“I could break a Mack truck’s windshield with some of these grocery-store tomatoes,” griped my gardener friend Shawn. “I hate buying produce at the grocery store. This is San Diego; I should be able to garden all year long! But last year, my little patch of earth just petered out on me — the tomato crop was heartbreakingly small.”

“Maybe you need to give back something of what you take,” I suggested.

“Don’t get all circle-of-life on me,” replied Shawn. “You want me to bury the first fruits of my crop in my raised bed or something?”

I told her I was talking about cover crops, and I told her I would get her some information. My first call went out to Tiger at Mission Hills Nursery (619-295-2808; missionhillsnursery.com). He explained that the concept behind a cover crop “is that it does something to the location, whether it be the soil or the environment, that will benefit the next crop planted in that location. For instance, you could plant marigolds in a location. They contain pyrethrines. Specific bugs — like nematodes, white fly, and aphids — don’t like pyrethrines. Once the marigolds bloomed, you would till them under a good six inches. Then, if you plant vegetables in that spot during the next season, the rotted marigolds in the soil would help prevent them from getting those pests. Marigolds bloom only in the summer, so you’d use it as a summer crop to prepare for a winter vegetable bed.”

Other crops, he said, could help make the nitrogen in the soil more bio-available for developing plants. “Beans [$1.59–$1.69 per packet] might be a good choice there; they’re high in nitrogen. And we sell catgrass [$1.59 per packet], which are oats, and mustard greens [$1.89 per packet]. They’re also high in nitrogen.”

Besides adding available nitrogen, these crops supply the soil with organic material. “Think about having soil in a jar and growing a plant in that soil. It takes soil to grow that plant. If you take the plant out, then you have less soil in the jar than you did at the beginning. By turning your cover crop over and letting it rot, you replace the lost soil.”

“You’re basically growing your own compost,” added Amber from Peaceful Valley Farm Supply in Grass Valley (530-272-4769; groworganic.com). “That’s why it’s referred to as green manure. Besides adding organic matter, nitrogen, and other trace nutrients, it helps to break up hard soil. For a home till-in cover crop, something you want for just one season, you would plant annuals” — perennials are better for commercial ventures that turn over crops more quickly.

“If you’re sowing a winter cover crop for a summer garden,” she said, “we have a winter soil-builder mix composed of seeds that are cold-season specific. There are bell beans, winter peas, a couple of different vetches, and oats. The oats act as scaffolding for the vetches, which are climbing vines. This particular mix can get really big — it’s created to provide the maximum amount of organic matter in a given space.” If you give it enough time, “It will get to four feet high. Once the plants flower, that’s the ideal time to cut them down. You can use a machete or a weed whacker. Of course, if you chop it up a lot, it will decompose more quickly and be easier to till into the soil. The important thing is that it has at least four weeks to rot and be incorporated into the soil before you plant anything else. Where you are, you could plant the winter soil-builder right now.”

Planting, said Amber, could be as simple as hand-scatting seeds. “Just make sure you cover those seeds with a half-inch of soil, and keep the soil moist while they’re germinating. You may or may not need to water, depending on rainfall. I also recommend an inoculant. That’s bacteria that colonizes the root system of the legumes in the cover crop and also increases the biological activity of the soil. It comes as a black powder — you moisten the seeds and then sprinkle on the powder before you scatter them. The winter soil-builder mix is $1.20 a pound, with a five-pound minimum. The seeds will last for several years. The inoculant costs $4 and will last 13 months. And we also have a summer soil-builder for $1.20 a pound with a five-pound minimum. That one includes annuals such as buckwheat and cowpeas.”