If you are about to plant a fall vegetable garden, but wonder if it’s worth the effort – wormy cabbage, bitter lettuce and puny broccoli are all you can count on – consider a cover crop instead.
True, you won’t be harvesting produce for the next several months, but as a result of the cover crop, the garden you plant next spring might yield healthier and more abundant crops than you ever have gotten out of the ground before.
The advantages of cover (also called green manure) crops – clovers, vetches and grasses – include weed and erosion control, attraction of beneficial insects and increased soil fertility. Most significant to those of us who struggle with compacted clay soil, however, are the channels created in the earth by cover crop roots, which provide aeration and decompaction comparable to that rendered by the action of a spading fork.
We tend to think of soil as a mass of sand, silt and clay particles. Yet air, which contains 20 percent oxygen, makes up one-fourth of the volume of well-aerated soil. Air is an integral component of any soil, arguably more important than the soil’s solid constituents. Plants, in fact, do not require soil to grow. Virtually any plant can be grown with its roots in nothing but water, as long as oxygen is bubbled in and minerals added. And it has been shown experimentally that plants grow best of all suspended in air, as long as their roots are periodically misted with a mineral solution.
It is vital to remember, then, that the growth of plants in general, and of vegetables in particular, is limited sharply by the amount of oxygen in the soil. The reason a spading fork is used to prepare planting beds is to add more air, and thus more oxygen, to the soil.
Here’s a useful and instructive exercise: Take a spading fork and start loosening and turning the top 10-12 inches of soil. As you loosen and turn over the soil, you will begin to see the soil level rise; in the process of loosening the soil, you have fluffed it up by adding air and increasing its volume. Before planting annual flowers or vegetables, this is the minimum soil preparation that should be done, in conjunction with addition of compost (12-15 cubic feet of compost per 100 square feet of planting area).
In addition to aerating the soil with their roots, leguminous cover crops (clover and vetch) add nitrogen to the soil through a symbiotic relationship with Rhizobium bacteria. These bacteria live in the roots of legumes, taking nitogen out of the soil air and “fixing it” with soil oxygen to make nitrate, which is the form in which nitrogen – needed for making leaves – is taken up by plants.
When you buy nitrogen fertilizer, it is often in the form of nitrate – calcium nitrate, potassium nitrate or ammonium nitrate. This nitrate is yours as a free bonus when you grow leguminous crops and plow them back into the soil. To ensure maximum nitrogen fixing by your leguminous crop, ”inoculate” seeds before planting with Rhizobium bacteria, available in pelletized form wherever clover and vetch seeds are sold.
Fava beans, which actually are produced by a vetch plant (Vicia Faba), grow well in the Valley, may serve as a cover crop and have a taste that, in the opinion of some, is superior to that of conventional bean varieties. Peas, too, which may be planted now, are a nitrogen-enriching legume. However, when legumes are grown for the nitrate produced in their roots, they should be turned under just as flowering begins, since this is when root nitrate is most
concentrated. This would indicate that legumes grown for their beans or peas – which are the end result of flowering – do not make the best cover crops.
Grasses – whether wheat, oats, rye or barley – also may be used as cover crops. They should be harvested and turned back into the ground just as their classic tassel flower spikes begin to form; this will prevent production and spread of seeds throughout the garden, which will germinate in weedy abundance.
Here are two sources for ordering seeds of cover crops: Bountiful Gardens, 18001 Shafer Ranch Road, Willits, Calif. 95490; and Peaceful Valley Farm Supply, Box 2209, Grass Valley, Calif. 95945 (catalog $2).
Tip: Early fall is the best time of the year to plant nearly all trees, shrubs and ground covers, as well as imported Dutch bulbs, cool-season vegetables and flowers. Pansies planted now and established before the cold and wet weather arrives may last until June; pansies planted later, and exposed to stormy weather soon after garden placement, tend to be short lived. Proper soil preparation with a spading fork and compost is the best buffer against unpredictable weather and pests.