Irrigation specialist outlines conditions for growing hemp

Two-thirds of the hemp planted in the United States is grown for CBD, cannabidiol, and agricultural techniques for the crop formed the focus of a virtual presentation during the 2021 World Ag Expo.

During the presentation, Kevin Stewart, sales director for the agriculture division of Jain Irrigation in North America, keyed on correct soil selection, how to improve organic matter in the soil, managing pH, implementing drip irrigation and other steps.

Stewart said hemp has "a pretty impressive root system" that can reach down as far as 6 feet. The plant itself grows from 6 to 15 feet high. He said it requires a lot of sunlight each day; if it gets below 10 hours a day, it seeks to grow from a vegetative state to flowering.

The talk opened by considering soil selection and the need for well-drained soil, a preference for loam soil and a pH level from 6.0 to 7.5. Heavy soil will cause issues, Stewart said, and so will damping off. He recommended a soil test and seeking an organic matter level above 3.5%.

The second step was tagged to planting dates. Hemp, he said, should be planted as early in the season as possible, right after the last killing frost in the spring. Some research suggests there is major yield reduction, up to 50%, with delayed planting into June.

"The goal for fiber is to get as much biomass for harvest as possible," Stewart said, adding that the growing period should be 100-120 days.

He said ideal seed depth is 0.5 to 1 inch. Ideal soil temperatures are between 46 and 50 degrees Fahrenheit.

The third point to consider is irrigation. Hemp water requirements vary between varieties, Stewart said, but most research indicated a water requirement between 20 and 30 inches for optimal growth to complete its life cycle.

The seedling/vegetative stage is critical for water management, he said.

"The first six weeks are very important to help develop the root system and get the stalk to 30 inches," Stewart said.

He said drip is the preferred irrigation method, and it can also provide fertigation; he said there are many fields with furrow and sprinkler irrigation, as well.

Stewart said it is important to select the right inside diameter of 3/5 inches or 7/8 inches and to determine the correct mil thickness, likely between 8 and 20 mil. It's also important to select the correct spacing, from 12 to 24 inches, and flow rate.

Another consideration is weed pressure. Stewart said limited herbicides are allowed for growing hemp for CBD, adding farmers spend $60,000 per season to hand-weed. He said drip irrigation can help reduce weed pressure by keeping water near the plant.

Plastic mulch is also used to help keep moisture in the soil and to warm it. It also serves as a physical barrier to weeds.

Another strategy to reduce weed pressure is to plant later in the season, Stewart said, which will risk lower yields but allow a treatment for weeds and before planting.

He recommended to get the plant to 12 inches in the first four weeks. This will provide 90% shade and prevent weeds from germinating within plants.

Regarding fertilizer, Stewart said hemp requires about the same amount as high-yield corn or wheat crops.

Nitrogen demand is about 100 to 130 pounds per acre, he said, noting that too much nitrogen creates poor fiber quality. Phosphorus demands are 45 to 70 pounds per acre, and potassium should be at 35 to 80 pounds per acre, Stewart said.

Hemp farmers should consider pH levels, he said.

"Take advantage of managing the pH in the soil and water by applying fertilizers and chemicals through the drip system," Stewart said. "The ideal pH range to target for the good hemp crop is 6.0 to 7.5. Small doses and frequent applications of acid or chlorine through the system can raise and lower the pH levels, helping sustain a desired range."

Regarding crop rotation, he called hemp a good choice for rotations but should be avoided following canola, edible beans, soybeans and sunflowers. He said there are issues with white mold following those crops, based on research with hemp in Canada.

Stewart said hemp can be grown back to back in a continuous cycle, but that poses the risk of insect and disease resistance. Common threats are root worm, borers, rot and mold.

Another consideration is keeping THC, tetrahydrocannabinol, below 0.3%. The amount of THC in the plant determines if it is hemp or marijuana: If it exceeds 0.3%, it is considered marijuana.

Factors that can cause hemp to exceed 0.3% or "go hot," he said, include environmental plant stress from drought, flooding or cold; perhaps too much nitrogen; higher elevation; cooler weather; and variety choice.

Stewart said there is no crop insurance for a crop that exceeds 0.3%. Growers can sample for levels by taking the top 3 to 5 inches for testing after the plant begins to flower.

The final consideration is harvest.

"The harvest is all about the flower," Stewart said. "Harvest when buds turn from white to an amber-milky color."

The harvest is mainly by hand and done with a machete. It's best to avoid the presence of mold and mildew, which will lower hemp floral biomass, Stewart said.

(Dennis Pollock is a reporter in Fresno. He may be contacted at agcompollock@yahoo.com.)