Crop rotation and companion planting

Hello all -

Following up on a suggestion by a reader of this blog, I have decided to write a few words about crop rotation, and the related concept of companion planting. I hope that guidance related to good crop "neighbors" has found its way into your gardening plans.

If you have grown vegetables and herbs before, you may have noticed that the various plants have their own spectrum of needs, ailments, and behaviors. Genetically-, or more specifically, phylogenetically-related plants take the same nutrients out of the soil and are vulnerable to the same pests and diseases. One example of this is the vulnerability of both turnips and arugula, Brassicaceae family members, to the flea beetle, a very small, black beetle that leaps from the soil onto greens such as these, leaving  scores of tiny, tragic holes in its wake.  Alternatively, some traits that significantly impact plant health may have no origin in closely-shared genetics. While not a Brassicaceae member, eggplant, in my experience, is highly prone to flea beetle attack.

The personalities of the crops we plant can inform us of how, when, and where to put them down. It is wise to separate crops with similar nutrient and pest profiles by time and space. One way to accomplish this is crop rotation, a practice whereby the same crop, or type of crop, is not planted year after year in the same spot but rotated to someplace else in the garden. For instance, one might choose to plant garlic, the scourge of many garden pests, in the ground to replace last year's arugula or turnip plantings, and rotate the planting of these Brassicaceae to a spot that most recently housed lettuce or carrots. Another method is companion planting, a practice by which crops that are "good neighbors" are planted in close proximity, and "bad neighbors", such as those that consume large amounts of nitrogen, or those that attract the same pests, are kept far apart. Companion planting also frequently involves the use of flowers, allia, and other plants you might otherwise not have cultivated, for the purpose of promoting or repelling various fauna, pests or otherwise. Both crop rotation and companion planting are good strategies to minimize loss and maximize output, without relying on pesticides or other chemical additives not permitted in organic gardening.

Two key consideration for both crop rotation and companion planting are 1) nutrients and 2) pests.

Nutrients

Some plants are givers, and some are greedy. The nightshades, including eggplants and tomatoes, are among the heavier feeders. A plot planted year after year with only eggplants and tomatoes will become nutrient-deficient, and eventually fail to deliver adequate minerals, or macronutrients and micronutrients, to ensure healthy, productive plants. While unrelated, corn and Cucurbitaceae (melons, squash, cucumbers) are also heavy feeders. "Greedy" crops such as these give you a good reason to invest in some soil amendments for your plot, including compost, leaf mulch, and even coffee grounds. However, even with soil amendment, it is in your best interest to move your Nightshade/Cucurbitaceae plantings around your plot from year to year, replacing them with plants that contribute positively to the condition of your soil. This year, for instance, I am replacing last year's sowing of eggplants, for instance, with green beans in the same spot. Green beans and other legumes are nitrogen-fixing, meaning that they host beneficial bacteria in their roots that convert gaseous nitrogen in the air to a useful, water-soluble form in the soil. I have also sown peas, another legume, into the spot where I grew my butternut squash last year, and fava beans to replace tomatoes.

People have been using legumes to supplement the soil nitrogen lost to the cultivation of heavy feeders for a very long time. Native Americans developed this technique long ago, and planted beans (for nitrogen), squash (for ground cover), and corn together under the name of the "Three Sisters". Whether you plant legumes and heavy feeders together in the same season, or rotate the former in to replace the latter from year to year, you will be using the inherent nitrogen-fixing capacity of the legumes to its full potential.

Even if you don't plan to take advantage of this property of legumes, it is a good idea to rotate tomatoes, eggplants, peppers, melons, squash, and corn around your plot from year to year. In addition to depleting the same nutrients out of the soil, these crops, when planted in the same place year after year, also promote the recruitment and establishment of serious garden pests.

Pests

Flea beetles, as I mentioned above, are a striking example of why you don't want to plant your broccoli, kale, and brussels sprouts in the same place from year to year. Flea beetles love the Brassicaceae family, and will hang out all winter in the soil, patiently awaiting your return to the gardens, and the paralleled return of their staple crops. (This is not quite as true of large, vernalized, overwintered kale plants, which generally have tougher leaves are are less vulnerable to the tiny bites of the flea beetle.) Flea beetles will also attack your eggplants, but in my experience have no idea what to do with tomatoes, lettuce, carrots, green beans, or most other crops. Move your Brassicaceae around and replace them with a crop that doesn't look like lunch to a flea beetle.

Another good example is the tomato hornworm, which is attracted to tomato as well as dill. Move those tomatoes around!

Some flowers and herbs are thought to be repellent to many garden pests, including the flea beetle. I find that insect pests completely avoid garlic, onions, chives, and shallots, just one more reason to plant Allia in the garden. Lemon grass, with its fragrant stalks, it also effective. Nasturtiums, beautiful sprawling flowers whose petals can be eaten in salads, recruit predatory insects to hunt down the offenders.


If you are interested in learning more about companion planting, there are extensive, dedicated resources available online. I find the wikipedia entry on companion planting particularly simple and helpful.

I will leave you with an excerpt from Robin Mittenthal's gardening manual, From the Ground Up:

"Rotate crops so related plants don’t stay in the same place. Broccoli, cabbage, Brussels sprouts, collard greens, kale, radishes, kohlrabi, rutabagas, turnips, and cauliflower are all very closely related (they’re in the same plant family, known as “Brassicaceae,” or the mustard family). As a result, they take the same nutrients out of the soil and are vulnerable to the same pests and diseases. As much as you can, move these crops around your garden so that no two related crops occupy the same space for two years in a row. There are other groupings of plants that should not follow each other: Potatoes, tomatoes, peppers, and eggplant are in the family “Solanaceae” (sun-loving family); Carrot, parsley, celery, parsley, dill, and parsnip are in the family “Apiaceae” (carrot and parsley family); summer squash, winter squash, pumpkins, watermelon, cantaloupe, and cucumber are in the family “Cucurbitaceae” (gourd family); Chicory, endive, salsify, dandelion, lettuce, Jerusalem artichoke, sunflowers, and globe artichoke are in the family “Asteraceae” (aster family); Beets, chard and spinach are in the family “Chenopodiaceae” (goatsfoot family); and onion, garlic, leek, and chives are in the family “Liliaceae” (lily family)."

Happy gardening!