If your vegetable garden seems like it's not producing as much as you think it should, you might need to follow ancient farmers' wisdom: plant cover crops.
Cover crops, also known as "green manure," have been used for centuries to enrich soil prior to planting and harvesting summer and winter crops. Unlike main crops that you can pick and eat, these plants are meant to be clipped, chopped and buried as compost in the soil.
"Basically what we're doing is feeding all the microorganisms that live in the soil and improving soil texture. And basically (the soil) becomes more workable, more nutritious for the plants you're going to grow in afterward," said Ann Burrell, a master gardener at the University of California, during a recent visit to the Palo Alto Demonstration Garden, where she and the other master gardeners keep a healthy edible garden brimming with bees, birds and colorful flowers.
Here, Burrell and other gardeners planted cowpeas as this year's summer cover crop to add nitrogen (a key nutrient for growing plants) to the soil. Other cover crops, such as grasses, turnips, bok choy and broccoli, are used to add carbon to the soil to make it more fertile.
"People complain about clay soils because if they're heavily compacted, they're hard as rock in summer, and they're dry and horrible to work with in the winter," Burrell said. "So adding compost, especially green cover crops like (cowpeas), will lighten the soil and make it workable."
Oftentimes, cover crops will grow so abundantly, they'll start growing out of their beds. When this happens, Burrell will clip the plant at the ground and cut it in pieces about four inches long with hedge shears before planting the pieces in the soil. The smaller the pieces, the better.
"If you can shred it into tiny pieces ... they would compost faster. Basically what we're doing is composting them in the soil," she said.
When it comes to timing, Burrell recommends planting cover crop seeds in May, growing them throughout the summer and turning them under three to six weeks before growing winter vegetables, such as onions and garlic. Winter cover-crop seeds should be planted in October, so they can be turned under in time for spring and summer plantings.
For summer cover crops, Burrell chooses cowpeas, sudangrass, buckwheat and other plants that add nitrogen since the much of the soil in this area is carbon-rich, she said. For the winter, she goes with fava beans, bells beans, veches and clovers.
"Clovers are particularly attractive because they make these beautiful red clovers that are just absolutely gorgeous as a cover crop," she said.
Burrell said one of the more difficult things with growing cover crops is resisting the urge to eat them.
"One of the standard cover crops is fava beans, and a lot of people like to eat fava beans, so you have to encourage them not to," she said.
If you have gardening questions, contact UC Master Gardeners at 650-329-1356 ext. 205.