Cover crops — crops grown specifically to improve the soil rather than to produce income — are a critical part of our farming practices at Terra Firma. In the winter, over half our land is normally “covered”. But since we also grow quite a few winter vegetables, there are times when we want to grow a summer cover crop in a particular field. A good example is our overwintered onion and garlic fields, which we finish harvesting too late in the spring to put into a summer crop. Meanwhile, the soil in those fields has often taken a beating from the winter rains and wind. It needs a rest.
Growing cover crops in the winter in Northern California is pretty simple: rough up the soil surface a bit, throw the seeds on top, and wait for rain. The grasses and legumes available for use as cover crops grow pretty quickly, generally faster than most weeds. But growing a cover crop in the summer is a bit more complicated, primarily because it needs to be irrigated — at the same time as all the other, rent-paying crops. Sometimes there just isn’t enough water for the cover-cropped fields to get a drink.
We’ve been experimenting for many years with finding fast-growing, drought-tolerant cover crops to grow during June, July and August. We settled on a couple of varieties of cover crop that will grow for two months with just a single irrigation. They are “cowpeas”, weedy, vining versions of Black-Eyed Peas. Within three weeks after planting, they completely shade the soil and smother out most weeds.
But you can’t just throw the seeds on the ground and hope they will grow. To get the Cowpeas established, you have to treat them just like a vegetable: prepare the soil the same way, irrigate the field before planting, and control the weeds so that it can get well established.
By the time the cowpeas are two months old, they are almost four feet tall — tall enough for a dog to get lost in! At this point, it’s time to either let the sheep eat the vines or till them into the soil. Either way, their roots have pulled in hundreds of pounds of nitrogen from the air by that point — fertilizer for the fall crop we plant after them. And the formerly cloddy and tight soil is soft, moist and loamy. It will require less water to grow the next crop compared to a field that had been left fallow during our long hot summers.
Strawberries are our most important fall-planted crop, and we make a point every year of growing a summer cover crop in the field where they are to be planted. Once the berry patch is planted with the cover crop, we try to get as many other fields as possible done, but it can be quite a challenge given all the other work that needs doing. This year we got behind with the wet spring and were only able to plant around 10 acres of cowpeas; other years we have done as much as 20.
Of course, we also grow a significant amount of green beans in the fall — almost 15 acres this year. While they are not technically a “cover crop” because we harvest them, they still provide many of the same benefits to the soil. The plants are not quite as tall and bushy as cowpeas, but in the end we still plow them into the ground after harvesting the beans. And being legumes, they produce their own fertilizer.
Growing cover crops to feed your soil is a hugely rewarding and immensely satisfying task. It’s good for the planet, good for the soil, good for the vegetables, and also profitable.