Recharge your soil with a cover crop

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Cover crops are highly beneficial in vineyards, orchards and vegetable gardens.

Many people are probably familiar with the idea of cover crops, having seen them planted between the vines in many vineyards, particularly the iconic mustard. In our area, usually annual plants such as grasses and legumes are used to protect and benefit the soil and the health of the crop plants. Some examples are grasses like barley, oats and triticale, and legumes like vetches, fava beans, clovers and peas.

Grasses serve to add a large amount of long-term soil organic matter and legumes act to fix nitrogen in soils. Other plants used or naturalized are mustard and radish, often in heavy soils. Sometimes annual wildflowers are included for pollinators and beneficial insects. Each plant or group of plants has specific roles to play for the crop, the soil, and the above-ground environment; the mixes used are based on the goals of the farmer.

Cover crops protect the soil from erosion, increase the soil’s organic matter, help develop good soil structure, add fertility for the crop plants, and often foster beneficial insects or pollinators. They are usually managed with equipment, but have equal application for our own vegetable gardens, both small and large. There are ways to easily manage them without a tractor.

An important benefit of cover crops in our climate is the protection they provide the soil from erosion from winter rains. They do this by breaking down leafy matter, which in turn breaks the force of raindrops, dispersing them and the flow of water. But they also serve the important function of physically holding the soil with their roots.

Cover crops help develop good soil structure and porosity. Soil texture is the mineral (nonliving or inorganic) component of the soil: the proportion of sand, silt and clay. Soil structure is basically the process of soil particles aggregating into complex structures composed of both solid and pore space (think of the structure of chocolate cake). The pore spaces are places where water and air infiltrate, and soil organisms live. Soils with good porosity readily absorb water. Soils with a high organic matter content hold it, increasing plants resistance to drought. Soil organisms — large and small — cycle organic matter into forms plants can take up.

A healthy soil has around 8-10 million micro-organisms living in each tablespoon of soil. They need organic matter and root exudates — protective organisms in the soil around a plant’s root structure — to feed on. Cover crops provide them with both. Soil structure is created through the interaction of plant roots, soil organic matter, soil macro and microorganisms and soil fungi.

Cover crops can add greatly to soil fertility. Legumes like clovers, peas, beans and vetches, fix nitrogen from the air with bacteria on root nodules. The organic matter provided by both the roots and top growth is cycled into nutrients plants can take up by soil organisms.

A cover crop mix of grasses and legumes is the best to gain the dual benefits of better soil structure, soil fertility and for beneficial insects. A diversity of plant roots leads to a diversity of soil organisms, which studies show are highly beneficial to our soil and plant health. Clovers and vetches are very important plants for pollinators.

A commonly sold and used mix of seeds is called Plowdown mix. It is composed of oats, bellbeans, vetchs, clovers and winter peas.

In our climate, cover crops are usually planted in the fall to germinate and grow with the fall and winter rains. They remain small over the winter and then grow rapidly in the spring. They are mowed and sometimes tilled during the spring as the plants bloom or the area is needed for spring planning. They can be irrigated into our home gardens to get them growing if fall rains are late. Much root growth happens in the fall while soils are still warm but air temperatures cool. They can also be planted in late January or early February, although root growth will not be as strong.

There are a couple of easy methods to managing cover crops in our home gardens. A reader who has a number of very large raised beds made of nonmortared cinder blocks said she doesn’t cut her cover crop. But three to four weeks before planting she just covers the boxes and cover crops with opaque plastic. Within weeks they are completely decomposed and her soil is soft and friable with no digging involved. Landscape fabric, cardboard weighed down with bricks, or old carpet can be used as well to smother the cover crop.

Another method is to cut cover crops in spring two to four weeks before you plant your garden. Mow or cut down to the ground. This is key. You don’t want the cover crops to regrow. They can be cut with a weed wacker. If cover crops are large, it may take two passes to get green matter cut down to the ground.

In small vegetable boxes, you can cut them with shears, pruners or scissors. Rake the green matter back up on the beds, then cover it with 1 to 2 inches of compost. Make sure to keep it moist. The green matter and roots will decompose quickly and the area should be ready to plant in two to four weeks. Any large remaining debris can be raked off. Some people compost the green matter and apply it later as compost. If this is done, cover the soil with an inch of compost to help roots decompose. Keep soil moist. This method avoids tilling. Not tilling (called no-till) is the best way to increase the organic matter content of the soil and develop soil structure. It also avoids the hard work of digging the soil.

Cover crop seed can be purchased locally at area farm supply stores and seed companies. Both Harmony Farm Supply in Sebastopol and Le Ballister’s Seeds in Santa Rosa have many excellent varieties and mixes for every situation.

Kate Frey’s column appears every other week in Sonoma Home. Contact Kate at:,, Twitter @katebfrey, Instagram @americangardenschool.

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