Chapter 14

Weed Management for Organic Crops

Mechanical Weed Control

Mechanical weed control is critical for managing weeds in organic systems. Mechanical weed control consists of methods that kill or suppress weeds through physical disruption. Mechanical weed control includes the use of pre-plant tillage such as plowing, disking, and field cultivating. Most organic growers cultivate a conventionally tilled seedbed before planting the crop. Mowing and rowing/crimping may also play a critical role in managing weeds in or non-crop areas. Repeated mowing or rowing/crimping reduces weed competitive ability, depletes carbohydrate reserves in the roots, and prevents seed production.

Tillage

A full-field system manages the soil uniformly across the entire field surface. Such conventional tillage systems typically involve a primary pass with a heavy tillage tool to loosen the soil and incorporate materials at the surface (e.g., fertilizers, amendments, weeds, etc.), followed by one or more secondary passes, often referred to as secondary tillage, to create a suitable seedbed.

Primary Tillage

Primary tillage is the initial step in seedbed preparation. Its objective is to prepare the soil for planting by reducing soil strength, covering plant material, and by rearranging soil aggregates. Primary tillage techniques is always aggressive and usually carried out at considerable depth, leaving and uneven surface. For weed species that are propagated by seeds, primary tillage can contribute to control by burying a portion of the seeds at depths that minimize seed germination. Primary tillage can also play a role in controlling perennial weeds by burying some of their propagules (e.g., rhizomes, tillers, or roots) deep, thereby preventing or slowing down their emergence.

Secondary Tillage

In secondary tillage, the soil is not worked as aggressively or as deeply as in primary tillage. The purpose of secondary tillage is to further pulverize the soil, mix various materials or manure into the soil, level and firm the soil, and control weeds. Seedbed preparation is the final secondary tillage operation except when used in the false or stale seedbed technique. Secondary tillage operations are performed with lighter tools such as the disk harrow, field cultivator, spring-tooth harrow, wire-tooth harrow, spike-tooth harrows, packer rollers, and rolling baskets.

Cultivation

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.

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.

Annuals

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.

Perennials

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.

Mowing

Mowing is another option used by organic growers in controlling weeds and cover crops to a manageable height in orchards and vineyards. It is a relatively fast operation that causes minimal soil disturbance, although soil compaction may become an issue where mowing is frequent. Mowing weeds or cover crops can be used to produce mulch material for weed suppression in the inter-row or intra-row areas. Growers should consider a program of mowing alternate rows, allowing the uncut rows to provide habitat and food sources for beneficial insects in the orchard or vineyard.

Rowing/Crimping

Cover crops can be killed using a roller-crimper (See Figure 14.11). As the name suggests, the roller-crimper is a large, heavy cylinder, with metal flanges coming off at an angle that are attached to a roller drum, one to two feet in diameter. As it rolls, it pushes down the cover crop and “crimps” the stems to kill the crop. Rollers can be front- or rear-mounted on a tractor. Roller-crimpers come in a variety of designs. Straight bar rollers have good killing action and are easy to make, but transfer vibrations to the operator and require slower tractor speeds. Curved or spiral blades on the roller drum enable the roller to stay in constant contact with the ground, thus allowing faster speeds and reduced vibration, but also have reduced crimping action.

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