Rotation of crops can stymie pests, boost yields
Q
In Southern California (and other warm-winter areas), we have two growing seasons instead of one per year. Do we still have to rotate our vegetable crops on a four-year schedule, or can we reduce that to a two-year schedule because of the additional winter growing season?
A
Many gardening guides tell you to wait four years before planting the same crop (or same family) in the same garden space. For example, if you plant tomatoes in one section of your garden, you should plant beans in that space the following year and squash the year after that, then chard the year after that. This is called crop rotation.
Crop rotation serves two purposes. When we grow vegetables and harvest them, we are ultimately taking nutrients away from the soil. Each type of plant uses different amounts of nutrients, so if we grow the same type of plant in the same spot year after year, the soil will soon become depleted of those nutrients. Of course, this can be corrected by adding compost, worm castings and other organic matter.
In other words, think about a bear eating berries in the forest. After the bear digests those berries, he does what all bears do in the woods, thus replenishing the soil nutrients. This is not a viable option for most of us, so we use compost instead.
The second reason for crop rotation is pest and disease control. Many, if not most, pest insects have adapted to their host plants’ seasonal growth habits. Tomato hornworms will go dormant in cold weather, then reemerge in the spring to eat your newest crop of tomatoes. If you look closely, you will probably find multiple chrysalises in the ground when pulling out tomato plants in the late fall. These are hornworms-in-waiting.
In the following spring, if you plant something that the hornworms can’t eat, those dormant caterpillars will starve.
Of course, there are many other pests and diseases that can remain dormant for more than a year. Some soil diseases can linger for even longer, so lengthening planting intervals can enhance the effectiveness of crop rotation.
So, to answer your question, I would recommend regular addition of compost and other organic materials to replenish your soil’s fertility. For the vegetable garden, every time a plant is removed the soil should get a little something back. Composting those spent vegetable plants can recycle at least some of those nutrients as well.
If you have had issues with pests or disease, it’s important to accurately identify which pest or disease is present. Many insect pests can be managed by garden clean-up and other housekeeping tasks. Soil-dwelling nematodes and soil diseases can be reduced through solarization (heat-treating the soil using the sun).
Many plants have been bred to resist certain diseases. If you have identified which diseases are lurking in your soil, purchase plant varieties that are resistant.
If you have the space, crop rotation is still ideal for keeping pests and diseases at bay. For many people, this is not an option (or is an option limited to every other year).
Have questions? Email gardening@scng.com. For more master gardening resources, visit ucanr. edu/sites/RiversideMG for Riverside County; mgsb.ucanr.edu for San Bernardino County; celosangeles.ucanr.edu/ UC_Master_Gardener_ Program for Los Angeles County; and mgorange. ucanr.edu for Orange County.