With the recent legalization of industrial hemp, there are suddenly thousands of opportunities and very little information to help us along the way. We have been working with the University of Kentucky and Murray State University’s Agriculture Programs on researching best growing practices and general hemp agronomy. We still have a way to go but in the mean time, this is a quick overview of growing industrial hemp.
General Industrial Hemp Crop
Industrial hemp is made up of varieties of cannabis sativa that contain less than 0.3% THC. It is an annual broadleaf plant with a taproot and can grow from seed to harvest in 120 days. The female flowers and seed set are indeterminate, meaning that the seeds continue to develop over an extended period.
Our fiber crop can grow 7 to 15 feet without branching. In dense plantings, the bottom leaves atrophy due to the exclusion of sunlight. The male plants die off after shedding pollen, leaving room for the females to grow. Inside the outer bark of the stem is the long, tough bast fibers. Similar in length to soft wood fibers, bast fibers are also very low in lignin content. These bast fibers give hemp the strength and durability advantages that it is known for. The core, or shive, contains the hurds, or short fibers (similar to hard wood fibers) that are useful in other applications like coreboard, composites, absorbents, or horse bedding.
Growing Industrial Hemp
Let’s start with the soil. Hemp grows very well in a well-drained, low-clay soil. Clay soils are easily compacted, and hemp is sensitive to soil compaction. The preferred pH of the soil is 7.0 to 7.5, but a pH over 6.0 is fine. Like most crops, young hemp is also sensitive to wet soil until it has grown about a foot off the ground. Having said that, there’s no need for extreme fertilization or soil amendment processes and tilling is equally low maintenance; with the option to pursue vertical tilling, minimal tilling, or even no tilling.
Once the soil preparation process is complete, a basic broadcast spreader, cultipacker, or even a modern-day air drill can be used to spread the seeds. It is best to plant the seeds early, as hemp is daylight sensitive and grows better with longer days. The best soil temperature at an inch or two deep for fast germination is about 45 to 50 degrees Fahrenheit, though the seed will germinate around 40 degrees.
As I mentioned before, hemp is best planted around the middle of Spring. We tell our Kentucky farmers that we like to get seeds in the ground beginning of May, but the optimal date would depend on region. Vegetative growth begins to slow about a month after the summer solstice, so the point is to give the plant ample time to reach its growth potential.
Harvesting, Retting, and Sending
Let’s fast forward a few months to harvest. Our industrial hemp is ready to harvest when the plant is finished producing pollen and seeds begin to develop. Beyond this point, the fiber can become tougher and coarser. The classic sickle bar mower is the best way to cut the stalks for retting, though we have heard of farmers getting away with hay swathers. Over the last three years of growing hemp in Kentucky, disk mowers have been proven ineffective for cutting industrial hemp. When attempting to cut the industrial hemp stalks with those mowers, the strong green hemp fibers wrap around anything that is twisting and turning, locking up the equipment and causing hours of downtime for equipment cleanout. The morale here: our farmers prefer sickle bar mowers and we suggest you keep the blades sharp.
Once cut, a rotary rake is needed for windrowing, and field retting can be used for the curing process. Retting is a microbial process that separates the bast fibers from the hurds and other plant tissues. We generally allow the farmers to decide which retting method they prefer. After that, the crop can be bailed in large, round or square bales and sent to us.