Managing Annual Weeds & the Weed Seedbank in Organic Fields

by Mark Dempsey, CFSA’s Farm Services Coordinator

Photo by Claire Keene

In no-till ground, where cover crops are used to suppress weeds, weed control after the crop is planted is limited to hand weeding or using specialized equipment like a high-residue cultivator.
Photo by Claire Keene

The summer cropping season is here, and, depending on where you are in the Carolinas, you may have planted your summer crops many weeks ago, or you may be just put them in the ground. For organic growers, the first 4-6 weeks after planting are the most important ones for beating the weeds: maintaining weed-free fields during this time gives your crop a chance to get established, canopy over, and stay ahead of the weeds throughout the cropping season. While this is sometimes easier said than done, weed control during this time is accomplished with weed-free planting conditions, robust transplants, rapid and uniform seedling emergence, and persistent weeding once the crop is up.

 

Cultivating weeds once the crop is up.  Photo by Claire Keene

Cultivating weeds once the crop is up.
Photo by Claire Keene

In tilled ground, this usually means a final weed-killing cultivation before planting followed by consistent between-row cultivation or hand-weeding while the crop establishes. In no-till ground, where cover crops are used to suppress weeds, weed control after the crop is planted is limited to hand weeding or using specialized equipment like a high-residue cultivator. You may not be able to keep your fields entirely weed-free during this time, but maintaining a substantial size difference between your crop and the weeds is the key to staying ahead of them.

 

The Weed Seed Bank

Once you’re beyond these critical weeks, your focus on weed control should turn to preventing weed seed production, and ultimately, to eliminating viable weed seeds from the soil (a.k.a. the weed seedbank). If you can handle preventing weeds from going to seed, then start thinking about how you can draw down the weed seedbank. There are two basic approaches to this: getting weed seeds to germinate and then killing them, or preventing germination and waiting for the seeds to die or be eaten by a variety of seed-eating animals.

Exposed soil allows more weeds to grow compared to covered soil, but options for weed control in no-till crops is limited once the crop is up. Photo by John Wallace

Exposed soil allows more weeds to grow compared to covered soil, but options for weed control in no-till crops is limited once the crop is up.
Photo by John Wallace

The first approach is usually accomplished with a false seedbed. This means preparing a seedbed about one month before planting, allowing weed seeds to germinate, then killing them by hand or with a field cultivator. The more often this can be done before you plant your crop the better, but you will inevitably be faced with soil conditions that are either too wet or too dry that delay false seedbedding, and that can make getting your crop planted on time more challenging.

 

Another way to do this is with a fall-planted winter crop that’s harvested before the weeds set seed the following summer. For example, in winter wheat the summer annual weeds that germinate in the spring don’t have enough time to flower and set seed before wheat harvest in June/July, which also effectively kills those weeds. While these techniques only kill a small percentage of the weed seedbank at a time (5-10%, which can be easily replaced by allowing weeds to set seed), they are one of many weed management tools to keep in your toolbox.

 

No-till soybeans.

No-till soybeans. Photo by John Wallace

Preventing germination is the other approach to drawing down the weed seedbank, and has the potential to be effective, but it is a long-term approach that takes advantage of ecological processes, and therefore, is linked to other farm management decisions. This is important to note because those decisions may seem simple (for example, when to till or which cover crops to use), but can affect the fate of weed seeds on your farm. Specifically, seeds left on the soil surface, (ie. those not turned under by tillage), are much more likely to die in a cold winter, or be eaten by a variety of insects, slugs, small rodents or seed-eating birds (a.k.a seed predators)1; and all but the birds require in-field habitat to keep their numbers high. That habitat is created by crop/cover crop residues left in the field and not tilled-in. Thus, by not tilling, seed predators have both habitat and a food source in the form of weed seeds. Generally, this means delaying tillage until spring, leaving crop residues in the field after harvest (e.g. leaving straw after grain harvest), planting into cover crop residue (winter-killed or roll-killed), or choosing a cover crop that decays slowly (e.g. cereals instead of more succulent legumes or brassicas). These practices have the added benefit of conserving moisture with crop/cover crop residue left on the soil surface2, providing habitat for beneficial insects that eat insect pests3, and potentially reducing pressure from soil-borne diseases through cropping diversity4.

 

Fall cover crops can suppress weed seed germination. Photo by John Wallace

Fall cover crops, such as ryegrass, can suppress weed seed germination. Photo by John Wallace

An additional technique for preventing weed seed germination is with a fall-planted crop or cover crop that shades the soil surface. The light that filters through the crop/cover crop canopy is often inadequate to induce germination, and is essentially tinted green – wavelengths in the visible light spectrum that tend to induce dormancy instead of germination in many seeds5, 6.

 

Another interesting approach to weed management is to aim for organic matter-building practices, such as soils rich in organic matter that may “buffer” the effect of weeds on crops. Research suggests that weeds affect crop yields less when organic matter is high and from diverse sources, as crops and weeds may use slightly different nutrient sources7. Thus, soil and weed management are inter-related, and targeting organic matter-building practices is good for both soil fertility and buffering the effect of weeds.

 

There are many approaches to weed and seedbank management in addition to tillage, and each management decision tends to relate to several others. Therefore, it’s important to take a step back every so often to look for opportunities, both small and large, to improve weed management, especially in ways that benefit other aspects of your operation. Remember, the practices discussed here that encourage weed control through ecology (e.g. seed predation and germination suppression) tend to have positive side effects such as building soil organic matter, conserving soil moisture, and potentially reducing insect and disease pressure. At the end of the day the goal is to find the practices that fit your operation.

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1. Schonbeck, M. Promoting weed seed predation and decay.

< http://articles.extension.org/pages/18544/promoting-weed-seed-predation-and-decay>

2. Keene, CL, Curran, WS. Unpublished data from research at Penn State.

3. Barbercheck, M, Rivers, A, Mullen, C. 2013. Epigeal insects in an organic cover crop-based reduced tillage cropping system. Conference paper presented at Entomological Society of America Annual Meeting, 2013.

4. Abdelsamad, N, Mbofung, GC, Robertson, AE, Liebman, M, Leandro, LF. 2012. Long-term crop rotations suppress soybean sudden death syndrome in Iowa. Research poster presented at American Phytopathological Society Annual Meeting, 2012.

5. Goggin, DE, Steadman, KJ, Powles, SB. 2008. Green and blue light photoreceptors are involved in maintenance of dormancy in imbibed annual ryegrass (Lolium rigidum) seeds. New Phytologist, 180: 81-89.

6. Anderson, RL. 2008. Weed seedling emergence and survival as affected by crop canopy. Weed Technology, 22(4): 736-740.

7. Smith, RG, Mortensen DA, Ryan MR. 2010. A new hypothesis for the functional role of diversity in mediating resource pools and weed–crop competition in agroecosystems. Weed Research, 50(1): 37-48.

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