Twelve Steps Toward Ecological Weed Management in Organic Vegetables
Introduction
Ecological weed management begins with careful planning of the cropping system to minimize weed problems, and seeks to utilize biological and ecological processes in the field and throughout the farm ecosystem to give crops the advantage over weeds. In addition, mechanical and other control measures are usually needed to protect organic crops from the adverse effects of weeds. This is particularly true in vegetables and other annual crops, for which production practices keep natural plant succession at its earliest stages, thereby eliciting the emergence of pioneer plants that can become agricultural weeds.
While tillage and cultivation can degrade soil quality and increase the risk of erosion losses, many other organic weed management tools (Table 1) are more soil-friendly. For example, a diversified rotation of vigorous cash crops and cover crops can enhance soil organic matter, tilth, and fertility, provided that a sufficient quantity and diversity of residues are returned to the soil to feed the soil life. Grazing livestock after a production crop to remove weeds or interdict weed seed set can add fertility in the form of manure, though intensive grazing can also compact the soil. In the interest of food safety, care must be taken to avoid direct contact of fresh manure with vegetables and other food crop. Mowing and flame weeding (if properly done to avoid excessive heating of the soil itself) are much easier on soil structure than cultivation, and can be just as effective in certain stages of weed and crop development. Mowing or rolling a cover crop to form an in situ mulch can enhance the soil benefits of the cover crop, compared to tilling it in, and can effectively suppress many annual weeds. Other organic mulches, such as straw and chipped brush, add organic matter, whereas synthetic clear or colored plastic films and weed barrier fabrics do not. All mulches are very effective in preventing soil erosion.
Ecological weed management consists of many-component strategies tailored to each region, cropping system, and farm. Matt Liebman and Eric Gallandt (1997) describe the process as using “many little hammers”, including “indirect controls”, such as crop variety, planting date, and nutrient management, rather than relying only on the “direct controls” or “large hammers” of cultivation and herbicides. In their words, “the use of a combination of methods can lead to (i) acceptable control through the additive, synergistic, or cumulative action of tactics that may not be effective when used alone, (ii) reduced risk of crop failure or serious loss by spreading the burden of protection across several methods, and (iii) minimal exposure to any one tactic, and consequently reduced rates at which pests adapt and become resistant.” (Liebman & Gallandt, 1997, p. 326)
The following list outlines twelve key steps toward successful organic weed management that are discussed in greater detail in a series of related articles that can be found on this website. Note that these steps do not comprise a precise linear sequence of instructions; rather they offer a conceptual framework within which each farmer can develop a site-specific strategy. This process requires systems thinking, which views the field as a complex system of interacting components—such ascrops, weeds, soil, insects, and microorganisms—that form a web of relationships, not a linear sequence of cause-and-effect. Similarly, the following steps are employed together in a synergistic manner, and thus differ from the sequence of instructions for assembling a car or a farm implement. For example, Step 6 (cover crops) can be seen as a part of Step 2 (minimize niches for weeds), and Step 1 (know the weeds) provides vital information for other steps, particularly steps 3 (keep the weeds guessing), 4 (design for effective weed control), and 7 (manage the weed seed bank). Biological processes (Step 9) include indigenous biocontrols that help reduce the weed seed bank (Step 7) as well as the competitive and allelopathic effects of cover crops (Step 6). Step 11 (observe weeds and adapt practices) is an ongoing feedback loop that informs and fine-tunes all the other steps. Utilizing this or another suitable framework, the organic grower selects and assembles a set of “many little hammers” that, working together, keep the farm’s weeds from becoming major weed problems.