BROADCASTER ARCHIVES

Organic and Sustainable Weed Control
By Jeff Gunderson

This article was first printed in the July/August 2010 issue of the Organic Broadcaster, published by the Midwest Organic and Sustainable Education Service.

Weeds are the most persistent pest farmers have to deal with, causing more annual yield loss than insects and disease combined. The conventional approach to weed management is predominantly dependent on herbicide application to eliminate weeds already established in the field. This is reactive weed management; it is based on waiting until the problem occurs and then moving to treat it. Sustainable weed management employs an entirely different strategy involving a more holistic and proactive approach. This approach focuses on the importance of understanding the root cause of weed problems, and with this information, developing an integrated system of preventative management options to control weeds.

These integrated strategies often mimic natural principles and therefore take advantage of ecosystem services for weed control. Ecosystem services are those services that a healthy ecosystem provides for free, such as nutrient cycling and, most important in this case, pest regulation. It is rare to see pest outbreaks occurring in nature. This is a direct result of the great diversity of natural ecosystems, which keeps them in a state of active equilibrium where no one species is using resources excessively. Sustainable and proactive weed management practices that take advantage of, and enhance, these ecosystem services will be discussed here. These practices include crop rotation, cover cropping, weed seed bank management, biological control through allelopathy and enhanced weed seed predation and decay, and probably the most fundamental of all: good soil quality and fertility for a competitive crop.

What is a Weed?
Before discussing these management strategies, there are two important questions that must be asked. The first being: what really is a weed? The answer to this question is extremely important when developing a sustainable integrated weed management system. A weed is usually defined as a plant growing where it interferes with crop production, or more generally, a plant growing where we don’t want it. From this definition it becomes apparent that what we define as a weed depends on context, on where it is growing. Monocultural crop production inevitably creates weeds by providing an environment in which crops have value and, of course, weeds do not.

The fact that most weeds of vegetable and grain cropping systems are highly opportunistic, seeming to crop up overnight by the thousands, indicates that our most common weeds have evolved to take advantage of the disturbances that are common conditions present in crop fields. Those that establish and grow quickly are more likely to successfully outcompete the crop. Weeds of pastures and hayland have a different strategy, slowly building reserves through large root systems to effectively compete.

This brings us to the second question: why do weeds occur? Weeds occur as a result of nature’s tendency towards diversity. Monocultures rarely exist in nature, especially on a large scale. Agricultural fields of a single crop invite weeds, as nature attempts to take advantage of ecological niches that the crop is not fully using. These niches can be thought of as opportunities, or any resources that could be exploited by a weed that are not being used efficiently by the crop. The key to preventative weed management is to eliminate these niches to the greatest extent possible.

Soil Fertility is Key to Successful Competition
The first step in eliminating opportunities for weeds to take hold is through good soil fertility management. Nutrient availability has a direct effect on crop health and ability to compete with weeds. Crops perform best when given access to not just N-P-K, but all 17 essential nutrients. If soil nutrition is not favorable to the crop, weeds are provided with opportunities to establish themselves. The law of the minimum states that your crop will only be as healthy as the most limiting nutrient will allow, but this is not the whole story. Unbalanced fertility, for instance that resulting from overfertilization with soluble fertilizers, can also directly affect crop health and weed performance. For example, excessive nitrogen in the soil environment can lead to an excess of N in the crop in relation to other nutrients, which in turn can lead to a metabolic bottleneck, reducing crop performance. Free nitrogen in the environment in excess of crop needs also gives an opportunity for weed growth, by providing a resource weeds can take advantage of. Furthermore, an excess of one nutrient in the soil can sometimes lead to reduced uptake of another nutrient. For example, excess potassium in the soil can reduce magnesium uptake. Many of the micronutrients also conflict with each other in terms of plant uptake.

Research has shown that even though weeds have a good deal of elasticity (meaning they can adapt to many different soil conditions), weed performance will vary along with changing nutrient ratios. This indicates that changing the relative nutrient composition of the soil can affect how well weeds will compete with crops. Ultimately, the competitive relationship between crops and weeds is complementary, meaning that when soil conditions promote healthy crops, they will have the competitive advantage, whereas, if soil conditions are less than ideal, weeds will have the advantage.

Using organic fertilizers is of benefit because these fertilizers may become available in a manner more in sync with crop needs, in contrast to soluble, synthetic fertilizers, which may leave excess N in the soil for weeds to use. The decomposition of organic fertilizers is enhanced by compounds exuded from crop plant roots, which stimulate microbial activity near the root surface. This microbial activity increases nutrient availability for the crop. While in organic form, nutrients can be thought of as in storage (and resistant to loss). As they become “mineralized” by soil biology, they become useable by plants. Good soil fertility management takes advantage of the process of nutrients cycling between organic and inorganic (available) forms. This is an important ecosystem service, provided by the soil biology, which limits excess nutrients in the environment and also ensures the efficient use of resources by the crop.

Weed Seeds in the Soil
The next important step to sustainably managing weeds is to understand the weed seed bank. The weed seed bank is the reserve of all the viable weed seeds in the soil. There are commonly thousands of weed seed per square foot, and it is the producer’s job to keep weeds from establishing, as well as to make sure that that seed bank is being drawn down rather than built up. One technique to reduce your weed seed bank is planting into a stale seedbed. This involves preparatory tillage a couple weeks ahead of planting. Initial tillage stimulates weed seed germination by exposing seeds to light. Follow-up light tillage is then done immediately before planting to kill the weed seedlings. Knowing what weeds are going to be dominant under different crops and management approaches can help farmers to adjust their stale seed bed practices, as well as time planting dates to enhance crop competitiveness. Care should be taken not to perform tillage when the soil is too wet. This can lead to soil compaction, and a tight soil can enhance weed seed germination. Leaving a loose soil at the time of planting also reduces the chances of crust formation, which can stimulate weed seed germination by increasing soil-seed contact.

Another important thing to consider when managing your weed seed bank is how to keep weeds from going to seed. This is very important if you want to draw down that bank account, especially since agricultural weeds don’t skimp on seed production. Common lambsquarters, for example, can produce up to 170,000 seeds per plant! The good news is that most weed seeds die or are consumed in the soil quickly, with a 50% reduction in giant foxtail seeds, for example, occurring in less than a year. If weeds are kept from going to seed, their populations will decline relatively quickly over time, although some will remain in the soil much longer than others.

Weed seeds are removed from the seed bank through degradation by microorganisms or consumption by insects and small mammals. Microbial degradation of weed seeds can occur, although its overall impact is not well known at this point. There is evidence to suggest that highly biologically active soils, which are common under organic management, may increase the degradation rates of weed seeds in the soil, a result of microbial degradation. “Weed seed predation” is the term used to describe the consumption of weed seeds by insects or small mammals. Insects, namely ground beetles or crickets, are usually the most common weed seed predators. Surprisingly, predation by these organisms is a significant fate of weed seeds on the soil surface. In fact, up to 80% of the weed seeds shed annually can be consumed by predators. Weed seed predation is greatest in environments with some residue on the soil surface to provide cover. Heavy residue, however, impedes the movement of small insects and therefore limits predation. The activity of weed seed predators is greatest in mid-summer, corresponding with the time weeds are shedding their seeds. Performing tillage at this time of year reduces the activity of weed seed predators, so managing in ways which decrease the need for tillage during mid-summer will maximize the benefit gained from this ecosystem service.

Rotating Crops to Interrupt Weeds
Crop rotation is another important weed management strategy. Farmers have been using crop rotation for thousands of years, and it remains a useful tool for weed control. In designing rotations for weed management benefits, “tight” rotations are best. This means not growing the same crop for more than one year in a row. The greater the length of the rotation, the greater benefit to weed control, if crops with the same growth habit are not grown in succession. Good rotations alternate between cool season and warm season crops, helping the communities of weeds associated with each of those crops from becoming well established and setting seed. Different weeds have evolved in sync with different crops. For example, weed communities in a small grain will tend towards cool season weeds because of planting date and the lack of post emergence cultivation, while in long day crops like corn, warm season weeds such as velvetleaf and ragweed can be more dominant (although these two weeds will be most abundant under differing soil conditions).

Cover Crops have Multiple Effects on Weeds
Cover cropping is an important addition to the crop rotation as a means to suppress and compete with weeds. Cover crops provide weed control both while the cover is growing, through competition or the suppression of germination, but also after the crop is terminated, especially when using crops with allelopathic properties. Allelopathic compounds are those that are excreted into the soil by certain crops which inhibit the germination of small-seeded plants in the adjacent soil environment. Since most weeds are small-seeded, planting covers with allelopathic properties can increase weed control in subsequent crops. Control by allelopathic residues is greatly reduced if residues are incorporated rather than left on top of the soil surface. Rye is one well known crop which exhibits allelopathy, although many crops, such as wheat, sorghum, sunflowers, field peas, and hairy vetch also demonstrate the ability to produce allelopathic compounds. Cover crop residue left on the soil surface can also provide a temporary mulch that can prevent weed seeds from germinating; however, this will only be effective if soil disturbance is kept to a minimum, so weed seeds are not stimulated to germinate.

Cover crops also help to increase soil fertility, especially when legumes are used. Covers also “catch” and retain nutrients in the upper soil profile and enhance soil structure. Given their relationship to fertility, it is important to know the quality of the cover crop residues that will be added to the soil. The carbon-to-nitrogen ratio of the cover crop is an important parameter to be aware of. A high C:N ratio indicates a material that can build humus content (and therefore nutrient and water holding capacity) but will add less readily available nutrients for immediate crop needs. Low C:N materials will have the opposite effect, cycling faster and providing more readily available nutrients, but contributing less to soil humus content. A general recommendation is that the cover crop consists of a legume/grass mix, preferably in the proportion of approximately 70% legume to 30% grass. This proportion of legume to grass will help to balance the C:N ratio, thereby providing a good supply of nitrogen from the legume, with the higher carbon grass helping to slow the release of this nitrogen. At the same time, if too much high carbon grass were to be included in the mix, microbial competition could reduce nitrogen availability for the crop.

Crop rotations and the use of cover crops inject diversity across time and space at the farm level. Encouraging a diverse agroecosystem mimics the tendency towards diversity in natural ecosystems. This in turn leads to an environment where resources are used more efficiently because the species involved have complementary roles. Less excess means less room for unwanted invaders to establish.

Managing the Weeds That Find Their Way In
The above techniques can be extremely effective in reducing weed pressure over time, and should form the backbone of a sustainable weed management program. However, even with the best preventative management, weeds are still going to sometimes find their way in. When that does occur, there are several control options farmers can try.

The most common tool used by organic farmers is cultivation. Depending on the crop, options include blind cultivation implements such as the rotary hoe and the harrow, or row cultivators with sweeps, baskets or fingers. Flame weeders are also commonly used. Another weed management approach is using physical barriers, such as plastic mulch, to prevent weeds from occurring, although this technique is most applicable to certain vegetable crops.

Another main strategy for reactive organic weed control is the use of approved herbicidal products. Under the National Organic Standards, allowed herbicides can only be used when preventative measures have failed and a documented weed problem is occurring. At the current time, no selective herbicides are available for use in organic systems, so options are limited to pre-emergence or post-harvest burndown applications, or selective application. Some products allowable in organic systems that are on the market include Matratec™, made from clove oil, or Weed Zap™, made from clove and cinnamon oil. Vinegar, or acetic acid, can be used as an herbicide under the organic regulations as well, but don’t buy the stuff off the shelf at the grocery store, because it won’t be effective accept at very high rates. Instead, look for at least 10% acetic acid or more. This kind of vinegar is available from the National Vinegar Company, although any source is fine as long as the product includes no other ingredients which are not approved for use in organic agriculture. Another product that may be allowed is AllDown™, made from citric acid, garlic oil and vinegar, but organic growers should check with their certifiers before using this product. All these products will be most effective on small weeds, and good contact with foliage is a must.

Conclusion
When beginning to plan an organic weed control program, it is important to step back and consider the big picture. Ask yourself what your problem weeds are and why they are a problem. Is your soil fertility where it should be? Is your rotation diverse enough? Are you working to draw down your weed seed bank? With the answers to these questions you can start to develop management approaches that will harness ecosystem services, which are performed free of charge by a healthy ecosystem, to prevent weed problems from occurring in the first place. While a farmer will never to be totally free of weed pressure, preventative management can go a long way to reduce the need for cultivation and herbicidal inputs, along with their associated costs.

Jeff Gunderson, MOSES organic specialist, has a Master's Degree in soil science and a background in agronomy. He may be contacted at jeff@mosesorganic.org.