Methods in General
Harvesting hemp is presently the most problematic and least understood part of hemp cultivation. Currently, there are two harvesting techniques used:
- The French method (applied to hemp cultivated for the paper industry).
- The one used for the textile industry
The paper-making method is well-developed, but cannot be uniformly implemented and standardized because of its single and dual purpose form. That is, it is also used to harvest seeds as well as fibers. The textile method is generally used by countries once belonging to the Soviet Union.
Hemp-harvesting equipment is continually being improved. Since 1996, the Kemper cutter, which the Dutch company HempFlax worked to improve, is now manufactured by John Deere.
Hempline, Inc., of Ontario, Canada, which planted Canada’s first modern hemp crop in 1994, has modified and improved several types of existing hemp-harvesting equipment.
Harvesting Seed Hemp
Harvesting hemp seed is difficult and tricky. Not only do the seeds mature at different rates on different plants, they can mature at different times on the same plant. When the lower seeds near the stalk are mature and have split open, the seeds near the top are not yet mature. The trick is to determine at what point harvesting should take place that will amount to a minimal loss of seed.
Harvesting hemp seed usually occurs in early October when the plant is roughly sixteen weeks old and when the flowers are ripe and full of seeds. By this time, most of the plant leaf matter has fallen to the ground where it will compost and give nutrients back to the soil for future years.
Hemp seed is harvested with a combine tractor and, almost immediately, cleaned to remove any stocks, leaves, and immature seeds. The cleaned hemp seed is then stored in grain bins, where it is kept cool and fresh until it is shipped.
Harvesting Fiber Hemp
Fiber hemp is ready to harvest about the time the plant is finished producing pollen and the first seeds start to develop. However, this does vary with the variety and maturity of the fiber desired. If left beyond this stage, the fiber becomes too coarse. Fiber from the male plant dies soon after pollination. It is coarse and good for fiberboard and other products since it is stronger than younger fiber.
Because hemp is sensitive to light, early planting will produce taller crops and thus more fiber. Stems must not be chopped or broken too much in the harvesting process since long fibers are more desirable.
Removing the leaves from hemp by hand is virtually out of the question. Leaves are an unnecessary burden, taking up a significant amount of space during transport and in the retting basin. This adds to the cost as well as imparting an undesirable discoloration of the stalks. Therefore, many farmers resort to chemical defoliation.
Chemical defoliation of fiber-hemp plants takes place when the male plants have reached 10-15% flowering stage or roughly five to eight days prior to technical maturation. Any earlier and the yield will be reduced, and any later will result in ineffective defoliation.
Roundup is one chemical defoliant. It contains 20% glyphosate. The main disadvantage with glyphosate is that it must be dispensed in significantly larger quantities than such others as Basta. This alone negates Monsanto’s claim that farmers will use fewer chemicals when using their products. In addition, Roundup cannot be used any closer to a lake or stream than 10 meters (33 feet) since it is toxic to fish.
Purivel is a supposedly gentler chemical that contains the active substance Metoxuron (by Sandoz). Purivel has a systemic effect; that is, once it is absorbed into the epidermis of the leaves, it hinders the synthesis of chlorophyll, primarily in the new leaves. The leaves begin to dry out within 5-8 days after spraying and fall off after 10-12 days. Diquat and paraquat defoliants are not considered because they diminish the tensile strength of the fibers as well as being health hazards.
There are products that are equally effective and supposedly more environmentally friendly. These newer products are produced from the natural raw materials derived from plants. Basta (by Hoechst) is one example. It contains the active substance ammonium glufosinate and applied when the male flowers are 10-30% open. A later application at the end of flowering is not effective. The active ingredient in Basta takes effect after four or five days and does not translocate, nor does it block the synthesis of proteins. However, Basta is still a definite health hazard. It is toxic to fish and may be used only on fields from which the flow-time is at least 50 days to the next water treatment facility.
Alternatives to Chemical Defoliation
Since the introduction of eco-type products, chemical defoliation has been dramatically reduced, but not eliminated. Ecologically-minded textile manufactures in western Europe are no longer accepting goods that have been subjected to chemical defoliation. In addition, a growing number of businesses are interested in certification of hemp crops, which means a higher market price than for uncertified products. Precise analytical techniques are now used to provide evidence that no chemicals have been used during cultivation.
Alternatives include labor-intensive manual removal of the leaves, mechanical removal (but new machines need to be invented), or water-retting without defoliation. All have disadvantages. Newer methods developed in the west do not require chemical defoliation since they no longer employ water retting.
Please note that hemp food products are not a problem since most hemp is grown without the use of chemicals; although, to be doubly sure, it should be advertised as coming from an ‘organic’ source. It is the fiber that is creating a stir among fabric producers and consumers. Some are discontinuing their hemp line in favor of organic cotton.
Others, like Rawganique, are finding a solution to this problem by contracting with certified organic growers who will not use the chemical retting process. Once consumers become more aware, they should be insisting on this from other companies as well.
Once the hemp is harvested, it must go through a process called retting in order to separate the fiber from the rest of the plant. This is not an easy process and can be accomplished through several methods where moisture, microorganisms, or chemistry break down the bark tissue that binds the fiber and non-fiber portions, making them easier to separate.
- Dew retting occurs when the stalks are left in the field so that rain, dew, or irrigation is used to keep the stems moist. This may take up to 5 weeks and produces a coarse fiber with a light brown color.
- Water retting occurs when stems are bundled and then submerged in water so that bacteria break down the pectin. This takes 7-10 days and produces a better quality fiber.
- Warm water retting occurs when bundles are soaked for 24 hours after which the water is replaced. Heat is then applied to warm the batch for the next two or three days. This gives a very uniform, clean fiber.
- Green retting is an all mechanical process that separates the components and used when the fiber is needed for textiles, paper, or fiberboard products.
- Chemical retting occurs when chemicals are used to dissolve the pectin, allowing the components to be separated. This shortens the time to as little as 48 hours when the next process can then be instigated. This produces a very high quality product.
After retting, the hemp fiber is decorticated, scutched, hackled, and combed – processes that separate and clean the component parts so each can be used. Equipment companies are continually working to improve these processes for efficiency and to lower costs.
The purifying process for hemp is different from wood fiber, which is very dense compared to hemp. This allows for it to be economically transported long distances for processing.
Since hemp has been banned for so long, there are few technological advances to make hemp processing less polluting on a large scale. Since the renewed interest in hemp, however, such problems may soon be overcome.
When hemp is grown for seeds in western Europe, the plant is predominantly cultivated for dual usage as seed and fiber and takes place in a two-stage process.
- First, the tops of the plants are cut off and threshed. The cutter from a modified combine harvester is set at the lower end of the seed-bearing sections of the plant. The harvester must be kept at a high speed to insure a trouble-free harvest. Only then will the cut plant pieces fall directly into the chopper, which should be narrowed to avoid blockages. Axial-flow combines have proven to be well-suited for this type of harvesting.
- The second stage involves harvesting the stalks remaining in the field. This step is essentially identical to the harvest of all fiber-hemp plants with the stems being cut, possibly trimmed, and pressed into bales after retting and drying.
Hempseed can be harvested when the seed husk is hard. The seed’s marble-like characteristic is easily identifiable, and the external husks are yellow to bright-green in color.
At this stage, the seeds do not fall off the plant if touched. Premature harvesting will result in numerous non-viable seeds, while a late harbest will result in significant yield reduction.
This is particularly common in fields where the row distance is more than 40 cm (16 inches). When the distance between the rows is within the recommended 20-40 cm (8-16 inches), maturity is relatively uniform.
When the seeds reach maturity, reaping should begin immediately. A combine harvester equipped with a dual-beam cutter can be used but only during dry, sunny weather after the early morning mist has dissipated. A combine harvester with an elevated cutting table is also recommended.
Cleaning seeds in the field is not very difficult. It can be done with a winnower. It is advised that a clod sieve with round slits of 5-6 mm and a seed sieve with elongated slots of 1.5-2 mm in length be used. Depending on the grower’s particular requirements, a seed operator could also come in handy. It is difficult to remove the sclerotia from seeds that have been improperly treated. Significant losses may also result when a winnower or separator is used for that purpose.
Because these are oil seeds, special care is required for storage. Initially, the seeds can be stored in thin layers only and must be turned frequently. If the moisture content is 12%, the seeds can be stored in sacks.
Seeds cannot be stored in a dry storeroom because they will no longer germinate. Hempseeds burst easily and can quickly become rancid or germ-infested. Loss of germination potential can be avoided if seeds are kept in cold storage (0-5°C or 32-40°F) with a very low humidity level.
Under these conditions, a germination rate of 80% can be maintained, even in the second year. Through storage at -10°C (14°F), the germination rate of the seeds after 5-6 years is 90%.
When farming for hempseeds, the remaining stalks must be destroyed – and there is no environmentally sound method of doing so as yet. In Hungary, the fields are burned; but since this practice is prohibited in Germany, the stems are chopped as fine as possible with a chaff cutter. However, experience with various pieces of equipment for this task is limited. The resulting product is left in the soil for added nutrients.