Syam Dodla, Ron Strahan and Peters Egbedi
Recent legalization of industrial hemp (Cannabis sativa L.) cultivation has increased interest among many producers who want to know more about its cultivation and opportunities. Industrial hemp belongs to the same plant family and species as marijuana, with the primary difference being that the concentration of delta 9-tetrahydrocannabinol (THC) is no more than 0.3% on a dry weight basis. Industrial hemp is grown for seed and biomass as well as its cellulose-rich bast fibers, which are strong, soft fibers from the phloem. It is an annual crop with fast growth and high fiber yield. Hemp fibers, which have low density and high strength and stiffness, help in alleviating environmental pollution by replacing synthetic fibers. Hemp is a dioecious plant with either a male or female reproductive system. When the primary goal of production is fiber, male-flowered plants are desirable because of their higher biomass and early senescence.
Field Selection and Preparation
Industrial hemp can be grown on several soil types with marginal to good soil fertility. However, hemp prefers sandy loam to loamy soils with deeper soil, good drainage and a pH between 6.0 to 7.5. Hemp is sensitive to both drought and waterlogging. Hence, avoid extremely light or heavy textured soils such as sandy and clay soils. Soil compaction greatly influences hemp seed germination and initial root growth; therefore, soil tillage is desirable to avoid soil compaction as well as minimize weed pressure.
Optimal Planting Period
In Louisiana and the Midsouth of the U.S., the optimum planting period for industrial hemp is from mid-to-late March to the end of April once the soil temperatures reach 50 degrees Fahrenheit. Later planting until June is still feasible. However, yields tend to be appreciably lower because of the drought sensitivity of early growth stages and high temperatures that cause plant stand failure. Also, late planting may require irrigation to avoid drought stress, early flowering, and enrichment of THC and to achieve good crop growth. Along with drought, late planting can lead to higher weed pressures, especially pigweed (Amaranthus sp.).
Fertilization
Currently, there is limited research-based information available in the United States on industrial hemp nutrient needs and fertilizer recommendations. Well-maintained soil fertility is critical for maximum fiber yields. For fiber production, most studies suggest an application of 50 to 100 pounds per acre of nitrogen depending on residual soil nitrogen and organic matter. Phosphorus and potassium should be applied as needed based on soil test results, similar to winter wheat. Excessive nitrogen applications are reported to yield similar fiber yields but often cause self-thinning due to competition for sunlight. Further, excessive nitrogen was reported to cause weaker fiber due to fast cell elongation that leads to thinner cell walls. Application of all the required nutrients, including nitrogen, closer to planting date followed by soil incorporation is most desirable. Studies that evaluated split nitrogen applications did not find any yield benefit over a single pre-plant fertilizer application. Also, narrow row spacing and taller plants make it difficult to carry out post-emergence fertilizer application.
Seeding Rate and Depth
For fiber production, optimum seeding rate ranges from 35 to 60 pounds per acre to achieve little over 1 million plants per acre. This high plant density is critical to improve the quality of fiber. The most effective way to plant hemp seed is using a seed drill with row spacing of approximately 8-inches. The grain drill setting for industrial hemp will be similar to that of grain sorghum. However, it is recommended to calibrate the seed drill before planting. The optimum sowing depth is 0.5 to 1 inch with a greater sowing depth (1 inch) recommended only under drier conditions to facilitate better moisture availability.
Weed Management
Weed management is one of the major concerns in hemp production due to the lack of labeled herbicides. Current research by Virginia Tech showed S-metolachlor (Dual II Magnum, for example) works very well as a pre-emergence herbicide with minimal effect on hemp germination and emergence. A study conducted at the LSU AgCenter Red River Research Station also had similar findings with S- metolachlor applied at 1.5 pints per acre. The same study from Virginia Tech also determined that sethoxydim (1.5 pints per acre), quizalofop (10 fluid ounces per acre), bromoxynil (1 pint per acre) and clopyralid (0.33 pint per acre) may be useful as post-emergence herbicides in hemp with minimal damage.
Disease and Insect Control
At present, there are no labeled synthetic fungicides available for use in industrial hemp grown in the United States. For diseases, it is important to choose fields that are well drained and have no prior history of disease. Hemp has been reported to be susceptible to fusarium, southern blight, phytophthora and pythium; therefore, avoid any hemp crop rotations to fields with potential for these diseases. Hemp grown in Louisiana is highly susceptible to damage from insects. However, no synthetic insecticides are labeled. and only a handful of organic options are available.
Harvesting
Optimum time for harvesting hemp for fiber production is when the initial sign of flowering is observed. Delayed harvesting causes a decrease in fiber quality as plants transform to support flowering and eventual seed development. Hemp takes about 90 days from planting to reach harvesting for fiber. However, if the plant stand is lower, earlier harvesting is required to avoid excessive stem diameter, which decreases the quality of fiber. Sickle bar mowers work best for harvesting industrial hemp. Cutting the plants 4 to 6 inches above the ground level will help in avoiding the coarse stump. The retting process, which is the microbial activity that breaks down the pectin layer between the fibers, can be conducted in the field prior to bailing.
Syam Dodla is an associate professor and agronomist at the LSU AgCenter Red River Research Station in Bossier City. Ron Strahan is a weed scientist and the AgCenter Northwest Region director. Peters Egbedi is a postdoctoral researcher at the LSU AgCenter Red River Research Station.
This article appears in the fall 2023 edition of Louisiana Agiculture.
A hemp farmer holds a stalk from a hemp plant and shows the soft bast fiber that can be used for many textiles. Photo by Kyle Peveto