Companion planting can be thought of as choosing a mixed and good neighborhood for your plants. Rather than planting marigolds in the flower garden, basil in the herb garden, and cabbages in the vegetable garden, you grow flower, herb, and vegetable plants together. And no solid blocks — mingle the cabbages freely with the tomatoes, the cucumbers with the corn.
The folklore on companion planting cautions against mixing plants too freely, though. Some plants might be hostile toward one another. Although they love the company of beets, you’re supposed to keep onions away from beans. Similarly, cucumbers love beans, but dislike potatoes. The likes and dislikes of plants are reflected in how well a plant grows, whether pests will attack, even how a plant tastes!
The whole idea of companion planting is so appealing. It’s almost as though plants were like, er, people. But folklore is often at odds with science. At its best, science tells us what really happens, not what we wish would happen.
As it turns out, companion planting is a mix of fact and fiction.
First, the facts: Science has shown that some plants do, in fact, dislike each other. The best known example is black walnut, which can put a natural chemical, juglone, into the soil that is toxic to many other plants, most notably tomatoes.
Rye is another example, a residue of which in the soil temporarily inhibits the germination of small seeds. And did you ever notice dead lawn beneath your bird feeder? That’s because a chemical in sunflower hulls is toxic to grass.
Science also has confirmed that the makeup of a plant community does, in fact, have an influence on pest problems. Just imagine that you are a female cabbage butterfly, flying around in search of cabbage leaves on which to lay your eggs. Which cabbage plant would you find first: one from an isolated row of cabbage plants, all staring skyward against well-tilled, brown soil; or one from a group of cabbage plants rubbing elbows with tomato plants, with colorful marigolds and aromatic mint plants tucked in here and there? You would think that it would be most easy to pick out a plant from that isolated lineup. Yes, it’s so.
Visual confusion is the main force at work in thwarting those cabbage butterflies (Astroturf would also probably be effective), but other forces might be at work in other situations. Aromatic plants might repel insects or mask the aromas of potential host plants. A plant particularly tasty to an insect might be grown as a sacrificial crop: radishes for flea beetles, nasturtiums for aphids. I perhaps began planting nasturtiums in my vegetable garden for aphid management, although I pretty much never saw nor see aphids on my nasturtiums or vegetable plants. I still grow the nasturtiums there because the plants are pretty — and tasty, both the leaves and the flowers.
Another plant family prominent for fostering beneficial insects are the daisies. Many of these plants, including cosmos, coneflowers, and coreopsis, sport nectaries on their stems and leaves. What a treat for beneficial insects!
Other plant families also attract beneficial insects in varying degrees. When deciding where to plant these plant attractants, don’t be straitjacketed by convention. There’s no reason not to also plant flowers in the vegetable garden or to also plant vegetables in the flower garden. The clumps of Little Gem marigold at the head of my vegetable beds dresses up my vegetable garden and keeps beneficial insects nearby.
Now, the fiction. First, most of the details of companion planting — the specific congenial and antagonistic plant associations — are unsubstantiated. Onions really do not dislike or grow poorly near beans; and beans don’t grow better near carrots.
Most of the alleged benefits of herbs in repelling pests also are unsubstantiated. Planting chives at the base of your rose bush will not thwart aphids (Are aphids even a problem on roses? Never on my roses). And basil plants will not enhance the flavor of tomatoes growing nearby, even if they do make a tasty combination in sauce. Nonetheless, my basil plants are intimate with my tomato plants; I grow them so that I can conveniently pick off a few basil leaves while I grab a couple of tomatoes for sandwiches.
A second problem with companion planting is that even if a particular plant combination does repel a pest, growing the two plants close together might cause more problems than the potential pest. Planting snap beans and potatoes together has been shown to reduce the numbers of Colorado potato beetles — but also the yields of potatoes and beans. I once planted mint between cabbage plants, as prescribed to foil cabbageworms. By the end of the season, a mesh of underground mint stems were threatening to grab hold of my entire garden. I spent more time getting rid of the mint than I ever did controlling cabbageworms (by either handpicking or spraying with an environmentally benign spray containing the bacterium Bacillus thurengiensis).
Nowadays, my cabbages do still socialize, but with beans, lettuce, and other plants convenient to grow between, before, and after the cabbages. And I have strategically planted tansy and yarrow near the vegetables, mostly for looks. Although most of companion planting advice is bunk, it generally does no harm. And diversity is good in the garden, both for looks and for a balanced ecology.
New Paltz writer Lee Reich, author of The Pruning Book, Weedless Gardening, Growing Figs in Cold Climates, and other books, is also a garden consultant specializing in growing fruits, vegetables, and nuts. He hosts workshops at his New Paltz farmden. For more information, go to www.leereich.com.
Join the family! 
