The basic idea of cover cropping is to sow seeds over ground that would otherwise be bare in the garden or on the farm during the rainy season. Cover crops are most often grown throughout the winter, however there are appropriate seed mixes for any time of year.
Why sow seeds on bare soil when it is not in use? The most widely recognized reasons for cover cropping are erosion control, nitrogen (and carbon) fixation and the development of organic matter. Cover crops can also increase habitat for beneficial insects such as pollinators and predatory insects, and they can trap pests like aphids, keeping them off of other winter crops like kale and broccoli. Crops with a large taproot such as radishes and mustards can even help break up hardpan clay soils.
Weeds can serve as effective cover crops too! The most relevant aspect to managing weeds as cover crops is understanding their reproductive cycles. If the weed in question is a perennial and can spread without the production of seeds, it should probably not be used as a cover crop. For most annual weeds, however, merely ensuring that they are tilled in or “chopped and dropped” as mulch before they produce mature seed is enough in the way of management. A diverse, even weedy, winter garden ensures a diverse and abundant soil food web to nurture healthier and more vigorous fruits and vegetables in the coming seasons!
Our favorite cover crop mixes are improvised. A mixture of grains and legumes is a good base for your seed mix. Grains (barley, wheat, oats, rye, etc.) have complex root systems, which provide erosion control and soil aeration (once they die and their roots decompose), and legumes (beans, peas, vetch, etc.) fix atmospheric nitrogen in the soil with the help of mycorrhizal bacteria. If you are not sure that you have the right mycorrhizal bacteria in your soil, it’s a good idea to purchase bacterial inoculant from a seed company, which you can sprinkle onto your seeds before planting. Note: you will know that you have the appropriate bacteria in your soil if you see round, white nodules on the roots of your legumes (see fava bean roots with nodules pictured below).
Instead of growing a plain mix of grains and legumes, try throwing expired seed into your mix and see what comes up! In many cases you can begin to grow your weeds of choice. Our favorite intended weed is arugula. Even late into the winter growing season in the San Francisco Bay Area, we will sow arugula, radishes, flax, cilantro and parsley and low-growing wildflowers around our winter lettuces, broccoli, fennel bulb, celery, chard, kale, onions and garlic. Sometimes the most random and spontaneous growth in the winter garden is the most beautiful!
Our inspiration for growing a diverse winter cover crop is borrowed from the theoretical principles of Masanobu Fukuoka, Japanese pioneer in sustainable agriculture. Fukuoka managed large production vegetable fields without tillage by sowing, harvesting and mulching all year round. Our favorite quote from his One Straw Revolution, which every earnest gardener should have on their shelf, reads, “If we throw mother nature out the window, she comes back in the door with a pitchfork!” His words seem especially poignant in today’s times of ecological turmoil and are specifically relevant when using a natural means for adding nutrients and complexity to the soil food web through the use of cover crops.
Finally, consider growing edible grains as your winter cover crop. You will not get the same nitrogen fixation as you will with legumes, but you will get the erosion prevention and the bonus of an early summer grain harvest. Even a small patch amount of wheat or barley can add up! Nothing is more satisfying than making an entirely homegrown sourdough bread, pie crust or batch of cookies!