Cover Crops – sometimes called green manures – are plants that are used primarily to help improve a location, primarily because of the advantages they bring to the soil. Cover crops are often used to help ‘repair’ soil that has been depleted or eroded. There are many benefits the permaculture gardener can get from using cover crop planting.
In permaculture practice, bare earth is something to be avoided. Ground that is exposed to the elements is at a greater risk of erosion by wind and water runoff. This can mean the removal of the rich topsoil and the compaction of the soil underneath, making planting much harder. Cover crops help to stabilize the soil, prevent runoff and both binding the soil together and improving its structure.
Improve Soil Structure
The roots of the cover crop will also help to improve the structure of the soil. The foliage of the plants helps to prevent compaction of the soil by protecting it from rain, erosion and, in some cases, livestock. The passages and pore spaces that the roots create allow for moisture percolation and aeration of the soil, as well as means by which insects and other microorganisms, which are themselves essential to the health of the soil, can move through it.
As permaculturists know, soil is improved by the addition of organic matter. Organic matter helps stimulate microorganism activity, gives nutrients to the soil, improves the structure and helps with moisture retention. Cover crops add to the organic matter of the soil, both when living as leaves drop to the floor, and when slashed or allowed to die back, when they form a natural mulch or compost. Combining cover crops and compost is one of the most efficient ways to maintain soil quality throughout the year.
Cover crops are sometimes referred to as ‘living mulches’; one of the reasons being their ability to suppress weeds. The roots of the cover crops compete vigorously with weeds for available nutrients, depriving the weeds of the elements they need to thrive. The leaves of the cover crops also compete for light and space above ground, typically shading out the weeds so that they cannot photosynthesize effectively. Furthermore, when crops die back or are slashed back, their perform a more conventional mulching function of smothering the weeds and their incipient seeds.
Planting a cover crop is an effective way to conserve and even increase the moisture content of the soil. Besides preventing runoff by limiting the erosion of the topsoil, the crops do this in two ways. Firstly, simply by providing a cover for the soil, they protect it from evaporation by the sun and the wind. Secondly, many cover crops send down deep roots, which can bring up moisture from lower down in the soil profile.
Another of the benefits that cover crops bring to the soil is to add valuable nutrients, such as nitrogen, an essential element that all plants need. Species in the legume family of plants have a special ability to ‘fix’ nitrogen in the soil. They have nodules on their roots that provide a habitat for certain nitrogen-fixing bacteria. Not only does this increase levels of nitrogen in the soil while the plant is growing, when the plant dies back, after harvesting for example, the nitrogen is released into the soil and becomes available for other plants to use, so if you are planting a food crop in succession after the cover crop, it will have a good nutrient load with which to get started.
Permaculture gardeners do not always have to let these leguminous crops grow through their life cycle; they can be periodically slashed back and the stems and foliage left to rot in order to release their nutrient load into the soil. In traditional agricultural methods the cover crop would be cut down then ploughed into the soil. To avoid this destructive technique, the cut plants can be mulched to quicken breakdown. Examples of leguminous cover crops include vetch, field peas and clover.
Cover crops also save the permaculturist time and energy. Given all the nutrients that they provide to the soil, there is no need for composting or mulching. This makes cover crops a good option when looking to improve the soil quality of a large area. And by suppressing weeds, it reduces the need to sheet mulch an area.
It’s not only the soil that benefits from the presence of a cover crop; it may add something to your kitchen as well. Certain species of cover crops can provide an edible harvest. Legumes such as peas and beans perform both functions, while mustard plants and daikon are also suitable cover crops that you can eat. For larger areas in zone 3 of your permaculture plot, you might consider a grain crop such as wheat, barley or rye.
Instituting cover crops adds to the biodiversity of your permaculture plot. All species of plants have their own unique characteristics, including how they interact with other plants (such as providing shade or fixing nitrogen) and organisms (such as attracting beneficial insects, or repelling insects that could damage neighboring specimens). The cover crops can also attract wildlife to your permaculture garden, by providing habitat, feeding opportunities (on insects attracted by the plants, for instance), and protection from the elements and predators.
This biodiversity is a major part of attracting a wide variety of insects to your plot. By planting cover crops rather than leaving bare earth, you will bring more species of insect to your site. Some insects will predate on others and so prevent populations booming which may impact upon your crop yield. Attracting insects also increases the number of pollinators on your site, helping propagate your garden plants. The increased organic matter and nutrients in the soil also feeds beneficial microbes that can keep fungal and bacterial infections in check, and limit the number of nematodes, microscopic organisms that feed on plant roots and stems, and which can carry viruses that they transmit to the plants.