GARDENING : Cultivating Friends: Plant Buddies Look Out for Each Other

Companion planting allows the gardener to become a matchmaker with plants, turning strangers into fast friends. The practice of companion planting goes back centuries, driven by the abiding belief that some plants do better when grown near other species, either for protection from pests or as a result of a chemical interaction beneficial to at least one of the partners.

The technique is unproven scientifically, although in its quaintness it may have merit, especially because each gardener’s preferences provide gardens of diversity and character.

In one plot, the aim might be to attract birds, in another, to repel raccoons. Companion planting in the modern garden is generally a casual undertaking, with few rules and an openness to new ideas and combinations. In fact, one of its basic truths is that where a combination might fail in one garden, it could prove indispensable in another.

Most people, whether they are new to gardening or have many seasons behind them, are familiar with the marigold’s reputation for warding off all manner of insect pests above and below the ground. Although this reputation may be overblown, it goes to the basic concept of companion planting: Plant marigolds among your vegetables, and you’ll have fewer bugs.

Marigolds, in field tests and in lab trials, seem to have a pretty good record in controlling damaging subterranean nematodes that invade plant roots.

But in most cases, no one knows why something does better when planted near something else. One gardener swears that planting radishes around her beans controls Mexican bean beetles. I have tried this and have seen little effect on the voracious bean pests.

While the variables from one garden to the next will dictate much of how successfully companion planting works, there are some fundamental principles that are universal and have been proved:

* Don’t plant tomatoes, eggplant and potatoes close together. All members of the nightshade family, they tend to share pests. Thus, if the potato beetle finds your eggplant near the potatoes, the pest probably will afflict the eggplant. Some gardeners find that eggplant is more vulnerable to beetles than are potatoes.

* Don’t plant parsley, carrots and cilantro near each other. These also share a family tree and the pesky carrot fly. Its grub prefers to tunnel into the carrot but will go to parsley and even cilantro if they are close at hand.

* Do interplant onions, radishes and even--yes--marigolds with other garden varieties. Apart from the practical aspect of sneaking space-saving onions and radishes among larger plants--they rarely compete with companion plants, and they do very well in the smallest of spots--they emit an odor that is repugnant to a wide range of insect pests. Ordinarily, radishes and onions are pulled before they bloom. However, if they are left to mature, their flowers are most attractive to predatory insects that prey on pests.

* Do combine dissimilar species. Eggplant with beans; corn with potatoes; lettuce and basil with tomatoes.

* Do intermingle herbs with vegetables. With the exception of fennel, nearly all herbs offer aid to garden plants by attracting predatory insects, repelling pests or simply providing a compatible growth habit.

* Do interplant flowers with vegetables. These will attract birds and insects that prey on many pests. The classic example of this is nasturtium, with its peppery, edible foliage and its aromatic, edible flowers, themselves a magnet for hummingbirds.

* Worth a try: Plant bush beans among tomatoes. I did this a couple of years ago and was delighted with the beans’ ability to keep down weeds. In addition, they will fix nitrogen in the soil, providing an important nutrient to tomatoes. As a living mulch, low-growing bush beans ensure an even moisture content in the soil, important in the prevention of blossom-end rot in tomatoes. One caveat on this companionship, however: My bean crop was poor, I think because the tomato vines robbed the beans of their full measure of sunlight.

* Worth a try: Combine corn with pumpkins, melons or other climbing squashes. The rough foliage of the squashes is reputed to dissuade raccoons from invading the corn at the time ears are ripening. The corn is thought to deter the squash vine borer moth from laying its eggs on pumpkin vines. The squash shades the ground around the corn and even climbs up stalks but does not hinder the corn’s growth and development.