Many organic farmers say dealing with weeds is their biggest challenge. It’s easy to understand why; weed management can be tough in organic systems, especially transitioning ones. But the idea that an organic field is a weedy field is outmoded. Skilled organic management can achieve near-total weed control even in challenging crops like soybeans, small grains, carrots, and strawberries.
Another misperception is that organic weed management relies entirely on mechanical cultivation. It’s true that you’ll be better off if you know your way around a cultivator. But cultural weed-management methods are just as important, if not more so. Crop rotations, adjusted planting dates, mulches, and other preventive methods can all be critical components of a successful organic weed-management plan.
Where weeds are concerned, an ounce of prevention is worth a pound of cure. Using a good mix of organic weed-management practices that work together to lower weed pressure is especially important when weather is uncooperative or other complications arise.
Organic Weed Management Fundamentals
The organic approach to dealing with weeds is best described as ecologically based weed management or integrated weed management (IWM). IWM brings together a wide variety of strategies – from well-designed crop rotations to adjusting planting methods to novel techniques like flame weeding – that together offer an effective weed-management program. Iowa State University weed ecologist Matt Liebman refers to this as the “many little hammers” approach – going after weeds with many small, varied strategies instead of a one-shot approach, as with herbicides.
IWM emphasises an ecological understanding of how weeds behave in farming systems. Virtually every field activity you engage in – from soil amendments to crop selection to tillage methods – can have an impact on your weed levels and how they interact with crops. There are a few basic principles of IWM to keep in mind.
Don’t let weed populations get out of hand. This means if you see a patch of weeds developing or if you notice that a certain field has become weedy, you need to increase the intensity of management in that area. This can be as simple as scouting and targeting patches of problem weeds or rotating a field into a weed-suppressive crop. Don’t let weeds get adapted to your operation. This means using multiple and diverse tactics to manage weed populations.
Weeds can easily survive and persist if the same management practices are done at the same time of year every year. The amount of damage weeds can do to a crop can vary. There are many ways to make crop plants more competitive so the negative impact on crop yield from weeds is reduced. Just as every field operation can affect weed levels, weed-crop competition can be affected as well. The key is to manage your system so that the crops are given as much of an advantage as possible over the weeds.
Focus on Prevention and Attention
Preventive weed management includes good sanitation and other practices to limit the spread of weed seeds on your farm. Be sure to:
- Use high-quality, weed-free seed. Commercial seed packages should state their weed seed percentage on the label. If you save seed, make sure you clean it well.
- Limit weed seed production in field margins and other areas. Some farmers clip weed seedheads over standing soybeans with a high cutter bar. An hour of bean-walking or thistle-stumping can be a good investment.
- Don’t spread weed seeds from one field to another via tillage equipment or other tools. Hose down muddy tires and cultivators if necessary.
- Avoid plowing down a fresh crop of weed seeds in the fall. Weeds in the top few inches of soil have a relatively low survival rate, while those deeper down are more likely to germinate when brought to the surface next season.
- Compost manures and plant materials thoroughly. Buy compost only from reputable sources. High-temperature composting with active microorganisms should break down any weed seeds.
High-value vegetable crops require particular vigilance, since acceptable weed thresholds are much lower. Vegetables are not very competitive with weeds, and the hand-harvesting involved makes weed management through cultivation and cultural practices your number-one priority. Cultivation is best done every week until you can no longer clear plants.
Rows should be marked with a mechanical cultivator if you are planting or seeding by hand. Flameweeding is very useful on slow-germinating vegetable crops such as carrots and beets prior to emergence. Insufficient attention to weed management in the early years of an organic operation can be fatal to the success of your overall enterprise.
Use Rotations to keep Weeds in check
Diverse crop rotations are among your best strategies for reducing weed populations. Alternating row crops with solid-seeded crops, including one or more years in hay or pasture, and filling gaps in your rotation with aggressive covers like rye and hairy vetch can all help reduce annual weed pressure and contributions to the weed seed bank in your soil.
By limiting and varying the baresoil periods during which weeds can germinate, rotations increase the likelihood that weed seeds will decompose or be eaten by predators before they have a chance to grow. Rotations also help with weed management by building soil fertility, tilth, and organic matter, creating conditions in which the crop plants can thrive at the expense of the weeds.
Another way to use rotations for weed management is to take advantage of certain crops’ allelopathic, or natural weed-suppressive qualities. Rye and sorghum -Sudangrass are strongly allelopathic; sunflower, sorghum and many brassicas are less strongly so. Certain varieties of wheat and rice are also thought to inhibit weed seed germination. Plants like these can be used as primary crops or plow-down smother crops to clean up a field with severe weed pressure.
Sod crops, when mowed or grazed regularly, also help manage weeds. Pasture and hay crops can go a long way to suppress weeds while also building organic matter and enhancing soil health. Paying attention to weed management in the early years of transitioning to organic is critical – a couple of bad seasons can give you serious weed problems for years to come.
Planting strategies to beat Weeds
Organic farmers use a number of different planting and seeding techniques to meet the challenges of herbicide-free farming. Planting at higher densities, on narrower row spacing, or at higher broadcast rates can increase crops’ competitiveness against weeds, compensate for losses during cultivation, and get the canopy closed faster.
Seeding rates can be as much as 10% to 20% higher than those recommended for nonorganic systems. If you can, choose crop varieties that canopy rapidly to shade and outcompete weeds. Most modern cultivars of wheat, barley, and oats have been selected for reduced top growth and straw length, but varieties with stronger vegetative growth may work better for organic production. Organic vegetable growers also shift planting methods to make weed management easier.
Transplanted crops are easier to cultivate than direct-seeded crops, for instance, so organic growers typically use transplants as much as possible. For direct-seeded crops, a precision seeder that will limit the need for thinning can be a big advantage. Straight, evenly spaced rows will make cultivation more efficient.
Vegetative and Synthetic Mulches
Mulches can be very effective at controlling weeds. Mulch changes the environment around the soil surface, making it difficult for weed seeds to germinate and grow. Vegetative mulches, such as rye straw, can also suppress weeds by allelopathy. Research shows that rye mulch can reduce weed seed germination by 75 to 95% while leaving large-seeded crops such as corn, peas, cucumbers, or beans unaffected.
Natural or non-synthetic mulches such as straw or leaves can have the additional benefit of boosting soil organic matter over time, and even adding fertility. Be careful whenever you import mulch (or compost) materials such as leaves, loose straw, or grass clippings that you avoid potential contaminants or debris. Avoid glossy inks from newspaper inserts and magazine waste.
“Living mulches” are cover crops of clover, grasses, or other species used to suppress weeds in orchards and some other types of cropping systems. These can be mowed regularly or seasonally to maintain the stand. Synthetic mulches: many organic vegetable growers use plastic mulches to manage weeds in crops like tomatoes, peppers, squash, and cucumbers. Plastic mulches can be laid down with specialised bed-former/mulch-layer implements, or, on a smaller scale, laid by hand after beds have been formed. Drip irrigation tape is usually laid down beneath the plastic.
A variety of different coloured plastic mulches (black is standard) can be used to influence soil temperature and other factors. Natural and synthetic mulches can also be used together, of course, with straw or clover used between raised beds covered in plastic. Synthetic mulches are classified as a “restricted” input under the National Organic Program Standards, meaning they must be used in compliance with the stated annotations. Regulations require that all synthetic mulches be removed from the field at the end of the growing season.
Summary
Organic weed management does require a shift in attitude. Striving for 100% weed eradication in organic systems is not essential. The fact is, most crops can tolerate a few weeds without showing any impact on yield or quality. Research at the Rodale Institute, moreover, has shown that organic crops actually have a greater ability to tolerate weeds than nonorganically managed crops do.
In almost 30 years of side-by-side trials, Rodale Institute’s organic plots have consistently yielded as well as the non-organic plots, even though the organic plots usually have heavier weed pressure. It may be that the organic crops suffer less competition from weeds because soil quality is better, making nutrient and water resources more plentiful.
Or, it may be that more complex ecological interactions are taking place. After all, weeds can play a variety of useful roles within your farming system. They can help protect the soil from wind and water erosion. They provide food and habitat for wildlife and beneficial species. They can increase biodiversity, aid nitrogen fixation, add organic matter, and catch excess nutrients. They serve as trap crops for pests and provide grazing for livestock. The basic goal is just to keep weed pressure below the economic threshold – the point at which it has an impact on the crop.
