Prepare soil for spring with a cover crop now

Even though daytime temperatures remain warm, the fall, with cold nights and shorter days, finds many of us pulling out spent summer vegetable plants. What to do with our now bare beds or boxes? Consider planting a fall cool-?season cover crop.

Soils are living systems, and many farmers and vegetable gardeners plant cool-weather cover crops for the benefit and protection of the soil.

Cover crops are usually composed of a mixture of annual plants such as grasses and legumes like those often seen in vineyards between the vine rows.

Some examples of commonly used plants are grasses like oats and triticale, annual plants like daikon radish and legumes like vetch, fava or bell beans, annual clovers and peas to capture nitrogen in the air via root nodules.

Some people include spring blooming wildflowers to provide nectar and pollen for beneficial insects and pollinators.

Each plant type provides a different benefit to the soil and above-ground environment and is usually tailored to the specific growing conditions and goals of the garden, farm or vineyard.

Cover crops benefit the soil by physically protecting it from erosion from rain or running surface water. When the stand is robust, these multi-purpose crops can out-compete weeds. Plant roots also help prevent and break up compaction.

Another critical benefit is increasing the organic matter levels in the soil. Many native soils in California have low levels of soil organic matter (OM) - only about 1%.

Our vegetables grow best with OM levels of about 6%. Both the roots and top growth of cover crops, particularly grasses, which are extensive and strong, create large amounts of organic residue that decay over a long period of time.

This increases organic matter levels and fuels soil biology, essentially feeding soil organisms that cycle nutrients and help develop good soil structure.

Plant roots and soil organisms both need oxygen just as much as we do, and well-structured soils are well oxygenated.

A diversity of plants in a cover crop mix helps create an environment that supports a diversity of soil organisms. The roots and decaying top growth essentially stimulate and feed soil life.

With sufficient amounts of organic matter, each teaspoon of soil has or should have hundreds of thousands of numbers and species of bacteria, fungi, actinomycetes and other microorganisms.

These organisms essentially hold nutrients like nitrogen and phosphorus in their bodies or cells, and when they die or are eaten, it is released over time.

Cover crop mixes increase the variety and supply of nutrients in the soil. Some initial recent studies show higher nutrient levels in food crops grown in soils enriched with diverse cover crops.

Plant roots also capture and essentially hold nutrients and prevent water soluble nutrients leaching into groundwater.

The roots and residues of the cover crop and action of the soil organisms improve the soil structure, or aggregation (think of a good piece of chocolate cake) and enable better water filtration into the soil.

The decayed organic matter holds water like a sponge, increasing the water storage capacity of the soil, decreasing needed water use and enabling plants to better withstand drought - an important issue in our area.

There has been a lot of new research on cover crops and no-till gardening and farming. While cover crops confer many benefits to the soil, the most effective way to realize these benefits is to not till the soil, allowing the roots and tops to remain in and on the soil.

Tilled soil increases the oxygen in the soil and causes oxidation or rapid decomposition of the organic matter in it. Organic matter doesn’t build up in tilled soils compared to non-tilled soils where levels build quickly, creating a sponge for water and enhancing fertility.

Tilling creates a boom and bust nutrient cycle instead of a steady release of nutrients. Mixing the soil homogenizes it and reduces the diversity of soil organisms - mostly confining them to those that rebound quickly. Soil that is not tilled has many micro-environments and levels where different organisms live.

A diversity of organisms is important because you want those that recolonize rapidly, those that compete with other organisms and those that survive under unfavorable conditions like drought or heat.

The more diversity of life in the soil, the better likelihood there is for the potential natural control of soil pathogens and plant pests.

How does no-till work in a home vegetable garden? Cover crops are usually planted in the fall and allowed to grow over the winter until spring when they are cut or mowed before they flower.

For vegetable farmers or gardeners, you will want to cut them three to four weeks before the bed area is needed for spring crops.

A local person who has a number of very large raised beds said she doesn’t cut her cover crop, but three to four weeks before planting - just covers the boxes and cover crops with opaque or black plastic. Three to four weeks later, the plants are completely decomposed, and her soil is soft and easily crumbled, with no digging involved.

Landscape fabric, cardboard weighed down with bricks or old carpet can also be used to smother plants.

Another method is to cut cover crops in spring two to four weeks before you plan on planting your garden. Mow or cut them down to the ground with a weed whacker or hedge trimmer.

This is key if you don’t want them to regrow. It may take two passes to get larger green matter cut down to the ground. In small raised boxes hedge shears will work. Rake the green matter back up on the beds, then cover it with 1-2 inches of compost. Make sure to keep everything 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.

The decomposing plant matter and added compost will help protect the soil from the sun, limiting evaporation of water and keeping soil cool in hot weather.

Farm supply stores, some nurseries and seed companies like Le Ballister’s in Santa Rosa carry cover crop seeds and mixes.

Mixes can be purchased, or you can create your own. Make sure to use annual species rather than perennial. Include both grasses and legumes.

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

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