Happy Halloween, and welcome to November. We are just about done planting vegetables for the year at Terra Firma, which means it’s time for planting something else: winter cover crops.

Most farmers in California leave their fields fallow during the winter, exposed to the wind and rain that erode the soil and cause runoff into both ground and surface water.

“Cover crops” are just what they sound like: plants that cover the soil and protect it from rain and wind. Their roots hold the soil in place and keep nutrients from being washed into the groundwater. And while some of our vegetable crops serve this purpose as well, such as beans and peas, those crops are harvested. Cover crops are not.

Instead, cover crops are “terminated”: mowed, mulched or incorporated into the soil without being harvested first. All of the nutrients they have collected from the soil through their roots are returned to the soil, to feed it instead of you or me. And the biomass of the plants themselves, much of it carbon, returns to the soil as well.

At Terra Firma, we primarily grow leguminous cover crops — related to peas and beans — that pull nitrogen out of the air through a complex interaction with their roots. When the cover crops are returned to the soil, the nitrogen stays in the ground where it is available for the next food crop we grow.

Food crops that are grown in fields following a cover crop always produce better than fields that don’t. The soil is easier to work, the plants are happier, and the yields are higher. We would love to plant cover crop on every acre of our farm, every year. But it’s not feasible. Why not?

First: Winter is the best time to grow cover crops, because they are usually irrigated by rain, but we grow quite a few vegetables in the winter as well. We can’t plant the whole farm to cover crops.

Cover crops can also be grown in the summer, but they need to be irrigated. It’s pretty rare we have much “extra” water available in the summer — or extra time, for that matter.

We can also get in trouble for the same reason if we plant too many winter cover crops and it doesn’t rain enough.

Second: One of the benefits of cover crops is also a downside — they keep the soil from drying out. But in order to plant our spring crops, we need the soil to dry out. If all our fields are planted in cover crops, it can make it impossible to get any cash crops planted in a wet year.

Third: Weather in the fall also plays an important role in determining how much cover crop we plant. We generally start planting cover crops around Halloween, and try to finish by Thanksgiving or the week after. But if it’s too wet, it limits how much we can get planted.

These complications are why most farmers in California don’t typically grow cover crops — even most organic farmers. But as their benefits have become increasingly well documented, the state started a program called “Healthy Soils” to provide monetary incentives for farmers to use them. A few farms around us have started growing cover crops in a limited fashion thanks to this program.

We’ve been growing cover crops at Terra Firma for thirty years. On our farm, they provide their own monetary incentive — higher yields — that more than make up for the (relatively small) investment we make in them.