Cover crops have been in the political spotlight of late (with President-elect Joe Biden vowing in his campaign that the government would pay farmers to plant carbon-sequestering cover crops), but growing them can be novel territory for some farmers. Planted before and after primary crops, cover crops can benefit hemp cultivation in several ways, from preventing erosion and increasing soil aeration to suppressing weeds and providing essential nutrients. However, some growers are not yet planting them, in part because they are uncertain which cover crops to choose.
Researchers have recognized the need to develop proven guidance on cover crops and are studying hemp with different cover crop rotations. Growers are also conducting their own on-farm research on which cover crops work best with their hemp—in rotation or as a living cover.
Part I of this two-part series spells out considerations for deciding which cover crop is best for your farm and explores two popular options. (Part II will explore other cover crop choices.)
Choosing a Cover Crop
Growers should weigh several factors when choosing a cover crop:
- The cover crops should work in their regions with their soils;
- The cover crops should have a low risk of disease or insect movement from cover crop to hemp;
- The crops should meet the growers’ specific goals, like weed suppression or nutrient building;
- The crops should fit into the growers’ budgets (cover crop seeds aren’t always cheap); and
- They should be readily available (shortages of certain crop seeds can occur).
Cover crop choice and usage will vary depending on a farmer's production model. For example, a grower using a transplant model may opt to have a living cover between or within rows of hemp. The grower may need to manage weeds around the plants to reduce competition between the hemp and the cover crop. A grower that is direct seeding with tight row spacing is likely going to plant a cover and then terminate it before seeding their hemp.
Research continues on the best cover crops for hemp, but two options are popular among agricultural producers—rye grass and hairy vetch.
Rye Grass (Secale cereale)
This cover crop is planted in the fall after harvest and will continue to grow the following year. Rye grass is a popular cover crop option because it is inexpensive and has some great benefits for the soil. It can help prevent erosion and increase overall soil structure. It can also suppress weeds and increase organic matter in the soil.
A hemp grower will want to plant a rye cover in the early fall for ideal establishment, but the time frame will differ depending on the region. Rye grass would be a good option for hemp growers harvesting later in the fall because it can handle colder weather. This crop will continue to grow in the spring, so timing on killing the crop is important.
There are a few ways to terminate rye.
Tilling the crop under is one method of termination, but it must be done when plants reach, at most, about 18-20 inches in height. Rye can have allelopathic properties, meaning it can release chemicals into the soil to suppress the growth of other plants. Because it has this ability, growers should not plant their hemp immediately after killing the rye. When growing corn, it is recommended that the corn is planted a minimum of 10 days after the rye is killed, but we do not yet have an ideal timeline for when to plant hemp after a rye cover crop. Hemp seeds are small and do not have strong seedling vigor, so the time between terminating rye and planting hemp may be longer than corn.
Mowing is another option for termination; growers may then bale or remove the grass from the field.
Using a roller crimper is another method. This option, which is more technical and more common in the organic growing community, crushes the plant’s stem and kills it if timed during late flower.
Herbicides are another way to kill a rye crop, but growers should make sure there are not any concerns that the herbicide will carry over to the hemp crop. Certain herbicide residues could prevent or delay germination and cause damage to growing hemp plants. While we do not have much research on herbicide carryover in hemp, we can use the herbicide label to guide us. Hemp falls into the “other crops” category on a herbicide label, which can be referenced for the rotation restrictions. Other factors contribute to how long an herbicide can persist, including rainfall and soil type. A grower should understand the label and the environmental factors that could contribute to how long residues may last in the field. Farmers should also check their state’s pesticide office to determine if they can apply herbicides before planting their hemp in the spring. Some states don't allow pesticide applications in a field where a grower intends to plant hemp. Each state has its own pesticide rules, guidelines, and pesticide label interpretations, so make sure to check.
Hairy Vetch (Vicia villosa)
Hairy vetch is another cover crop planted in fall. Early fall planting will result in better establishment. This cover crop has many benefits, one of the largest being the supply of nitrogen this legume provides. Legumes, including vetch, have the ability to fix nitrogen and make it available to subsequent crops. Vetch can also suppress weeds and help with erosion control, but a thick stand must be achieved to reap these benefits. Vetch, if allowed to flower, will be visited by honeybees and other pollinators.
This crop can be tilled under using a variety of tillage types. Strip tillage, or keeping the vetch between rows as a mulch, can provide additional advantages, like providing a habitat for beneficial insects. However, there are diseases that can infect both vetch and hemp, including downy mildew and gray mold. This could be especially problematic for hemp growers that have a vetch living cover between their rows of hemp. Other termination methods include roller crimping and herbicide application.
Purdue University and Rodale Institute are currently researching both rye grass and hairy vetch as a cover crop planted before hemp. The research is funded in part by a USDA Organic Agriculture Research and Extension Initiative Grant.
Once growers weigh these considerations, they should spend some time digging into the details of how to successfully incorporate cover crops. It’s also important to note cover crops are just one facet of a crop management plan; growers still need to assess and control weeds, diseases and insect pests.