A brief look at the varieties of industrial hemp and their uses
by Anne Delling
As we know, hemp has thousands of uses making it a truly super plant. While a single plant can provide seed, oil, fiber and hurd, specific cultivars exist for each of these end products. It would be ideal if the stalk from grain/oil varieties could still be utilized for hurd or fiber, even if the output is much less. The “waste” can even be used for animal bedding or fuel for pellet stoves.
An important distinction to note is the meaning of varieties and cultivars. An easy way to differentiate is to remember that cultivar means “cultivated varieties” – meaning they have been cultivated by humans to enhance specific qualities of the plant. Varieties are natural plant variations.
There are 2 main varieties, Dioecious, which have male and female flower parts on separate plants and Monoecious, which have male and female flower parts on the same plant and are a result of breeding efforts in France, Poland, Ukraine and Romania for high seed/grain production. There is also a dioecious variety known as Female Predominant that has 85%-90% female plants. In order to be legally grown as hemp (where it is legal to grow), the THC content must be less that 0.3% under normal conditions. Fiber cultivars mature in 60-90 days and grain in 110-150 days.
There are several cultivars from France and Romania that are considered dual-purpose. These are tall varieties and they can provide both grain and fiber from the same plant. They do present some harvesting challenges in that the growers need to know the intended use of their crop. If using for grain (30-40 days longer maturation), in some areas the weather may not be as conducive for retting and drying of the stalks for fiber. Retting is the process of soaking the stalks to help facilitate the separation of the fiber from hurd.
Hemp for Fiber (Bast)
It is estimated that air-dry stalk yields are 1-5.5 tons/acre, depending on conditions.
3-4 tons of good quality, dry retted straw can be decorticated (separated) into approximately 1 ton of bast fiber and 2-3 tons of hurd. The fiber yield is dependent on both the ton/acre yield, as well as the fiber content of the stalk (see illustration below). Dioecious varieties from southern Europe give the highest stalk yields.
For finer textiles, the hemp should be harvested early in the flowering stage, before the seeds have set. Hemp fiber that is cut after seed harvesting will have lignified (becomes woody) and can only be used in some non-woven applications.
Hemp for Hurd (Biomass)
The short, fine fibers of the soft inner core of the hemp stalk are called hurd and can be up to 85% cellulose. The high cellulose content makes hurd very absorbent with great thermal and acoustic properties. It can be used as highly absorbent animal bedding, a green building material (hempcrete, hemp-lime), insulation, biodegradable garden mulch, paper, lightweight board, acoustical ceiling and wall tiles, and in bio-ethanol fuel. Hemp hurds can also be used to make hemp plastic, as cellulose is the basic building block of plastic. Hemp hurds can be processed into cellophane packing material, or they may be manufactured into a low-cost, compostable replacement for Styrofoam.
How the hemp is planted will make a big difference in the bast/hurd ratio and will depend on the grower’s intended use.
Hemp for Grain/Oil
The Grain (seeds) yield is affected by a number of factors including climate, soil type and nutrients, the date sowed and seeding rate and harvesting time. Aside from the environmental factors, there are important genetic differences in the hemp varieties used. Monoecious varieties often yield up to 70% more grain than Dioecious types. These cultivars tend to be shorter and bushier.
If you are using your grain for oil (a rich composition of 80% essential fatty acids), which is the main by-product of hemp seed, the proportion of extractable oil will be an important factor to consider. If you are using your seed for hemp hearts or nuts (de-hulled hemp seeds) the size of the seed will be important as the larger the seed, the easier it is to de-hull and the greater the yield.
Industrial hemp seed is harvested when the seed begins to shatter. At this optimum harvest time, about 70% of the seeds are ripe and about 22-30% moisture.
Hemp for Paper
Most of the modern paper we use is a `chemical pulp’ paper made from trees. Hemp pulp paper can be made without chemicals from the hemp hurd. High-strength fiber paper can be made from the hemp bast, also without chemicals. Most hemp paper made today uses the entire hemp stalk, bast and hurd.
Hemp for Bio-fuel
This fuel additive is derived from cellulosic biomass and hemp hurd is an excellent source of high quality cellulosic biomass. You could also use the oil from the pressed hemp seed, but that oil has more useful purposes on it’s own.
The type of hemp planted and the growing methods used both play a very important roles in how the hemp can be utilized. It is necessary to know ahead of time your intended use, whether you are a grower, a processor or in search of a specific product.