Chapter 14

Weed Management for Organic Crops

Controlling Weeds by Cultivation

As previously discussed, mechanical weed control starts with the annual primary and secondary tillage practices. 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 leads to the shredding of the weed biomass and further dislodging of shallow-rooted weeds. 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 more effective in weed control after the planting of crop when the weeds are relatively small than when the weeds are large. Therefore, cultivating a field early in the season when the weeds are young will give better results than waiting until later. This section provides a more in depth discussion on cultivation practices for pre-emergence and post-emergence weed control, implements used for in-row and for between-row weed control, and addresses other factors—such as soil and weather conditions—that enter into decisions about weed management.

Blind Cultivation

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 14.12). 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 (See Figure 14.13) are extremely aggressive, but they are sometimes used for weeding. 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. They can both uproot and bury weeds (See Figure 14.14). 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.

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 14.15).

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 (See Figure 14.16). 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.

Weed Management after Planting

Cultivation is one of the most effective mechanical practices that can be carried out after planting. Many different types of cultivation implements have been developed to control weeds in and/or between the crop rows. The necessity for reliable mechanical weed control has led to the development of many innovative cultivators. This section focuses on some of the more commonly used cultivators for in-row (i.e., intra-row) and for between-row (i.e., inter-row) weed control.

In-row Cultivation

In-row cultivation, also called intra-row cultivation, is accomplished with cultivators that controls 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.

Finger Weeders: 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 14.17). 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.

Torsion Weeders: 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 14.18). 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.

French Plow: The French plow or grape hoe consists of a cutting blade that undercuts weeds working just a few inches below the berm surface and a sensor system to rotate the blade out of the row to avoid contact with the vines (See Figure 14.19).

Retracting Tree/Vine Weeders: These cultivators have developed mechanical cultivators for orchards and vineyards (See Figure 14.20). Many of them use a trigger bar to pull the device around the trunks and posts, allowing weeding in the tree-row.

Between-row Cultivation

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 five weeks 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.

Basket Weeders: Basket weeders, also referred to as rolling cages, are cylindrical, made of quarter-inch spring wire, and ground-driven (See Figure 14.21). 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.

Brush Weeders: Brush weeders uses flexible brushes made of fiberglass or nylon rotated about vertical or horizontal axes (See Figure 14.22). These weeders mainly uproot, but also bury and break weeds. The soil must not be too hard or too fine. When the soil is too hard, the brush weeder will remove only the part of the weeds above the soil, and the weeds will readily regrow.

Rolling Cultivators: 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 14.23). 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.

Rotary Tillers: 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 14.24). Driven by the tractor’s PTO, the cultivators have a vertical, horizontal or oblique axis.

Field Cultivators: Field cultivators are the most common machine used for mechanical weed control for row crops when they a few inches tall (See Figure 14.25). 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 (duckfoot, goosefoot, shovel, sweep, knife, hilling disc, etc.) 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. The width of the toolbar and the number of blades depends on the width of the working.

Flex-tine Harrows. Flex-tine harrows use a series of flexible tines mounted in overlapping rows vibrate as they drag through soil, ripping out small weeds. 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).

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.”

Click on the following topics for more information on weed management for organic crops.