Companion planting is based on the observation that some plant seem to have synergistic relationships with certain other plants One or both of them grow better, yield more, and sometimes even taste better when they grow near one another. I have found, however, that there's a lot more to companion planting than simply pairing up plants that benefit from one another's company. My wife Sylvia and I have spent years trying different combinations, and our garden now looks quite a bit different from the garden we had in our first companion planting years. In fact, every year's garden is an experiment. I can't take you on the whole companion journey, because we're still en route ourselves. But I can show you how we started while sharing some of things we learned along the way.
Eliminate the Negative
Companion planting begins with the liberating notion that we can sow different plants in the same garden space. It's important to remember, however, that while certain plants will help others thrive, some plants inhibit their neighbor's growth. The first step, then, in planning a companion garden is to eliminate the combinations that don't work. This is especially important if you plant in beds rather than in rows.
In a traditional garden, the root systems of plants in adjacent rows are kept apart by a wide band of compacted soil that roots cannot penetrate. In beds, on the other hand, plants may be only inches apart, so roots can spread and intermingle. Thus, it is particularly important to avoid antagonistic relationships — those in which the growth of either or both plants is inhibited. Antagonists should have at least six feet between them in a bed. Better yet, put them in separate beds.
You can avoid many problems by keeping a few entire plant families away from each other. For example, members of the cabbage and onion families don't get along with members of the tomato and pea families.
Sometimes, plants within families don't grow well together. They may be afflicted by the same pests or compete for the same nutrients. Close cousins carrots and parsnips don't get along; neither do potatoes and tomatoes. A cabbage planted among broccoli will attract cabbage moths as though it was among other cabbages.
Some insect pests have a taste for several different kinds of plants. Pairing them up offers predatory insects a one-stop smorgasbord. Take care to avoid the following combinations:
Corn and tomatoes Call it a corn earworm, call it a tomato fruitworm — it's the same beast in either case and likes both crops equally. Keep some distance between the corn and tomato patches.
Potatoes, tomatoes, peppers and eggplant The striped Colorado Potato Beetle is a nightshade nightmare and will feed on any plant related to potatoes, such as tomatoes, peppers or eggplant.
Cucumbers, squash and melons Pickleworms prefer cucumbers, but will happily munch their way through a nice squash or melon.
Accentuate the Positive
Once I have a list of plant combinations that are likely to fail, I look for the opposite: plants that thrive side by side. Plants work in synergy for a number of reasons. Some plants provide a better physical environment for one another in terms of nutrient-, sun- and shade-sharing; some plants act as natural insect repellents while others attract friendly insects; and some beneficial combinations are simply inexplicable mysteries.
The Summer Sandwich Sometimes we plant things together just because they're likely to be harvested together, such as basil and tomatoes-complementary ingredients for a salad or sandwich. There are other reasons, however, why this might be a good pairing. Tomatoes are big, heavy-feeding plants with deep root systems; basil plants are relatively small, light feeders with fairly shallow roots. Both plants, however, are very sensitive to cool temperatures at either end of the growing season and need similar protective coverings, such as plastic tents or row covers.
The pest-repellent nature of certain plants has a counterpoint: Some plants attract insects that are beneficial to the garden
Deep-rooted plants thrive near shallow-rooted neighbors because they draw nutrients from different places. Thus, car rots make suitable companions for leaf lettuce, arugula, garden cress and onions.
This pairing works the same way aboveground: Plants of similar heights may compete for sunlight, while those of different heights can share. Corn and potatoes can grow peacefully in adjacent beds, but corn and indeterminate tomatoes will shade one another.
Plants that require most or all of the growing season to mature can have short-season companions. We sow a bit of spinach between the rows of garlic soon after the garlic shoots emerge in the spring. While it's there, the spinach provides a living mulch for the garlic, discouraging weeds and conserving moisture in the soil. By the time the garlic needs all the space in the bed, the spinach is long gone. Likewise, a small sowing of radishes, which will detract some pests from more fragile plants, can go almost anywhere in the garden and be up and gone long before it competes with slower-maturing companions.
Sow Confusion with Interplanting
Plant-eating bugs and some plant diseases love gardens laid out in rows. Carrots here, tomatoes there, and all the onions together somewhere else offers pests a minimall of end-to-end munching.
Mixing things up makes it harder for diseases or pests to gain a toehold. An onion disease harbored by some of the onions grown from a set will not infect plants grown from seedlings if the seedlings are in a separate bed some distance away. Cabbage butterflies that have to flit all over the garden in search of cole crops will get more predatory attention than butterflies that can settle in one bed. The greater the distance any bug has to cross from one feast to the next, the more likely it will itself become prey.
Better Home in Garden
One of our favorite companion planting combinations didn't come from a list, but from figuring out through observation how two plants might meet one another's needs.
When we started growing peas in wide beds instead of narrow rows, we created a situation that begged for a companion planting experiment: The peas did not need the space a 30- or 36-inch-wide bed had to offer. At about the same time the peas began to climb their trellis netting, a batch of lettuce seedlings we started in the greenhouse were ready to move to the garden. We paired them up in the bed and found the combination quite beneficial to both. The peas and lettuce didn't compete for sunlight — a bit of shading from the peas actually helped the lettuce stay cool later in the season so it didn't go to seed as quickly. The lettuce acted as a living mulch for the peas, keeping the soil moist and cool for the pea roots. Peas grown this way were at least as productive as those grown alone, and they produced over a longer season. The lettuce matured quickly, producing very large heads that were both tender and tasty.
We have refined the combination over the years. At first, we planted the peas down the center of the bed with lettuce on both sides. The lettuce on the shady side grew more slowly, but it ended up getting about as big and looking almost as nice as the lettuce grown on the sunny side. But it didn't taste as good. The shady-side lettuce heads were older (and hence a bit tougher) and not as sweet. Now we plant the peas closer to the northern edge of the bed — and the lettuce all goes on the southern side.
Adding flowers to the vegetable and herb mix in a garden does have one possible negative effect: Some flowering plants might contain poisonous elements, which you don't want to harvest along with the food plant. None of the flowers we use or recommend are poisonous, but you're probably going to want to try your own combinations. If you want to experiment with combinations not on this or another list, check to make sure the plants you try aren't dangerous. Some examples you're likely to encounter are lily of the valley, pasque flower and monkseed.
Companion Planting for Pest Control
Certain bugs favor specific plants and seek them out to feed upon. Those same bugs find other plants distasteful and avoid them. If we mix the plants bugs deem tasty with ones they don't care for, the bugs will become confused and avoid the plants they like along with the ones they don't.
All plant-eating bugs avoid members of the onion family — probably because they stink. We don't know if the odor repels pests, or if the onion-family aromas mask other plant smells and prevent pests from finding them. In any case, most garden plants will be less bothered by insects if they live among onions or their relatives. Onions, chives and garlic repel ants, aphids and flea beetles. Garlic offends Japanese beetles, vegetable weevils and spider mites.
Celery deters the white moth that begets the green caterpillar that eats cabbage, broccoli and other cabbage family crops. All heavy feeders with shallow root systems, they'll need extra compost.
In adjacent beds, beans and potatoes make mutually beneficial neighbors. The beans repel Colorado Potato Beetles, and the potatoes reciprocate by driving away Mexican Bean Beetles.
Radishes should go anyplace flea beetles are a problem, since flea beetles would sooner eat radish leaves — especially young and tender ones — than they would any other leaves. Although radishes may suffer from defoliation, they can withstand greater damage than most. They can also deter striped cucumber beetles from cucumbers.
When an Ounce of Prevention Yields A Mess
Mint discourages cabbage moths and ants, and horseradish repels bean beetles, but both mint and horseradish are hardy perennials that, left uncontrolled, will quickly take over your garden and become pests themselves. You can still use mint and horseradish as insect repellent companions in the garden, but make sure they are securely contained. Plant them in large clay pots and set the pots in the appropriate beds of vegetable plants.
Say it with Flowers
Many plants give off volatile chemicals that are attractive to the pests that feed on them. If you grow stronger-scented plants among your vegetables, you may be able to disguise or hide the crops' scents and keep the pests from finding them. Some of the possible strong-smelling companions are other vegetables, but a lot of them are herbs and many more are flowers. We began to make our biggest breakthroughs in companion planting when we started combining our vegetable, herb and flower gardens.
Marigolds are useful against aphids, Colorado Potato Beetles and whiteflies.
Rosemary deters carrot flies and cabbage moths; sage repels both.
Hyssop, wormwood, thyme and sage all repel the cabbage moth.
Rue and white-flowered geraniums are offensive to Japanese beetles.
Tomato hornworms don't like borage or basil.
Marigolds repel whiteflies, tomato hornworms, bean beetles, cucumber beetles and asparagus beetles.
Neither squash bugs nor whiteflies can abide nasturtium.
Luring the Helpful Insects
The pest-repellent nature of certain plants has a counterpoint: Some plants attract insects that are beneficial to the garden.
Flowers of all sorts and some herbs attract pollinating insects, and the more pollinators there are in your garden, the more likely that fruiting vegetables will be pollinated in a timely fashion. Borage and cleome lure bees; dill, fennel, cosmos and marigold attract butterflies.
Some pest-eating predators, like lady beetles, are helpful to the garden both as larvae and as adults. Others, like hover flies and lacewings, as larvae, feed on insects, but as adults subsist on nectar or pollen.
Lady beetles like morning glory vines, angelica, coreopsis, cosmos, sweet alyssum and tansy.
Hover flies — the larvae of which eat aphids, various beetles and caterpillars, leafhoppers, mealybugs and thrips-like chamomile, asters, cornflower, black-eyed Susans and marigolds.
Many of the flowers that attract or provide shelter and food for helpful insects are perennials — a fact you can use to your advantage by locating your perennial garden near your vegetable garden.
Very little is known about why some of these plant relationships are antagonistic or beneficial. It is clear, for instance, why carrots benefit from proximity to onions, leeks, rosemary, wormwood or sage: Carrot flies, whose larvae attack young carrot roots, are repelled by the aromas those plants give off. But why do carrots paired with tomatoes or lettuce grow to mutual benefit? Why do celery, parsnips and dill inhibit carrots? Nobody knows for certain.
I have a few general rules I use to evaluate plant combinations, then I check my results against the lists. Consider all the advice, including the suggestions here, as a starting point, rather than gospel. Keep track of the results and compile your own list of garden companions. Knowing the companion lists are mostly somebody's guesses, I'm inclined to use my own best guesses as a basis for garden experiments.
The Best Reason of All
Shortly after sunrise, dew still shimmering on the asparagus ferns, I head for the garden, coffee cup in hand, to see if there's a morning glory blooming in the corn. There is. So long as I'm there, I may as well pull a few weeds, check for Colorado Potato Beetles, and maybe thin the carrots.
Companion planting, particularly with flowers, has made our garden a more pleasant place to be and enticed us to spend more time there, which has had a beneficial effect on all that grows there ...and on us.
Ed Smith gardens with his wife, Sylvia, and their children in northern Vermont. When he's not gardening or writing about the subject, Ed makes cabinets and furniture and teaches in Goddard College's Sustainable Living Program. Ed wrote The Vegetable Gardener's Bible (Storey Books, 2000).