What to Think About When Using Cover Crops for Weed Control

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Cover crops can provide many benefits, including preventing soil erosion, improving soil structure, alleviating compaction, nutrient management, and weed control. The most successful approach with cover crops is to identify your goals, and then select the best cover crop(s) and actively manage the cover to reach the goal(s). When considering cover crops to help with weed control, consider a cover crop species that will not only produce a lot of biomass, but also produce biomass that does not breakdown quickly. Local research has shown that weed density can be reduced by more than 50% with a cover crop and cover crops can outcompete weeds for light resulting in small weeds that are easier to control with other tactics, including herbicides. Use of cover crops has shown promise for resistance management by helping to reduce selection pressure when herbicides are used. Most cover crops in Delaware are planted in the fall and terminated in the spring, so they have the potential to affect both winter and summer annual weeds.

For fall planted cover crops, cereal rye or triticale are the best two species for weed control from our research (annual ryegrass also has many similar attributes, but it can be difficult to kill in the spring). They both shade the ground quickly in the fall because they emerge fast and grow late into the fall, and they grow more prostrate than other grass species to provide ground cover very soon after emergence. Legume cover crops are not as effective for winter annual weed control because they are slow to establish and often do not produce significant amount of biomass until the spring. In addition, legume biomass tend to breakdown much quicker than winter cereals, reducing their ability to suppress late emerging summer annual weeds. Winter sensitive cover crops, such as forage radish, produce a significant amount of biomass in the fall, and are a good option for control of winter annuals such as henbit or chickweed. However, the biomass breaks down quickly in the early spring and they provide very little suppress of summer annual weeds.

Both have cereal rye and triticale have a wide window for planting and quite adaptable to a range of soil conditions. Forage radish has a very short window for successful planting, while legumes have a wider window, though is not as wide as cereal species. Getting a uniform stand across the field is critical for maximum weed control or suppression. Where the cover crop stand is not uniform, the weeds are not subjected to the plant competition, the weeds are more numerous and larger, and can be harder to control than those plants growing amongst the cover crop. Achieving this uniform stand is best with a drill due to the improved soil to seed contact. Other seeding methods can sometimes provide good stands, but the results have not been consistent. Many farmers are compensating for the challenge of achieving a good stand by increasing their seeding rates compared with rates used when drilling.

Most of the local research has used 2 bu/A of cereal rye (or triticale) because that was the seeding rate for cost share programs, but experience tells us we can reduce that seeding rate without sacrificing weed control. We do not have enough local research to say how low we can plant rye and still get the same level of weed suppression. We are asking that question with ongoing research.


Herbicide carryover: what herbicides were used throughout the summer (and when)? Is there a risk that the herbicide can slow the growth or reduce the stand of the selected cover crop species? Forage radish and legumes tend to be more sensitive to herbicide carryover than the cereal species.

Cover crop termination (when): the maximum benefit of the cover crop for weed control occurs when the cover crop is terminated as late as possible. Terminating the cover crop at or close to flowering/heading time allows for the high levels of biomass, but also allows for the maximum production of lignified tissue (which is the tissue most resistant to breaking down). Terminating the cover crop more than 2 weeks before planting lessens the weed control benefits. Earlier termination of the cover crop results in less biomass production, less lignified tissue produced, and longer opportunity for the cover crop to breakdown (and less weed suppress after planting). UD Weed Science is examining the weed control benefits of planting green, and we have some promising results, but more work is needed.

Cover crop termination (how): ask yourself how you will terminate the cover crop in the spring. It is best to know how difficult it might be to kill the cover crop; what herbicides are needed to control the cover crop; and will the herbicides used to kill the cover crop interfere with planting your spring crop. If planting a cover crop mixture, consider how you will kill all the species. Do not plant something that you cannot successfully terminate in the spring. Annual ryegrass is one of the species that can be challenging to kill in the spring.

Is an herbicide needed at planting: Are weeds present when you are planting your cover crop, how big are they? If winter annual weeds have emerged before you plant your cover crop, you may see benefits from a burndown herbicide. Winter annual weeds emerging after planting are seldom a problem with a competitive stand of cereal rye or triticale, but weeds up before planting could compete with the cover crop and limit it from reaching its full biomass potential.

Fertility levels: some cover crop species’ growth can be reduced if there are low levels of soil nutrients (including micronutrients). Tillage radish is more sensitive to fertility levels than some of the cereal cover crop species.

Multi-species mixtures: using more than one species is a great strategy for some soil health objectives. However, as more species are added to the mixture (and the amount of any one species is reduced) the benefits for weed control could be reduced. Terminating a multi-species cover crop could be difficult.

In the early spring newsletter we will discuss related topics to using cover crops for weed control, including challenges:

  • Cover crop/cash crop competition (moisture/light/nutrients)
    ● Volunteer cover crops
    ● Pests (slugs)
    ● Additional expense / management
    ● Establishment a good crop stand