Crop Rotation

Growing the same plants in the same place each year can be a bad idea.

You can break the life cycle of many soil borne pests and diseases by growing plants from different families in the same beds, at different times, in a process called crop rotation. Intercropping is similar to crop rotation, except that intercropping refers to growing different plants in the same area at the same time. Many false claims have been made about the “companion planting” concept, but the benefits of crop rotation have withstood the tests of time and science.

Benefits of rotating crops

While it may be convenient to grow the same plants in the same place each year, in a method called monoculture, many soil borne pests and root diseases can be thwarted by moving crops around. Also, growing different types of crops in an area can reduce nutrient depletion, and sometimes, growing nothing at all is the best choice.

Going fallow

In agriculture, allowing land to rest is referred to as going fallow. This means nothing is grown - no crops, no weeds, nothing. This period of inactivity starves out many agricultural pests. And, honestly, we all need a break now and then. Having a fallow period is an important aspect of crop rotation.

Traditional crop rotation

In Biblical times, farmers were urged to leave an area to its own devices every seventh, or Sabbatical, year. Other methods have also been tried:

• Two-field system - one field is planted while the other rests
• Three-field system - rye or wheat is followed by oats or barley, in one field, while a legume crop is grown in a second field, and the third is left fallow
• Four-field system - wheat, turnips, barley, and clover are rotated year round

Modern agriculture uses chemical supplements to replenish lost nutrients and counteract soil pests, but this method has its own drawbacks, as many of those chemicals then find their way into our food and water supplies.

Crop rotation goals

The goals of crop rotation are to break the life cycle of soil borne pests and diseases, to control weeds, to increase the amount of nitrogen in the soil, to improve soil structure, and to add organic material to the soil. Depending on what you are working with, different crops will help you reach those goals. The first thing you need to do is to identify the soil borne pests and diseases in your garden.

Common soil pests and diseases

Verticillium wilt and downy mildews are the most common Bay Area soil borne diseases. Phytophthora tentaculata is a new threat to many garden plants. Root maggots and many destructive nematodes, such as root knot nematodes, are the most common pests. Take the time to find out which pests and diseases are affecting your plants. Then, look those troublemakers up and learn what you can about the plants they prefer, which plants are unaffected, and which plants can help eliminate the problem. For example, research has shown that Verticillium wilt, which commonly attacks tomatoes, can be reduced by planting broccoli in the same bed, in the following winter. In the same way, root knot nematodes may not impact corn and onions, but they can devastate berries, grapes, and many fruit and nut trees. Growing members of the cereal grain family, such as wheat or barley, in an affected area can reduce nematode populations.

​Planning the right rotation for your garden

The simplest way to incorporate crop rotation is to think of why each crop is being grown. Are you harvesting roots, leaves, or fruits? Simply switching up the type of harvest can be enough to break the disease triangle, just be sure to add in a legume crop for the nitrogen in this most basic rotation. For a more science-based crop rotation, you will need to put on your thinking cap.

First, identify each unique planting area. Then, list all of the plants you normally grow in a year. Next, label each plant as nitrogen adding or depleting, also biomass adding or depleting. You will also want to add notations about any particular pests or diseases that affect each plant. Personally, I am a visual learner, so I break out a pad of graph paper and some colored markers for this exercise. I use slips of paper for each plant, and then add color-coding to represent each of the different conditions. Then, I move the slips of paper around on a sheet with each of my planting areas roughly drawn on it. Your method may be very different from mine, but it is worth the effort to find a way that works for you.

Successful crop rotation can result in harvests that are 10-25% larger than crops grown in a monoculture system. While scientists have not yet figured out why this happens, they have named it “The Rotation Effect” and you can put it to work in your garden today!