Successful revegetation projects rely on the use of high quality seed. Seed from harvesting machines or hand collections will include a mix of stems, leaves, chaff, appendages, empty glumes, and seeds of various sizes and quality. The extraneous materials must be removed to insure a quality product and to allow uniform dispensing through seeding equipment. This chapter deals primarily with machinery used by USDA NRCS Plant Material Centers (PMC) and researchers. Manufacturers have developed numerous seed cleaning devices for small lots as well as very large lots. With experience and some ingenuity, operators can adjust the cleaning machinery to produce a quality seed product with high purity levels. Pre-cleaning is the rapid removal of excessive, extraneous materials by scalpers, hammermills, or debearders prior to basic cleaning with air-screen separators. The air-screen separator, properly adjusted, can supply a finished product for many grasses, forbs, and shrubs. However, finishing machines are also available to separate seed by density, length, width, thickness, and shape. These include specific gravity separators, pneumatic separators, velvet rolls, spiral and indent cylinders, indent disks, magnetic separators, electrostatic separators, vibrator separators, stoners, fresh fruit separators, and others. Moisture content may be high in freshly harvested seed and drying may be required prior to cleaning. Processed seed should remain viable for several years if stored in a clean facility where temperature and humidity are kept low.
Grass seed can be harvested by several types of commercial machines. Hard slick seed is relatively easy to harvest while chaffy seed with awns and hairs that impedes the flow of seed through machinery is difficult to harvest. Timing of harvest is of utmost importance because seed development between medium dough and seed shatter can last from a few days to two weeks. Conventional grain combines are widely used alone or following swathing. Combining is a once-over treatment and must conducted when the greatest number of seeds are mature and remain on the plants. Brush harvesters have become popular for many of the chaffy seeded species. They can strip plants multiple times to capture seed crops that mature over a long period. Small plot combines and handheld units are also available. Seeds from shrubs are usually hand stripped and collected in lightweight hoops. Freshly harvested seed is usually high in moisture content and requires drying before processing to prevent damage to the seed.
Descriptions of equipment and techniques related to hand seed collecting, pelleting or coating seeds, and seed storage are in process. Until then, please see the Appendix for useful references and resources.
De Vitis, M.; Hay, F.R.; Dickie, J.B.; Trivedi, C.; Choi, J.; Fiegener, R. 2020. Seed storage: Maintaining seed viability and vigor for restoration use. Restoration Ecology. 28(3): 249-255.
Frischie, S.; Miller, A.L.; Pedrini, S.; Kildisheva, O.A. 2020. Ensuring seed quality in ecological restoration: Native seed cleaning and testing. Restoration Ecology. 28(3): 239-248.
Houseal, G.A. 2007. Tallgrass Prairie Center's native seed production manual. Faculty Book Gallery 102. Cedar Falls, IA: University of Northern Iowa. 122 p.
Karrfalt, R.P. 2012. Low-cost, high-tech seed cleaning. In: Haase, D.L.; Pinto, J.R.; Wilkinson, K.M., tech coords. National Proceedings: Forest and Conservation Nursery Associations - 2012. Proceedings RMRS-P-69. Fort Collins, CO: USDA, Forest Service, Rocky Mountain Research Station: 53-57.
Karrfalt, R.P. 2008. Seed harvesting and conditioning, Chapter 3. In: Bonner, F.T.; Karrfalt, R.P., eds. The woody plant seed manual. Ag Handbook 727. Washington, D.C.: USDA, Forest Service. 1228 p.
McCormack, J.H. 2004. Seed processing and storage - Principles and practices of seed harvesting, processing, and storage: An organic seed production manual for seed growers in the Mid-Atlantic and southern US. Washington DC: USDA-CREES and Southern SARE. 28 p.
Pedrini, S.; Gibson-Roy, P.; Trivedi, C.; Galvez-Ramirez, C.; Hardwick, K.; Shaw, N.; Frischie, S.; Laverack, G.; Dixon, K. 2020. Collection and production of native seeds for ecological restoration. Restoration Ecology. 28(3): 228-238.
Pedrini, S.; Balestrazzi, A.; Madsen, M.D.; Bhalsing, K.; Hardegree, S.P.; Dixon, K.W.; Kildisheva, O.A. 2020. Seed enhancement: Getting seeds restoration-ready. Restoration Ecology. 28(3): 266-275.
St. John, L.; Ogle, D.G.; Scianna, J.; Winslow, S.; Tilley, D.; Hoag, C. 2010. Plant materials collection guide. Tech Note 1. Boise, ID: USDA, Natural Resources Conservation Service. 16 p.
Simonson, D.B.; Cornforth, B.; Ogle, D.; St. John, L. 2006. Function and operation of a machine to lay weed barrier fabric. Tech Note 25. Boise, ID: USDA, Natural Resources Conservation Service. 8 p.