Hemp is traditionally known as a fiber plant and most historical cultivation of the plant in the United States from the 17th to mid-20th centuries was with fiber use in mind.
Two kinds of fibers are derived from the hemp plant’s stalk. These are long (bast) fibers and the short (core fibers). The long, strong bast fibers are similar in length to soft wood fibers and are very low in lignin content (lignin is the “glue” that holds plants together). The short core fibers are more similar to hard wood fibers.
When grown as a fiber crop, hemp grows to a height of 6-12 feet without branching. Dense plantings (as many as 300 plants per square yard) help ensure that the plant grows straight. An ideal sized fiber plant has the same diameter as a #2 pencil (about ¼ inch or 6 mm). Male plants die after shedding pollen, but fiber crops are usually harvested before or during flowering.
Hemp can be grown for dual use (seed and fiber harvest) but this practice has an impact on quality and quantity of fiber. A dedicated fiber crop yields the highest quality bast fiber for textiles and composites.
Hemp Fiber Uses
Hemp fiber has many qualities including strength, durability and absorbency that make it very desirable to use in a wide range of products. Not all fibers are created equal — given their differing physical properties, bast and core fibers have different ideal end uses.
The economics of using hemp fibers in many products are a subject of ongoing debate, research and development, and business analysis. While the uses of hemp are manifold, bringing these products to market at a price that customers are willing to pay can be rather challenging. Hence, not all possible products may be readily available.
A long standing use for bast fibers are their use in textiles. Bast fibers can be cleaned, spun and then woven or knitted into many fabrics suitable for durable and comfortable clothing and housewares. Hemp fibers also can be blended with other fibers, such as cotton and linen, for specific textures and performance. Well-crafted hemp textiles are durable, breathability and have strong thermal qualities, as well as being mildew-resistant and hypoallergenic.
Rope and Twine
Cordage is an age old use for hemp fiber. While its use in the marine world has largely being replaced by cheaper, long-lasting and lighter synthetics, hemp rope still has its uses. Some people prefer hemp rope’s coarser texture as the rope can bind against itself for better knot stability. It some situations, this is very useful.
Hemp twine performs very well for beading, macramé, and other crafting projects because it makes attractive and firm knots. It is also great for gardening and landscaping for unlike many other fibers, hemp is round by nature, so as a twine it is easy to work with and does not cut plants or the hands tending them. Importantly, for those of us with green thumbs, is also fully biodegradable.
Hemp yarn is smooth, consistent and very strong. Its softness makes it both easy to work with and very comfortable when used for crafted jewelry that rubs against the skin.
Hemp’s long bast fibers are ideal for pulping into high quality pulp. Due to their tensile strength, they are good for such high end specialized paper products ranging from tea bags, currency paper, cigarette papers or speciality filters.
Core and whole stalk can also be used to make lower end paper products, depending on available pulping technology that is tooled to process hemp efficiently.
Similar to textiles, hemp fibers can be used as blends with other pulp fibers such as wheat straw or flax or even recycled wood, in order to increase paper performance, strength and recyclability.
Hemp core fibers have been successfully formed into medium density fiberboard (MDF), hemp building blocks, and hemp cement or hemp concrete. “Hempcrete” is drawing considerable attention among do-it-yourselfers for its thermal properties, low cost and ease of use. In some places, hemp bales have been used for straw bale style building. Hemp building materials also trap CO2 (carbon dioxide), making their use very attractive from an atmospheric perspective.
Harvesting & Processing Hemp Fiber
Harvesting hemp fiber is a bit of a tricky business. The fibers can wrap among a harvester’s moving parts, leading to mechanical failures, so machinery has to be adapted to deal with the plant. As well, harvester knifes and blades must be kept sharp and in good condition in order to cut through the hemp stalk.
Once cut, hemp fiber is usually left to be field retted — retting is a controlled process of decomposition that begins the separation of the bast and core fibers. After retting the fiber is baled and transported to a processing facility where its put through a mechanical process called decortication. Decortication machinery consists of a series of crushers and rollers that separate bast fiber from core fiber, then each fiber are further cleaned, combed and graded.
About one ton of bast fiber and 2-3 tons of core material can be separated from 3-4 tonnes of good quality, dry retted straw. However, fiber yields are very dependent on variety, growing regime, harvest practices and processing equipment.
Research into methods that produce cotton-like fiber are advancing. This “cottonized” fiber will especially be well suited for textile applications. Cottonized hemp fiber is as soft as cotton while retaining hemp’s positive characteristics which include strength, and resistance to both bacteria and UV light. These advances in hemp processing allow for a cleaner separation of fibers without the bother of field retting, making for a stronger and superior bast fiber. In comparison to cotton, hemp fiber does not require pesticides, herbicides, or irrigation.