Companion Planting: The Do's and Don'ts of Growing Plants Together

Knowing which plants complement each other, and which don't, is part of traditional gardening knowledge — but it's firmly based in science. It works because of the different ways plants interact with soil and pests. All plants require nitrogen, but some, like leafy greens, lettuces and Brussels sprouts, need more of the element than others.

There are lots of ways to add nitrogen to the soil, but using companion planting, you can have other vegetables do it for you. Nitrogen-fixers, including peas and beans, actually deposit nitrogen back into the soil. One of the basic companion planting tenets is to plant nitrogen-needers next to nitrogen-fixers.

When it comes to pests, one of the best defenses is an herbal remedy. Lots of herbs deter bugs that feed on vegetable plants, and attract natural predators and pollinators. Some of the best for this are umbelliferous plants, like dill, cilantro and parsley, which bloom with clusters of small flowers on short stems. They attract ladybugs and wasps, which are the primary predators of veggie eaters like aphids and cabbage moths.

One of the earliest and best-known examples of companion planting is what the Iroquois called "the three sisters": corn, beans and squash. They were three of the first domesticated crops in North America. Corn was a primary source of nutrition, and the Iroquois believed it had to grow in "community," rather than alone.

The three plants are ideal companions. The cornstalk grows quickly, creating a natural pole for the bean plant to climb. The squash spreads its leaves and shallow roots around the base of the bean and corn plants, keeping down weeds and shading the soil. The beans provide nitrogen that helps the squash grow.

Some vegetables and herbs just seem like they're meant to be together. This is especially true for tomatoes, basil, peppers and parsley. In other words, salsa ingredients!

This is a classic companion planting formula, because the veggies and herbs offer each other mutual benefits. Basil repels pests like tomato hornworm and aphids, while parsley attracts parasitic wasps and ladybugs. Basil grows better when planted alongside tomatoes and peppers and tomato plants that have basil growing near them produce more fruit.

Peppers and tomatoes are more flavorful when they've been grown with basil and parsley. And, of course, the best bonus of all is that these four taste great together on the plate.

There are a few varieties that happen to be almost universally good companion plants, and they have the added bonus of giving your garden a boost of color. Nasturtiums and marigolds, which flower in shades of yellow, pink, orange and red, both make fantastic pest repellants.

Marigolds keep away deer and rabbits that might otherwise snack on your veggies; they find the flowers' odor unpleasant.

Nasturtiums also help with pest management, but in a different way. Bugs like aphids and cabbage moths love the flowering plants, which is a good thing when you're trying to keep them off your vegetables. The pests prefer the sacrificial nasturtiums, so you want to plant them close — but not too close — to things like tomatoes, kale, cauliflower and broccoli.

If you've ever grown a zucchini or summer squash plant, you know that by midsummer the plant is taking up quite a bit of garden real estate with thick stalks and wide leaves. Take advantage of the shade by planting greens like spinach — which usually can't stand up to the heat and sun of high summer — in the squash's shadow.

Be aware of what else you plant near squash and zucchini, though. Other vegetable varieties, like pumpkin, can cross-pollinate a zucchini or squash plant and affect its production for the rest of the season.

While there are many plants that grow well as companions to members of the allium family — think onions, garlic, scallions and chives — sometimes they make bad neighbors.

Beans and most other legumes can be stunted if they're grown too close to garlic. The same is true for strawberries. Parsley and sage will both stunt garlic, and asparagus and garlic don't do each other any favors: The garlic can stunt the growth of asparagus shoots, and the asparagus, in turn, can make the garlic taste funky.

Onions are also no friend to legumes. They have some natural antiseptic qualities that make them great for when you have a cold, but an enemy to the beneficial bacteria on the roots of beans and peas.

Alliums also don't do well when planted with their own kind. There are a number of allium-specific pests, and if they're planted too close together, it makes spreading infection or infestation a little too easy.