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  A Publication of UVM Extension's Vermont Vegetable and Berry Program

Managing Weed Seed Rain

by Vern Grubinger
Vegetable and Berry Specialist

Annual weeds are a challenge to manage on vegetable farms. Hairy galinsoga, lambsquarters, large crabgrass, and red-root pigweed are just a few of the many species that farmers commonly do battle with. Cultivation, crop rotation, and cover cropping are key strategies for managing annual weeds, and herbicides may also be used. All of these strategies are more effective if the size of the weed seed bank, which is the reservoir of viable weed seeds in the soil, is small. In other words, if you have a large weed seed bank and you kill 99% of the weeds, the 1% that’s left is still enough to create a lot of weed pressure.

Seed banks and seed rain. Just like with money, the way to reduce your weed seed bank account is to withdraw more than you deposit. Cultivation, herbicides, mulches, and mowing can all reduce the weed seed bank by allowing weeds to grow and then killing them and/or by keeping weeds from going to seed. When a weed goes to seed, it produces ‘seed rain’ which refers to seeds falling from the mother plant. Seed rain is the primary way that populations of annual weeds are maintained in farm fields, which makes sense given that most annual weed species can produce thousands, or tens of thousands, of seeds per plant, and these tend to fall to the ground in the nearby area.  

Stopping weed seed rain is an essential part of weed seed bank management. For example, you may kill a thousand weeds in a field but miss one weed that then produces ten thousand seeds. Your weed seed bank account in this spot just got bigger – even thought the field looked pretty clean. However, all is not lost.  The key point is to minimize these “deposits” or “credits” to the seed bank, and maximize the “withdrawals” or “debits.”

Seed rain and cover crops. Much of what I’ve learned about weed seed rain comes from the work of Dr. Eric Gallandt at the University of Maine. His research is helping growers develop better whole-farm weed management strategies. One of his recent research papers has a conclusion that really made me think; it reads: “Single-season cover cropping practices including three or more unique soil disturbance events resulted in a marked reduction in the germinable weed seedbank. Despite their apparent competitive ability, and likely benefits to soil quality, full season cover crops lacking soil disturbance may result in considerable weed seed rain and therefore an increasing weed problem in subsequent years.  While we do not discourage growers from considering these full-season cover crops, they must be monitored carefully so that they are terminated prior to production of viable weed seed.” (From: Exploiting weed management benefits of cover crops requires pre-emption of seed rain; by Eric Gallandt and Tom Molloy, University of Maine, see: http://orgprints.org/12296.)

In other words, while many people like me have advocated the use of cover crops to reduce weed pressure, if weeds are going to seed within those cover crops your weed seed bank is going in the wrong direction. So, my new advice is that timely cultivation (soil disturbance) before weeds can produce seeds is more important than cover cropping as a weed control strategy, although the two strategies can and should be combined in a way that prevents weed seed rain. On farms with lots of weed pressure (large weed seed banks) that means use of short-term cover crops interspersed with tillage events during the growing season. Of course, winter cover crops can still be left in place for many months to protect the soil and add organic matter - this is not the time that annual weeds are setting seeds (although perennial weeds like quackgrass may be gaining strength).

Here are some ideas on approaches to balancing soil health, plant health, and nutrient management with weed management.

Perennial cover crops. Alfalfa, red clover, grass sods left in place for one or more growing seasons are well know to contribute to improved soil quality on vegetable farms, and the legumes are an important source of ‘free’ nitrogen. However, such long-term cover crops may increase weed problems by preserving the seed bank of relatively persistent weeds and by allowing a lot of weed seed rain. Thus, perennial cover crops make the most sense in fields where strategies to reduce the weed seed bank have already been successfully implemented.

Post harvest management. After cash crops are harvested, the prompt incorporation of residues promotes their decomposition which can help reduce plant disease inoculum and allows for speedy sowing of a subsequent cover crop to protect the soil. However, if many weeds have gone to seed in the field, it may be more beneficial to leave the soil undisturbed in order to keep weed seeds on the soil surface to encourage predation by birds and weed-seed eating insects such as certain beetles. There is evidence that such predation can lead to significant reductions in the weed seeds that would otherwise be tilled in as a new deposit to the seed bank.

Timing tillage to germinate and kill weeds. Farmers are busy, weather is unpredictable, and equipment and labor are needed for lots of different tasks. As a result, cultivation of weeds sometimes happens when it can be squeezed in – often when the weeds are getting out of hand. To make progress in reducing weed seed banks, farmers may have to be more intentional about the timing of tillage in order to maximize weed seed germination so weeds can subsequent be killed by mechanical means. The article “Manipulating Weed Seed Banks to Promote their Decline” on the eOrganic web site (http://www.extension.org/article/18528) describes several tillage strategies to accomplish this:

Stale seedbeds. The soil is tilled to prepare for seeding the crop, then planting is delayed for two or three weeks to allow a flush of weeds to emerge. Just before sowing the crop, emerged weeds are killed with no or minimal soil disturbance. Organic farmers can kill weeds by flaming or cultivating as shallowly as practical, though any cultivation will stimulate some additional weeds to germinate with the crop. Conventional farmers normally use herbicides at the end of the stale seedbed period, an option that may become open to organic producers in the future with the development of natural-product postemergence herbicides that are economically viable at the field scale.

False seedbeds. Weeds emerging in response to tillage are killed by two or more additional shallow cultivations at weekly intervals. The crop is planted immediately after the final cultivation. Because small weed seeds germinate better when the soil is firmed to enhance seed–soil contact, rolling is recommended after all except the final cultivation.

Final cultivation before crop planting. This should be done as shallowly as practical to avoid stimulating further weed seed germination. Leave the soil surface loose and open, forming a dry, crumbly layer from which weed seeds are less able to take up moisture and germinate. Note that good soil tilth promoted by high organic matter and biological activity is essential for these tactics to work effectively. Light duty implements like flexible tine weeders cannot effectively penetrate crusty, cloddy or compacted soils, and stale seedbed can fail to yield weed management benefits in these conditions.

For more information on ecological approaches to managing weeds, see the eOrganic articles and video clips at: http://www.extension.org/article/19642 and the blog called Weed Management for Organic Farmers, practical research projects from the University of Maine Weed Ecology Group at http://gallandt.wordpress.com/.
Published: August 2010
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