Are You Covered?

Despite cover crops’ many benefits, encouraging farmers to integrate them can be a challenge, especially when considering the costs to plant additional crops rather than leave the field fallow. Cover crop economics is its own topic of discussion that should not be skimmed over.

Cover crop production costs can range from $14-$100 per acre, according to an Ohio State University Extension presentation on cover crop economics. (This presentation, along with a breakdown of various cover crop costs, is available at bit.ly/OSUCoverCrops.) For large-farm owners, this could seem to be too hefty of a price for some farmers to consider. However, both short-term and long-term benefits exist. They can far outweigh the cost of seeding a cover.

Benefits of Cover Cropping

Cover crops can bring both fiscal and environmental benefits to growers. The short-term benefits are easier to measure and may be the focus for the majority of growers. Some immediate gains will directly affect the next crop planted. These could include weed suppression, adding available nitrogen to the soil through nitrogen fixation, and scavenging nitrogen that would have been lost and then releasing it as the cover crop decomposes.

Cover crops also bring important long-term improvements to the field and environment, though they are more difficult to measure. These include protecting the soil from erosion, improving soil structure, and increasing organic matter, insect diversity and carbon sequestration.

The potential of government-issued carbon credits, as proposed by President Joe Biden's new administration, could increase the value of rotating with cover crops, as it provides farmers a financial incentive for regenerative practices. Planting a cover crop also naturally adds inputs growers may otherwise purchase, such as herbicides or nitrogen. Calculating how much would be spent on these inputs without the cover crops is one way to think about how these costs are offset.

Even with these benefits, growers should try to minimize factors that could drive up the cost of planting a cover. Consider the cost of seed; planting, managing or terminating the crop; and any inputs that may be necessary to get the crop established.

Popular Options

While research continues on the best cover crops for hemp, here are additional popular options farmers are investigating. (Two popular choices, and considerations for choosing a cover crop, were featured in part I of this series.)

White Clover (Trifolium repens)

White clover, sometimes called Dutch white clover, is in the family Fabaceae (legumes), like hairy vetch. It can prevent erosion and is nitrogen-fixing. This means the plant, like other legumes, hosts bacteria in its roots that help it extract nitrogen from the atmosphere—something most other plants cannot do.

White clover is generally planted in the spring or summer. It can be planted in the late summer or early fall, but it should be sowed 40 days before the first killing freeze. This crop is appealing to many cannabinoid hemp growers as a between-row cover. Rather than terminate the crop, a hemp grower can let the clover continuously grow throughout the season, mowing occasionally. It is a perennial, so it will keep coming back. It tends to be a lower-growing cover and provides floral resources for pollinators. In fact, white clover is a very popular choice for vegetable and fruit growers as a living mulch.

If farmers choose to terminate this crop, they will need to uproot it. It can be tilled under using a variety of tillage types, including a chisel plow or field cultivator. Herbicides may also be an option to terminate the cover if hemp is not currently planted in the field.

Growers who want to use this as a living cover have a few additional considerations, including how to manage the clover directly around the hemp plants. In a 2018 vegetable and living mulch study conducted at the University of Wisconsin-Madison, researchers found that having a cover crop growing in the immediate area around certain vegetable crops reduces yield. This could best be explained by competition between the primary crop and the cover for resources like water and nutrients. Rather than plant hemp directly into a field of clover, growers could create a bare-ground strip (strip tillage) in the clover where they want to plant. These strips are typically 6-12 inches wide, so this may work better for those who are direct-seeding. Growers could also incorporate clover into their raised-bed system by planting it in the area between beds. This would be ideal for cannabinoid producers who are transplanting seedlings or clones with several feet between each row.

Clover also poses the potential risk of harboring pests. One of the concerning insects that could move from white clover to hemp is the potato leafhopper (Empoasca fabae)—a pest that I discuss further in the January/February 2020 issue of Hemp Grower.

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Oilseed Radish (Raphanus sativus L.)

Oilseed radish, sometimes called daikon radish, is a member of the Brassicaceae family (mustards). This crop is planted from late summer to early fall after the primary crop's harvest. This could be a great option for hemp growers who harvest early. This radish can produce a huge taproot that grows as far down as 6 feet and up to 2 inches in diameter in the upper portion of the root. This characteristic makes oilseed radish a great option for compacted soils.

Another advantage of a massive taproot is the ability to scavenge nitrogen that leached deep into the soil. As the plants decompose, they release nitrogen, making it available to the next crop. Plants in the Brassicaceae family release a compound called glucosinolates into the soil. These root exudates have both disease- and nematode-suppressing properties. This cover crop’s biofumigant properties are promising, but more research is needed.

In northern climates, oilseed radish winter kill, meaning there is no need to follow up with a termination method. However, in southern areas or if the winter is mild, termination must occur before the plants produce seeds. Once seeds are mature, they run the risk of becoming a weed pest for subsequent years. One major disadvantage (in some people’s opinions) is the rotten-egg or sulfurous smell the radishes give off as they decompose.

Cover Crop Mixes

I would be remiss if I did not mention cover crop mixes. These are plant populations that will consist of two or more cover crops grown in the same area.

Deciding to plant a combination of species provides some important advantages. If one species isn’t as successful for weather-related reasons, growers won’t have a complete crop failure if the other species in the mix perform well. Growers may also be able to meet more of their cropping goals if they incorporate multiple species. However, this is a balancing act, as cropping goals may also be stifled due to competition between the different covers.

Cover crop mixes also increase the biodiversity in the field, which provides an enhanced habitat for beneficial organisms. Ultimately, deciding on a single cover crop species versus a mix of species is going to depend on goals, costs and suitability for the field.

There are numerous environmental and economic benefits of integrating cover crops into a hemp rotation. But with limited research focused on optimal rotations with hemp, growers will need to figure out through trial and error what does and does not fit well.

Marguerite Bolt is the hemp extension specialist at Purdue University’s Department of Agronomy. She received her M.S. in entomology from Purdue University and her B.S. in entomology from Michigan State University. Bolt’s research has focused on hemp-insect interactions and plant chemistry.