Dr. Mark Schonbeck, Virginia Association for Biological Farming
Agricultural weed communities continually adapt to weed control tactics and other farming practices, as well as changes in soil fertility, weather, and climate patterns. Part of successful organic weed management is observing changes in weed species and populations, and modifying tillage, crop rotation, and other weed management practices based on these observations. The following article explores how organic farmers can build on their successes and fine-tune their weed management systems year to year to meet new and emerging weed challenges.
Ecological weed management is a never-ending learning process. Farm ecosystems change and evolve constantly, and the weeds do likewise. Every farm decision and field operation can elicit changes in the weed community, as can weather variations and long-term climate changes. “Reading” the weeds each year becomes an information feedback loop, guiding weed management decisions for the following season.
No matter how diligently you plan out your organic weed management strategy, expect some of its components to be quite effective and others less so. A given weed control tactic may knock existing weeds back, yet open new niches for other weeds. Certain weeds may become more prominent at certain seasons or phases of the crop rotation. Thus, a key step in successful organic weed management is to constantly observe weeds, weed–crop interactions, and the efficacy of each preventive and control measure.
Recording, Interpreting, and Responding to Trends
Note and record any changes in weed species composition or weed pressure in each field and at each phase of the crop rotation. What weeds are becoming more abundant or troublesome, and which ones seem to be declining? Do these changes seem to be linked to particular practices or tactics? Be sure to take note of any new weed species, and consider how they might have arrived on the farm. At the end of the season, review your observations, assess weed management outcomes, and address any new or emerging weed problems by planning to modify practices in the coming season. These can include changes in crop rotation, cover cropping, tillage, cultivation, and other weed control tactics.
For example, increasing problems with certain annual “weeds of cultivation”—especially small-seeded annuals like pigweeds, galinsoga, and common chickweed—may indicate a need to reduce tillage, modify type or timing of tillage, or diversify the crop rotation. Organic mulches can be especially effective against small seeded broadleaf annuals, blocking the light stimulus for their seeds to germinate, and hindering emergence by those seeds that do germinate. Simply shifting the time of year at which tillage is done can also reduce germination and growth of some of these weeds. This can often be accomplished by rotating the field to a vegetable crop that is planted at a different time of year.
If large seeded summer annuals are creating increasing weed pressure, and you have been using the moldboard plow or other inversion tillage regularly, consider shifting to noninversion tillage or minimum tillage.
An increase in invasive perennial weeds may indicate a need for more vigorous tillage until these weeds are brought under control. Choose those tillage tools and other tactics that are most effective against the particular weeds that are becoming problematic. For example, the shallow, horizontally spreading rhizomes of weeds such as quackgrass, johnsongrass, nutsedge, or Bermuda grass can be worked to the surface with a chisel plow or field cultivator, thereby stressing them through desiccation and temperature extremes.
Strategic use of cover crops can impose multiple and timely stresses that contribute to weed control, depending on the season and life cycle of the weeds that are on the increase. If summer weeds are becoming more troublesome, rotate to cool season vegetables and plant highly competitive summer cover crops such as buckwheat, pearl millet, or sorghum-sudan. If winter annual and biennial weeds are interfering with early spring vegetables, rotate to warm season vegetables followed by a high biomass winter cover crop like hairy vetch + winter rye.
If a particular field or bed has been intensively cultivated for annual crop production for several years, and populations of one or more annual weeds are becoming overwhelming, consider rotating to a perennial cover crop, pasture or hay, in which the soil will not be tilled for one or more years. On the other hand, if a perennial planting such as asparagus or blueberry becomes overrun with perennial weeds that resist efforts to cultivate or mulch them out, consider rotating the area into annual vegetables for a few years. If the perennial plants are healthy and productive, move them to a clean bed in early spring or fall; otherwise, obtain new stock.
When Weeds Seem Out of Control
When weeds seem to get the upper hand despite a diligently planned and executed weed management program, the problem may be that the existing weed pressure is simply overwhelming the set of practices being deployed. Intractable weed problems can also indicate that crops are not thriving for some other reason, and therefore cannot compete effectively with the weeds. In these situations, it may be worth taking the land out of production for a season or two to focus on weed control and soil improvement. Specific steps might include:
- Test the soil to identify any imbalances, nutrient deficiencies or excesses, acid or alkaline pH, or salt buildup.
- Check the field for poor drainage, anaerobic conditions, a subsurface hardpan, surface crusting, lack of soil life, or other adverse soil conditions.
- Apply compost or organic or natural mineral soil amendments as indicated by soil test.
- Chisel to loosen hardpan, grow cover crops, and implement other practices as needed to correct physical soil quality problems.
- Evaluate the soil, microclimate, and site needs of the crops being grown in relation to current conditions in the field in question. Consider changing crop mix if there is a wide disparity that cannot be addressed through natural soil amendments or organic soil improvement practices.
- Exhaust perennial weed reserves and disrupt reproduction and growth through repeated timely tillage, choosing implements and methods that target the major weed(s) present.
- Draw down heavy weed seed banks through cultivated fallow periods.
- Follow each tillage or cultivated fallow period with a fast-growing, weed-suppressive cover crop planted at high seeding rates.
- On small areas, it may be feasible and economical to kill weeds by spreading a thick organic mulch over a layer of cardboard or newsprint (avoid glossy colors), or an opaque black plastic film mulch or weed barrier. Leave mulch or plastic in place until the weeds are dead, which may take six to twelve months.
Learn from Your Successes
Sometimes, a certain crop managed with a particular set of practices seems remarkably weed-free, or grows especially well despite some weed pressure. Some of these successes occur by chance, while others may be related to the combination of soil conditions, crop sequence, and weed management tactics used. Try the successful combination again next year and observe carefully to see if it gives a similarly favorable outcome; if so, consider adopting it as part of your weed management strategy. However, note that it may not be wise to repeat the same strategy year after year in the same field, as this creates a predictable niche that some weeds will exploit. As a result, weed species that have historically been unimportant on the farm may become common and troublesome.
Keep track of the cost-efficacy of different tactics. If one practice or set of practices is cheaper or easier to do than another, and it gives comparable crop yields and long term weed control, you might take the easy way even if it looks a little weedier at certain stages of crop development. For example, if the field looks a lot neater with four cultivations than with just two, yet crop yields are similar in both treatments, it may be more economical, and kinder to the soil, to cultivate just twice. However, watch closely to be sure that the less intensive, more labor-efficient strategy does not allow increased weed seed set or vegetative reproduction that will cause greater problems next year. Evaluate weed populations in the field during the season following a side-by-side comparison of the two systems.
Finally, evaluate your weed management program within a wider context of ecological and organic production. For example, it may not be wise to save money by omitting a cover crop, even if planting it did not measurably suppress weeds. The cover crop may make important contributions to soil quality or biological insect pest control, and thereby enhance long term productivity. Assess your weed management program in relation to soil quality, insect management, moisture and nutrient management, and overall biodiversity, as well as crop yields and weed control.