A: Companion planting has been around for a long time, so most of the early evidence was anecdotal in nature. Eventually, companion planting was scientifically proven to be beneficial in several ways.

Some companion plants can serve as insect attractants that lure pests away from crop plants. Nasturtiums tend to attract aphids and are good companions for beans, cabbage, broccoli, carrots, cucumbers, winter and summer squash, and radishes.

Garlic’s strong scent can drive pests away from beets, cabbage, lettuce and potatoes. Basil, oregano, and rosemary can repel insects due to their aromatic compounds. Marigolds discourage the onion maggot fly and cucumber beetles. (Sadly, I have found that they do not repel rabbits.)

Some companions attract beneficials (particularly tiny, nectar-drinking predatory wasps). Dill, alyssum, parsley, buckwheat and other plants bearing multiple tiny flowers provide easy access to nectar.

The traditional “three sisters” planting of squash, beans and corn is one of the most well-known examples of companion planting. Corn provides a convenient place for pole beans to climb, and in turn, beans improve the availability of nitrogen in the soil. Squash plants shade the ground, discouraging weed growth and confusing the squash vine borer.

Interplanting may enable you to extend the growing season, especially for cooler-season crops. Since we grow our indeterminate tomatoes on a trellis, we can plant lettuce under the vines, where it is protected from intense sunlight. Although they don’t last as long as lettuce planted in the winter, the plants can grow large enough to harvest before they begin to bolt. Radishes can be tucked in between slower-growing eggplants or peppers because they will be ready to harvest before the bigger plants get too big. Don’t plant radishes when they are subject to hot weather, since temperatures over 95 tend to make them especially angry and unbearably hot.

Some crops should never be planted near each other. Sweet corn and popcorn will cross-pollinate, resulting in starchy, inedible sweet corn. Tomatoes should not be planted near corn or brassicas (cabbage family members such as kale, broccoli, cauliflower or collards). All of these are heavy feeders and will deplete the soil of nutrients to the detriment of everyone. Avoid planting crops from the same family (carrots and dill, for example) because they attract the same pests.

Including flowers in your vegetable garden not only makes the space beautiful but attracts pollinating insects that will improve productivity. Most of these flowers are easy to save seeds from, which can save some money for next year’s garden. Alyssum is probably the easiest to propagate — when pulling up the spent plants, just shake them and thousands of tiny seeds will fall out and resow.

Have questions? Email gardening@scng.com.