Crop rotation key to healthy garden

Early spring is the time to plot out your vegetable garden. But before you buy from seed catalogs and plant sales, consider this mantra: leaf, root, flower, fruit.

By rotating your crops — that is, planting leafy vegetables one season, followed by root vegetables, then flowering, then fruit — you're making your soil healthier, reducing insect and disease problems and giving your plants the nutrients they need.

Rodale's Organic Gardening, the encyclopedia for backyard growers, explains that susceptibility to pests and diseases runs in plant families. Further, "some crops are heavy feeders, taking up large amounts of nutrients as they grow, while others are light feeders."

So knowing which of your favorite vegetables belong to the same family, and how heavy a "feeder" each one is, will help you plan crop rotation.

Rotation: a family affair

You might not guess from the edible parts, but tomatoes and potatoes are in the same botanical family, Solanaceae, also known as the nightshade family. Peppers and eggplant are other commonly grown nightshades. Rotating these plants into the same garden spot in successive years would be unwise.

It might be a good practice to never replant in the same place, but since many garden spaces are small, it's not realistic. Common advice is to leave at least two years between using that same plot of land for plants of the same family. Three to seven years is preferred.

It also turns out that tomatoes are heavy feeders, so they will suck up more soil nutrients than other crops. This affects what gets planted next in their spot. A light-feeder such as a salad green might be a good choice. When the tomatoes come out at the end of summer, a winter crop of hardy greens could take their place.

Now, consider the mantra: leaf, root, flower, fruit. Using this progression can balance out the nutrient needs and what is most likely available in the soil. Tomatoes fall into the fruit category, and greens, of course, in the leaf category.

So by planting greens in last year's tomato spot, you're practicing crop rotation. It's easy to put most vegetables into their category, but consult a garden-center expert if unsure.

After the winter greens are done, it's time for a root crop. Perhaps beets or carrots, but not potatoes. A spring beet crop could be harvested by mid-summer, at which time you might decide to let this plot of land rejuvenate. Plant a flowering cover crop, completing the rotation.

Susceptible plant families

Other popular vegetable families that are especially in need of rotation include the Brassicas (broccoli, Brussels sprouts and cabbage), curcurbits (squash, cucumbers and pumpkins) and onions (a family that also includes garlic and leeks).

Like plants in the Solanaceae family, Brassicas are heavy feeders. They also attract garden pests, such as cabbage maggots, that can build up to unhealthy populations if the plants are grown in the same place for a number of seasons.

The larvae will stay in the soil or nearby plant material if they have a regular, preferred food source. A disease such as club root, a mold that affects the roots of brassicas, also remains in the soil once established.

The curcurbits are susceptible to powdery mildew, which also may overwinter in the soil. If other crops not bothered by the disease are planted in that soil for a couple of years, it should not build up.

Onions are susceptible to a number of soil-dwelling fungi and should be rotated for the same reasons.

Considering the nutrient needs of different plant families helps you add the proper amount of soil nutrition, too.

Onions want more organic matter, so composting their bed before planting is recommended. Many root crops are light feeders, so can be inserted into the rotation to give the soil a rest before the heavy feeders are planted.

Legumes to the rescue

You can further boost soil health by becoming friends with the legume family. Beans and peas have fewer problems than many vegetables, and planting them can actually improve the soil. Many legumes will "fix" nitrogen in the soil by pulling it from the air and storing it in their roots.

Instead of pulling nitrogen from the soil, like many leaf and fruit crops, legumes will at the very least leave the nitrogen level as-is, and possibly improve it. Growing legumes as cover crops, then digging the roots and chopped plants into the soil, releases that extra nitrogen, where it is available for the next crop.

In the rotation mantra, replace "flower" with "legume" to ensure an extra soil boost between other plantings.

If all this sounds daunting, as though pests and diseases lurk beneath every plant plunked into the soil, take heart. These practices are guidelines and nature is somewhat forgiving. Simply applying some of these concepts can increase the chances of healthy soil and bountiful gardens.

Bill Thorness is a freelance garden writer in Seattle: bill@thorness.com