Hemp Cultivation Through the Ages
Cultivators have tailored Cannabis sativa L. (hemp) for unique purposes since the dawn of recorded history. Even today, farmers and horticultural scientists are driving the plant in new directions.
Ancient China relied on hemp for textiles, food, and medicine as long as 10,000 years ago. Before the turn of the common era, China produced its first agricultural treatise on hemp cultivation, Si Sheng Zhi Shu. Other texts followed.  Ancient cultivation insights include:
- Sow immediately before or after rainfall
- Fertilize with silkworm excrement when the plant reaches 33 centimeters
- Soak seeds in rain water rather than well water
- Intercrop hemp with turnips or millet, but not soybeans
- Remove male plants after flowering (not before) to maximize seed production 
The word “canvas” comes from “cannabis” because, during the Middle Ages, sailors relied on hemp for sails. In 1535, Henry VIII mandated that all landowners must grow hemp.  Early Virginians were also obligated to grow hemp to help the colony survive. Colonists introduced Cannabis sativa L. to North America and mastered their own style of cultivation:
- Two fall-plows; two plows with harrows in Spring
- Seeds were hand scattered
- Tight patches and tall plants (14+ feet)
- Few branches and few flowers
- Plants were cut at 13 weeks and dried
The purpose here was to create strong fiber. Moving forward in time, many industrialists (such as Henry Ford) saw great potential in hemp. But the 1937 Marihuana Tax Act made all cannabis a federal crime pending registration and stiff payment. Harry J. Anslinger, Commissioner of Narcotics, scored this victory largely through anti-immigrant sentiment (Mexican) and propaganda. Hemp declined.
Then in World War II, the United States desperately needed hemp for its versatile industrial applications (esp. ship rigging). The government launched a campaign with a simple message: hemp for victory! (check out the original short film here). Farmers answered the call: in 1943 alone, they planted 176,000 acres. Hemp was planted in dense rows spaced about four inches apart. Slender stalks allowed for easy processing.
Socio-political maneuvering turned cannabis/hemp into a prohibited narcotic in 1970 with the Controlled Substances Act. This completely silenced public cultivation for 48 years. Finally, the United States passed the 2018 Farm Bill to re-legalize Cannabis sativa L. with less than 0.3% of tetrahydrocannabinol (THC). This non-psychoactive variety would be known as hemp.
- Well-drained soil with a pH of 6.0-7.0
- Avoid soil with heavy clay content
- Medium-tall plant genetics better compete with weeds
- Mechanical weed control required (no herbicide laws)
- ½ to ¾ inch planting depth
- Optimize nitrogen and phosphate with fertilizers
The University of Kentucky offers these insights:
- Use soil rich in organic matter
- ¼ inch seeding depth
- Avoid seeding when soil temperatures may drop below 46 °F
- Tall plants may suffer environmental stress that boosts THC content
- Fiber quality suffers as seeds ripen
Fiber plants are grown in tight patches to maximize height. If seeds and leaves are desired, plants will generally be spaced out to allow for branching.
Nonetheless, much of the demand on hemp now relates to therapeutic phytochemicals. Farmers mining for cannabinoids, namely cannabidiol (CBD), mimic a tactic long used for psychoactive cannabis: feminization of seeds/plants. Un-pollinated female plant flowers continue to produce cannabinoids and terpenes, maximizing desirable chemical profiles.
As Scientific American points out, “So far, it seems that coddling CBD hemp helps, but the best way to nurture the plants is unclear.” Farmers are learning through trial and error, much like psychoactive cannabis cultivators have done (in secret) for decades. Hemp breeders are even developing low-THC, high-resin genetics to meet the demand for potent full-spectrum CBD. Hemp cultivation may come to closely mirror psychoactive cannabis cultivation.
Humanity continues to experiment with and explore the horticultural potential of cannabis. In the grand scheme of hemp history, prohibition is a mere blip. Cannabis sativa L. is a powerful plant, and it won’t settle for less than victory.
- Liu, Fei-Hu, et al. “Ethnobotanical Research on Origin, Cultivation, Distribution, and Utilization of Hemp (Cannabis sativa L.) in China.” Indian Journal of Traditional Knowledge, vol.16, no.2, 2017, pp.235-242. Journal Impact Factor = 0.920, Times Cited = 2 (ResearchGate)
- Walton, P. “Textiles,” English Medieval Industries: Craftsmen, Techniques, Products. Edited by John Blair and Nigel Ramsay. Hambledon and London, 2001, p. 322. Google Books