Weed control can be one of the most challenging aspects of production for organic growers. While there are several different tools available for weed suppression, they have to be used with one another to work effectively.
One highly effective control measure for weed suppression is the use of cover crops. By competing with weeds for nutrients, inhibiting the growth of weeds through allelopathic compounds, and blocking out light that stimulates weed growth, cover crops are the backbone of weed management for organic growers.
Julia Gaskin, Sustainable Agriculture Coordinator at the University of Georgia, shares tips on how to best use them to keep weeds at bay.
Why Cover Crops?
In addition to their ability to suppress weeds, cover crops provide a variety of additional benefits to production including adding organic matter to the soil, supporting microbial populations, and nutrient cycling, to name just a few.
Cover crops are excellent at scavenging nutrients in the soil that might not have been utilized by your cash crop, thereby helping to prevent those nutrients from leaching out of the system. However, Gaskin explains that one of the most important things cover crops do is prevent erosion.
“Most of the fertility is in our topsoil, and with organic production, one of the drawbacks is that we need to use cultivation for weed management,” she says. “Destroying organic matter in the soil and leaving the soil bare can make it vulnerable to erosion, so cover cropping is a way for us to try to keep something growing on there for as much of the year as possible.”
How Do They Suppress Weeds?
Cover crops work in three key ways to help control weed populations, and the first way Gaskin mentions is through direct competition with weeds.
“Simply put, if you are growing a winter crop and plant your cover crop before winter weeds emerge, the cover crop will compete with the weeds and help keep them from growing,” she says.
The second way they function is to inhibit the germination of weed seed through allelopathic compounds. Gaskin notes that cereal rye and sudangrass sorghum hybrids, in particular, have compounds that are released as the cover crop breaks down. These compounds inhibit germination.
However, she warns these compounds also may inhibit germination of cash crop seeds, so it’s important to be aware of what you’re planting and when.
“Small-seeded crops, in particular, are vulnerable. For example, you probably would not want to direct seed lettuce after you have incorporated a sudangrass sorghum cover crop because of that allelopathy,” she says.
The third way cover crops control weeds is through blocking sunlight, which stimulates weed growth.
“For example, if you grow a winter cover crop, you could roll and crimp it down so it forms a thick mulch, and you would then transplant into it. That mulch is not only acting with the allelopathic compounds to inhibit weed seed germination, but it’s also block-ing light, which is a stimulant for many of these weed seeds to germinate,” Gaskin explains.
An Integrative Approach
Gaskin stresses that while planting cover crops is an effective strategy to control weeds, it must be done properly and in combination with other weed-control strategies for optimum results.
“You should use cover crops in combination with crop rotation so that you’re not letting the same window for the same weeds stay open every year,” she explains. “If you have several years of the same summer cash crop in the ground, you will start to see a buildup of weeds.”
She also suggests using drip irrigation so that you’re only watering the crop and not the plant beds where weeds tend to be a problem.
Proper Seedbed Management
When using cover crops, Gaskin stresses that the overall goal should be to avoid letting any weeds go to seed. Starting with a stale seedbed can aid in this process.
“If you can see baby weeds when you are seeding in your cover crop, they are going to out-compete the cover crop,” she says. “Also, I know people don’t normally irrigate cover crops, but if you’re not going to get rain, it helps to irrigate them enough to get a healthy stand.”
Gaskin also suggests planting with a high seeding rate to increase biomass, which will help increase the capacity of the cover crop to suppress weeds.
Because some cover crops, such as cereal rye or sudangrass sorghum, can have up to 10,000 pounds of biomass per acre, it is critical you have the proper equipment to be able to manage it.
According to Gaskin, questions to ask before you plant these high biomass cover crop include:
• How are you going to kill it?
• Will you roll and crimp it down?
• Do you have heavy enough equipment to do it all?
“If you’re doing really small-scale production and permanent beds, you probably don’t have the equipment to manage a really high biomass cover crop. In this case, I would recommend something like millet or a millet/soybean mix as an alternative,” she says.
While these crops may be easier to mow and incorporate, Gaskin says to keep in mind that they aren’t going to be as effective in controlling weeds, because the biomass in the field is directly correlated with weed suppression.
The Right Cover Crop
Choosing the appropriate cover crop for your circumstances will depend on a variety of factors. Gaskin says one of the best references for choosing cover crops is “Managing Cover Crops Profitably,” published by the USDA Sustainable Agriculture Research & Education program, which can be downloaded for free or purchased as a book at: https://is.gd/cover_crops.
While the book is a good place to start, Gaskin suggests talking to your county Extension specialist to further narrow down the specifics to suit your particular region.