Weed Management for Organic Crops
Controlling Weeds by Tillage and Cultivation
Primary tillage, such as moldboard plowing of the soil leads to the uprooting and shredding of large weeds that have grown in a field during the fall to the early spring season. Moldboard plowing can also bury the weed seeds deep within the soil where they will not be able to emerge. Secondary tillage, such as disking and harrowing, will lead to the shredding of the weed biomass and further dislodging of shallow-rooted weeds. Often, secondary tillage is used repeatedly to enhance weed germination to deplete the weed seed bank. Although both primary and secondary tillage often lead to a quick destruction of weeds in a field, they do not provide a lasting solution, especially if weed seeds are still present close to the surface of the soil. Follow up practices requiring cultivation are needed to dislodge and uproot the weeds that emerge after tillage as well as creating a good seedbed for uniform crop establishment, which is a critical part of a crop’s ability to compete with weeds. Cultivation is performed after the crop is planted and is probably the most widely used weed control method in organic farming operations. Cultivation kills weeds by digging them out, burying them, breaking them apart, or drying them out. Shallow cultivation usually is best, since it brings fewer weed seeds to the soil surface. Cultivation is more effective in dry soils because weeds often die by desiccation and mortality is severely decreased under wet conditions. Cultivating when the soil is too wet will damage the soil structure and possibly spread perennial weeds. In addition to controlling weeds, cultivation can break up soil crusting and thus can increase crop emergence, water infiltration, mineralization of nutrients, and soil aeration. This section provides a more in depth discussion on cultivation practices for preemergence and post-emergence weed control, implements used for in-row and for betweenrow weed control, and addresses other factors—such as soil and weather conditions—that enter into decisions about weed management.
Tillage and Cultivation Practices in Managing Weeds
The use of tillage and cultivation practices as a means in controlling weeds has required the organic growers to become knowledgeable not only in annual weed life cycles but also in the life cycles of perennial weeds.
The first line of defense against annual weeds is cultivation. The ideal time to cultivate is when weeds are in the “thread stage,” when they’ve just germinated and are no thicker than a thread. At that point, they are easily destroyed with a series of quick cultivation passes. Cultivation operations should be as shallow as possible to avoid bringing new weed seeds to the soil surface.
The strategy in controlling perennial weeds is just the opposite of that for the annuals. Perennials have the ability to reproduce by seed as well as vegetatively—a unique characteristic that promotes the survival of a perennial species. In vegetative (asexual) reproduction, a new plant develops from a vegetative organ such as a stem, root, or leaf. Several modifications of these organs are common in perennial weeds, such as underground stems (rhizomes), above-ground stems (stolons), bulbs, corms, and tubers. The combination of these factors can make perennial weed control a difficult process especially since reproductive structures can be produced any time a plant reaches maturity. Control of perennials is usually achieved through multiple management tactics and at different times than annuals. For example, a moldboard plow can be used to control of deep-rooted perennial weeds to sever the taproot deep in the ground and to completely bury the weed’s crown.
Stale (or False) Seedbed Tillage
An often overlooked weed management practice is the stale seedbed technique; a weed management practice in which weed seeds just below the soil surface are allowed to germinate and then killed prior to planting the cash crop while minimizing soil disturbances. The stale seedbed technique is based on the premise weeds that germinate and emerge before the crop is planted, are easier to manage. Stale seedbed works by targeting weed seeds in the shallow layer (i.e., germination zone) of the soil. When adequate moisture is available, most weeds will emerge from the top 2.5 inches (1 cm) of the soil profile. These non-dormant seeds are allowed to germinate and then flamed or very gently cultivated just prior to planting the cash crop.
Blind cultivation can be performed before the crop emerges. To minimize crop damage, blind cultivation should be done before the crop has emerged and/or once it is well rooted. Blind cultivation takes advantage of the difference in size and sprouting depth between crop and weed seeds. Most weed seeds are smaller than crop seeds, and they germinate shallower in the soil. The crop seeds are safely below this layer and are not hurt by a shallow weeding before emergence.
Crops Suitable for Blind Cultivation
Certain types of crops tolerate blind cultivation better than others. In general, crops that quickly develop large taproots after germination; larger-seeded crops, including corn and soybean; and crop seeds that are planted an inch or deeper will tolerate the blind cultivation.
Timing of Blind Cultivation
The success of the first blind cultivation is extremely important because it must give the crop an initial head start. The intention, of course, is to remove the weeds without harming the crop. Usually, the first blind cultivation pass is done right before crop emergence, with a second pass done about a week later, depending on conditions.
Weeds Susceptible to Blind Cultivation
Weed species vary in their vulnerability to blind cultivation. Broad-leafed weed seedlings with their growing point above ground are easily killed when their tops are broken, while grasses with growing points below the soil surface need to be uprooted and desiccated.
Implements Used for Blind Cultivation
Implements used for blind cultivation are designed to be able to contact crop plants without significant damage, yet remove young weed seedlings. Examples of implements used for blind cultivation include: (1) flex-tine harrows, (2) spring-tooth harrows, (3) spike-tooth harrows, (4) chain-link harrows, and (5) rotary hoes.
Flex-tine Harrows. Flex-tine harrows, also known as tine weeders, are the most widely used tools for blind cultivation (See Figure 14.3). Flex-tine harrows have multiple rows of flexible metal tines that are mounted on a toolbar, wiggling slightly as they are pulled along, uprooting or dislodging very small weeds. The spines vibrate perpendicularly to the direction in which the tractor is moving. The tractor speed will increase the vibration of the tines as they are pulled through the soil. The vibrating tines uproot and shake the soil loose from the newly germinated weeds, bringing them to the soil surface to desiccate and die. Many makers allow individual tines to be raised up over crop rows while other tines are down for inter-row, post-emergence cultivation.
Spring-tooth Harrows. Spring-tooth harrows are extremely aggressive, but they are sometimes used for weeding (See Figure 14.4). Because of their potential to do crop damage, spring-tooth harrows are generally only used in emergencies where the crop will otherwise be lost.
Spike-tooth Harrows. Spike-tooth harrows are very effective weeders (See Figure 14.5). They can both uproot and bury weeds. The angle of the spikes can usually be adjusted with a handle from straight up and down to angled back at a flat angle to the soil. Rocks are a big problem with spike-tooth harrows.
Rotary Hoes. Rotary hoes work before or after crops are up, as long as crop seed is more deeply rooted than weeds and crop tissue damage is not too severe. Weeds must be very small or not yet emerged for good control. Rotary hoes work primarily by uprooting weeds and/or by loosening the soil from the tiny roots of the weed seedlings. Rotary hoes are used for “broadcast” cultivation, i.e. lightly tilling their full width at one to two inches deep without regard to crop rows. They are very gentle on the crop and can be used when more aggressive weeders cause too much crop damage. Crops with strong, flexible stems suffer the least damage. Crops such as corn, soybean and various field beans tolerate one or several cultivations with the rotary hoe.
Chain-link Harrows. Chain-link harrows have short shanks fitted on chains rather than a rigid frame, so that they hug the ground (See Figure 14.7). They are especially effective on light soils and prior to crop emergence, or in short crops. Chain harrows are best for light soils and before crop emergence. Chain-link harrows are especially well suited for crops with rapid initial growth (e.g., peas, string beans and sweet corn).
In-row cultivation, also called intra-row cultivation, is accomplished with cultivators that control weeds within the crop row. Mechanical weeding within the crop row requires selectivity between the crop and weeds, and can be a difficult job given that both are often at similar growth stages at the time of cultivation. In-row weeders require very precise operation and are often operated at slow speeds with narrow (one or two crop row) equipment. Weed control is greatest when weeds are very small—often with two leaves or less. These tools usually need to be combined with more aggressive between-row cultivators. Cultivators for intra-row cultivation include specialized precision tools such as finger weeders, torsion weeders, retracting tree/vine cultivators, and the French plow.
The finger weeder is a simple mechanical intra-row weeder that uses two sets of steel cone wheels to which rubber spikes, or “fingers” are affixed (See Figure 14.8). These fingers point horizontally outwards at a certain angle and operate from the side and beneath the crop row with ground driven rotary motion. The rubber fingers work the soil just below the surface, uprooting small weeds located very close to the crop. There are a very large range of options on the basic design, including different diameters/sizes, a wide range of materials used for the weeding fingers, from steel, through a range of plastics, fabric reinforced rubber and even brushes.
The torsion weeder is a very simple, affordable design consisting of two steel rods, one on each side of the crop row to uproot small weeds while pushing soil into the row (See Figure 14.9). Torsion weeders work by breaking up the soil in the intra-row, but with more of a shattering effect than the mixing/churning effect of finger weeders. Torsion weeders use spring tines connected to a rigid frame and that are bent so that two short tine segments are parallel to the soil surface and meet near the crop plant row. This arrangement allows crop plants to pass through the tine pairs.
Retracting Tree/Vine Cultivators
These cultivators have developed mechanical cultivators for orchards and vineyards (See Figure 14.10) . Many of them use a trigger bar to pull the device around the trunks and posts, allowing weeding in the tree-row. They are mounted to a side frame or a rear 3-point hitch. The device is mounted on the right side of the tractor, providing operator visibility. They can also be outfitted with other implement heads, such as sweeps, disks, and rakes making it a versatile piece of machinery for orchard and vineyard operations.
Between-row cultivation, also called inter-row cultivation, controls weeds that grow between the rows, and therefore is only used in row crops. Inter-row cultivation is done three to fiveweeks post planting. Inter-row cultivation is low risk to the crop compared to intra-row (i.e., within the row) operations. Generally, cultivation is performed at depths less than two inches so that crop roots are not damaged and soil moisture is conserved. If the young crop is in danger of becoming buried by soil or weeds during cultivation, shields can be used on the cultivator. Implements used for inter-row cultivation can include basket weeders, brush weeders, rolling cultivators, rotary tillers, field cultivators, and flex-tine harrows.
Field cultivators are the most common machine used for mechanical weed control for row crops when they a few inches tall (See Figure 14.11). Commonly used cultivation setups consist of a shank, which is typically long and narrow (either straight, C-, or S-shape) attached to a toolbar, with a cultivating tool (e.g., duckfoot, goosefoot, shovel, sweep, knife, hilling disk) attached to the bottom of the shank. Usually there are three to five shanks, called a gang, mounted on a toolbar. The distance between the crop rows and the precision of the implement determine the working width of the gangs.
Rolling cultivators have gangs of three to five “spider wheels” (wheels of strong, curved, cutting teeth radiating from a center hub) that mount independently on a toolbar (See Figure 14.12). The angle that they work the soil, and thus their aggressiveness, is usually adjustable. The number of gangs grouped together determines cultivator width, and these are usually rear-mounted, but pairs of gangs may be belly mounted to work a row or two. Soil can be thrown into row to bury small weeds or away from the crop row, depending on angle of the gangs.
Flex-tine harrows, which can also be used for between-row cultivation consists of a series of flexible tines mounted in overlapping rows vibrate as they drag through soil, ripping out small weeds (See Figure 14.3). As previously mentioned, these tools are most effective when weeds are in the “white-thread” stage (single white roots are visible on weeds when soil is disturbed, often before leaf appearance).
Basket weeders, also referred to as rolling cages, are cylindrical, made of quarter-inch spring wire, and ground-driven (See Figure 14.13). Basket weeders consist of two rows of metal baskets that roll across the soil surface at different speeds. The first set of baskets loosens the soil and the second pulverizes it, uprooting young weed seedlings. The first row is ground driven, which also drives a chain to power the rotation of the second row.
Rotary cultivators, also known as rotary tillers, have blades anchored on rotors that are bolted directly to a single horizontal power shaft that is 8 to 12 inches (20 to 30 cm) above the soil surface (See Figure 14.14).
Brush weeders uses flexible brushes made of fiberglass or nylon rotated about vertical or horizontal axes (See Figure 14.15). These weeders mainly uproot, but also bury and break weeds. The soil must not be too hard or too fine.
Weather and Soil Conditions Suitable for Cultivation
Weather and soil conditions play a very important part in the success of cultivation. Ideally, soil conditions should be dry and warm to desiccate and kill the weeds on the surface. Dry soil will enable the cultivator to be effective at uprooting the weeds without creating clods or “root balls.”
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