Most fiber-producing varieties of hemp require a frost-free growing season of 5 months or longer to produce seed and approximately 4 months for fiber production. Hemp will endure light frosts in the spring and survive frosts in the fall better than corn. It grows best when well supplied with moisture throughout its growling season and especially in its early stages of growth. Drought conditions, if accompanied by high temperatures, appear to hasten maturity before the plants are fully grown.
The vegetative growth of hemp should be uniform. This growth is noticeably affected if the soil is flooded or saturated with moisture for too long a period. The leaves turn yellow, and the plants die. Rainfall, well distributed during the growing season, is, therefore, desirable for uniform vegetative growth. Hemp should be planted only on well-drained soils and not on flat, heavy, impervious soils.
Climate is important not only in the growth of the plant but also in the preparation of the crop after harvest. It influences the method used in handling the crop and the labor requirements, which determine the cost of production. In the United States the common practice (known as dew retting) is to cut the crop and let it lie on the ground. Exposure to the weather causes the fiber in the outer part of the stem to separate. Light snows and alternate freezing and thawing seem to improve or make the retting more uniform.
How to grow it
Hemp should not be grown on poor soils. To obtain good yields and fiber of high-quality, it is necessary to have a growth of uniform stalks 6 to 8 feet long. Short stalks, from poor nonfertile soils, seldom produce a high-quality fiber.
Fiber hemp grows successfully on soils of the Clarion, Tama, Carrington, Maury, Hagerstown, and Miami series, which, in general, are deep, medium-heavy loams, well-drained, and high in organic matter. Artificially drained areas of the Webster, Brookston, and Maumee series also give satisfactory yields. These soils are among the most productive soils of the Corn Belt. They produce average yields of 50 to 70 bushels or more of corn per acre. If land will not produce from 50 to 70 bushels of corn per acre, it should not be planted to hemp for fiber production.
Muck or peat soils are not recommended for the production of high-quality hemp fiber. The quantity of fiber produced per acre on these soils may be very high, but experience has demonstrated that the fiber lacks strength, which is the first requirement of hemp fiber for good cordage.
The inexperienced farmer usually gets advice from an experienced hemp-mill superintendent in the selection of the right soil. In fact, the farmer's contract to grow hemp usually specifies the exact field that it has been mutually agreed should be used for the hemp crop. This type of supervision by the company contracting for hemp has helped to prevent many crop failures.
Hemp should not be grown continuously on the same soil, for the same reasons that many other crops are not adapted to such practices. In Wisconsin, fields previously used for a cultivated crop are selected for hemp planting in preference to ones upon which small grains have been grown. In Kentucky, bluegrass sod, if obtainable, is selected. Old pastures plowed up are well suited for hemp culture. Fields previously cropped to soybeans, alfalfa, and clover are excellent for hemp. A good rotation is to follow corn with hemp, and in Kentucky a fall cereal may follow the hemp.