is discrimination implied against those not mentioned.
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