Editor's note: The following article is the second of a three-part series featuring the marketing and agronomic considerations involved in growing hemp for cannabidiol – CBD – production.
Farmers and researchers are working to find the best agronomic practices for growing hemp for cannabidiol products.
“After 70 plus years of prohibition everything is pretty new,” said Ashley Walsh, founder of Pocono Organics. “Lots of knowledge has been lost; we’re starting from scratch again.”
Prohibition also has applied to research so there’s scarce scientific data on the best cultivars, planting dates, spacing, nutrient requirements, pest management and other agronomic issues for Midwestern growers. When there is data it’s usually from states and countries where cannabis has been legal for a longer time. It’s unclear how the data will translate to climate and soils in the Midwest.
Hemp for cannabidiol products can either be transplanted or direct-seeded. Growers start with feminized seed or female transplants because female flowers produce significantly greater quantities of cannabinoids. But starting with feminized seed doesn’t guarantee there won’t be male flowers. Therefore growers should regularly scout fields to remove male flowers.
Many farmers have used a corn planter with a sorghum plate for direct seeding. Transplanting hemp is no different than transplanting other crops. Vegetable-production equipment such as water-wheel transplanters can aid in transplanting. Hemp is commonly transplanted into “plasticulture” or a living cover-crop mix that’s mowed throughout the season. Plasticulture involves growing plants in plastic-mulched drip-irrigated beds.
Spacing depends on various factors. Some data show yields per acre are greater with 1-foot by 1-foot spacing. But there doesn’t appear to be information comparing cost per acre and yields in different spacing situations. Many farmers plant cannabidiol hemp 4 to 6 square feet apart. Because even the cheapest feminized seed costs more than $1 per seed, farmers may use wider spacing to manage production costs. Wider spacing also increases airflow around plants to limit mold and other disease issues.
Patrick McHugh has a certified-organic farm near Onalaska, Wisconsin. He grew 40 acres of hemp in 2019 for cannabidiol. He started his first transplants mid-May, transplanting every week. He switched to direct-seeding at a tighter spacing later in the season. That helped stagger the workload for planting and harvesting. It also helped optimize limited space for drying the crop.
Most hemp varieties for cannabidiol production have been bred in the drier climates of Colorado, Oregon and California. It’s unclear what varieties will work best in the more-humid Midwest. Leah Sandler, education director and research agronomist at the Michael Fields Agricultural Institute, is evaluating which varieties work best in Wisconsin. And she's organizing a survey to learn which worked best for growers.
A common misconception is that hemp can grow anywhere. It should be grown on well-drained soils and fertility should be closely managed. Managing crop health is the main way farmers can grow hemp with good cannabidiol content and a reduced level of tetrahydrocannabinol. Cannabidiol hemp has nutrient requirements similar to corn, according to many farmers.
Some farmers and researchers recommend splitting fertilization between a pre-plant application and a July side-dress application. They apply between 125 and 200 pounds per acre of nitrogen. Sandler recommends applying 100 to 120 pounds per acre of nitrogen at planting, followed by about 50 pounds about one month later. The second application is timed to meet the plant’s increased nitrogen requirements for flowering.
Potassium uptake is at its peak at the start of flowering. The hemp plant needs about the same amount of potassium as nitrogen.
There are currently no herbicides approved for hemp. Weed management begins with field preparation. Cover crops – either tilled or roller-crimped – can help build soil and suppress weeds at the start of the growing season.
Spacing is a second major consideration. Wider spacing allows for mowing a living groundcover. Plasticulture allows for mowing or cultivating between rows, and hand weeding next to plants.
Some growers in drier climates have used straw mulch. But that may contribute to moisture-related disease issues in more-humid climates, Sandler said. Availability of labor and equipment are other key issues. Weed management will largely depend on one’s system.
Managing pests and diseases in any new crop can be challenging. Scouting is the first step to successful pest management. While there aren’t any pesticides approved by the U.S. Environmental Protection Agency for use on hemp, some state departments of agriculture recommend using 25(b) minimum-risk pesticides. Those pesticides are considered to “pose little to no risk to human health or the environment,” according to the EPA.
Powdery mildew and botrytis – gray mold – are two main diseases in hemp. They’re the result of moisture on leaves. Proper plant spacing and ventilation may help. Disease control is important because even a small amount of mold in hemp flowers can make the crop unsaleable.
The same organic pesticides used on powdery mildew in vegetable crops can be effective in hemp. Examples include products containing neem oil or potassium bicarbonate. Organic growers are advised to contact their organic certifier and their state’s 25(b) minimum-risk-pesticide list to ensure a product is allowed for use. Bacillus thuringiensis can be used against borers and worms.
To manage variegated cutworms in his hemp crop McHugh removed them by hand and placed them in a bucket of soapy water. His plan for managing pest pressure is to create optimal plant health, and to allow four to five years between plantings of hemp or other host crops.
The preceding article was originally published by the Midwest Organic and Sustainable Education Service's "Organic Broadcaster."