Hemp Production, Purdue University
Types of Hemp: Industrial hemp is composed of three types: Oilseed, fiber, and hybrid (dual purpose), which produces both fiber and seed, but not to the yield or quality of single purpose cultivars.
Hemp Growth and Development: Hemp is a short day plant. Short-day plants develop flowers only when the day length is less than about 12 hours (August in many states). For best results, earlier planting will promote greater vegetative growth resulting in more robust plants for seed, and in taller plants with higher fiber yields. Days begin to get shorter after June 21st, and approximately four to five weeks after this date, vegetative growth slows and flower development is triggered.
Hemp is dioecious, meaning plants can be male or female. Differences between male and female plants in growth rate and development are large (Van der Werf and Van den Berg, 1995). Male plants tend to flower and senesce (die) earlier. To minimize the impact of this on production, many cultivars are bred to be monoecious, resulting in plants that are mostly females (a small percentage of males plants are included for pollination), to harvest more seed (since male plants do not set seed) and greater fiber productions (since male plants die after flowering).
Soil: One common myth is that hemp can be grown anywhere. Hemp grows best on a loose, well-aerated loam soil with high fertility and abundant organic matter, with a pH of 6.0 - 7.5. Well-drained or tiled clay soils can be used, but poorly-drained clay or poorly structured soils often results in establishment failures, as seedling and young plants are prone to damping-off. Sandy soils can grow good hemp with adequate irrigation and fertilization but these additional costs often makes production uneconomical.
Planting Date: Although the seedlings will germinate and survive at temperatures just above freezing, soil temperatures of 46° - 50°F are preferable. Generally hemp should be planted after danger of hard freezes. Good soil moisture is necessary for seed germination, and plenty of rainfall is needed for good growth, especially during the first six weeks, for establishment. Rainfall should be about 25 - 30 inches per year, but is most important in the first six weeks.
Planting Depth: Seedbed preparation requires considerable effort. Fall plowing is recommended, followed by careful preparation of a seedbed in the spring. The seedbed should be fine, level, and firm. Seed is best planted at 0.75 - 1.25 in (although deeper plantings will be tolerated, they are more susceptible to damping-off).
Plant Rate and Row Width. Seeding rate is specific to each variety, and this information should be sought from the supplier. Industrial hemp is normally planted using a standard grain drill. Both oil and fiber hemp is typically planted in 6 - 7 inch rows, using every run of the drill.
Oil Seed: As per other seed crops, plants are direct seeded at a rate of approximately 25 - 40 lb/acre. There are approximately 27,000 seeds per pound. Competition between plants is minimized to produce the highest quality seed and oil. For grain production, desired final plant population is around 10 - 15 plants/sq. ft.
Fiber: Fiber crops are planted at a rate of approximately 40 - 80 lbs per acre. This could be higher if germination is low or seeds are large. This high density is needed to produce higher quality primary bast fibers, as opposed to core fibers. The bast fiber content increases with plant density, but optimal densities have not yet been established in Indiana. Large quantities of hemp seeds must be planted to establish optimal crop density, and suppress weed competition. However, it is important to recognize that difficulties may also result from plant competition. Final stand density should be approximately 30 - 35 plants per square foot for fiber.
Hybrid: Densities for seed production for tall, European, dual-purpose cultivars are less than for short oilseed cultivars.
When hemp is planted on good draining, fertile loam with appropriate temperature and moisture conditions, seed will germinate quickly and reach 12 inches in 3 - 4 weeks from planting. At this stage it will give 90% ground shade and suppress the growth of weeds by shading. Rapidly growing hemp at a density of approximately 20 plants/sq ft, will suppress nearly all weed growth.
Fertility [Editor's note: Conventional Big Ag Perspective]: Another myth regarding hemp production is that it doesn’t require additional nitrogen or potash inputs: Hemp production requires inputs of up to 100 - 130 lb of nitrogen/acre, 45 - 70 lb/acre phosphorus, and 35 - 80 lb/acre of potash (to keep potassium levels in a medium to high range of >250 ppm). Hemp particularly requires good nitrogen fertilization, more so for seed production than fiber. Phosphorus levels should be medium to high (>40 ppm), sulfur good (>5,000 ppm), and calcium not in excess (<6,000 ppm). In addition to well aerated, loamy soils, hemp does best when organic matter greater than 3.5%. To provide perspective, Hemp requires about the same fertility inputs as a high-yielding crop of wheat or corn. Editor's note: crops grown in natural organic programs require less fertility inputs.
Hemp Rotations: Hemp can be successfully grown in continuous rotation for several years on the same land. However, the risk of pest buildup, particularly root worms, borers, and rots, makes this a risky proposition. Hemp could be used to diversify current rotations of bean, wheat, or alfalfa. Based upon reports from Ontario, Canada, it has been recommended that hemp not follow canola, edible beans, soybeans or sunflowers due to the risk of white mold and other pests and diseases.
Pest Management: Like most plants, hemp is prone to insects and pathogens. As the acreage of industrial hemp increases, the number of insect pests and pathogens will tend to increase, as well.
Disease: Historically, the fungal pathogens gray mold (Botrytis cinerea) and white mold (Sclerotinia sclerotiorum) have been reported to infect and impact industrial hemp production. In Indiana, white mold in particular, is expected to be a major pest north of highway 70, particularly when soybeans are grown in adjacent locales, or in rotation with hemp. Hemp is also prone to numerous fungal and bacterial leaf spots, viruses, and Pythium root rot and blight during establishment.
Insects: European corn borer, armyworm and grasshoppers have done some damage to hemp crops in North America.
No pesticides (insecticides, herbicides or fungicides) are registered for use on hemp in the United States. Editor's Note: Many natural organic pesticides such as PureGro Bug Stop are exempt and available for use. For now, crop rotation is the only management option available to avoid disease build-up until more is known about hemp’s susceptibility to disease organisms. A 4-year rotation is recommended. Do not grow hemp on the same fields following canola, edible beans, soybeans or sunflowers.
Baxter, J. 2000. Growing Industrial Hemp in Ontario. Agdex# 153/20. Available at http://www.omafra.gov.on.ca/english/crops/facts/00-067.htm#fertility
Van der Werf, H.M.G. and W. Van den Berg. 1995. Nitrogen fertilization and sex expression affect size variability of fibre hemp (Cannabis sativa L.) Oecologia, 103: 462–470
Small, E. and D. Marcus. 2002. Hemp: A new crop with new uses for North America. p. 284–326. In: J. Janick and A. Whipkey (eds.), Trends in new crops and new uses. ASHS Press, Alexandria, VA