Polyculture cover crops are the key to sustainable fertility and productivity in a permaculture system. Cover crops provide a number of benefits including reduced weed competition, decreased risk of disease and pests, reduced erosion, soils enrichment, and high quality habitat. Cover crops can also be harvested to provide hay and/or green manure depending on species composition.
Regarding cover crops we should look to nature not agriculture for inspiration. Traditionally, cover crops have been over-harvested monocultures e.g. alfalfa, buckwheat, or clover. Cover crops grown as a monoculture are susceptible to disease and pest outbreaks, provide low quality habitat, and produce few if any secondary products or benefits. In place of a monoculture a species mix should be designed that limits pest and disease interactions, provides maximum seasonal diversity (flowers and growth during entire growing season), provides quality habitat, produces secondary products (herbs, fruits, etc.), and meets site specific goals (e.g. increasing soil cation exchange capacity and nitrogen content).
In Illinois, the natural systems that built our soils long ago were perennial prairies and forest systems. These systems were disease, drought, and pest resistant. Native systems enriched the soils year to year and were highly productive; they are ideal systems to emulate and provide a large number of species to use in polycultures. It is possible to create an entirely native cover crop system in Illinois; the only impediments are cost and seed availability.
There are many considerations in choosing the species composition of a polyculture cover crop for a site. Soil, moisture, and light conditions are the first considerations. What species can thrive in this area? What are the soil enrichment goals? Are the primary targets carbon, nitrogen, or soil mining? Will the system serve as bee forage? Some species impart a negative taste to honey. Will the cover crop be used for hay? Some native species, especially legumes e.g Astragalus (vetch) or Oxytropis (loco) can poison grazers. How will weeds be controlled? You don’t want to compromise species productivity or reproduction. How much of the system will be harvested? Will the system be cut to encourage growth? All of these questions and more should be asked so that you can more perfectly engineer a polyculture system that meets and exceeds your site’s requirements.
Once a cover crop species mix is chosen and planted the system must be managed. Management can vary dramatically depending upon cover crop utility and species composition. You may need to introduce different species at different points of your system’s maturity to ensure their persistence. Removal of too much biomass can result in soil and habitat degradation, but failure to cut cover crops can potentially stifle growth and limit productivity. Failure to properly time or scale harvests can dramatically impact cover crop quality and species balance. Ultimately, your management must fit your own polyculture system and this will take time, observations, and become a reiterative process. The combination of species, soil types, stochastic events, and goals create an endless variety of systems. Management of these systems cannot be prescribed as simply as a monoculture because their nature is more complex. Both engineering your polyculture cover crop and management of the system can be difficult. That being said, a mature well-engineered polyculture system will develop resilience capable of making the system independent. Mastering the use of polyculture cover crops is undoubtedly one of the most important and rewarding aspects of polyculture; a mature properly engineered polyculture cover crop will beautify your permaculture system from early spring to late fall while enriching it in innumerable manners.