Do you believe that your garden vegetables, herbs and flowers have a spiritual life? For now, you can simply answer yes or no.

Why does it matter? On the subject of companion planting, gardeners who see plants as metaphysical beings often follow companion planting charts which came into being in the 1930’s, when two interesting trends merged – one social and one scientific. Followers of the "spiritual science" of anthroposophy sought to relate to plants on an energy level. One such believer, Ehrenfried Pfeiffer, was also a chemist and gardener. Using a crude form of the procedure known as chromatography, Pfeiffer rated the energy levels of extracts taken from individual plants, and from various mixtures of plants. Pfeiffer believed that mixtures which created exciting chromatographs represented companionable (high energy) plant groupings, so he started making lists. Most companion planting charts seen today are based on Pfeiffer’s work, which took place in his lab rather than in his garden. By modern scientific standards, his methods were hopelessly flawed.

Does this mean that companion planting is worthless? Not at all! Few gardeners go a season without noticing some new benefit to companion planting in their gardens, and this is certainly true at my house. But instead of trying to capture ethereal energies that might be wafting about between plants, I use a common-sense approach to companion planting. Here are several companion planting tactics too logical to ignore.

Companion Planting for Diversity

A few years ago, researchers in the UK studied the effects of 24 companion plants on the egg-laying behavior of cabbage root fly and onion fly. Traditional thinking would suggest that aromatic plants like mint would be clear winners, but not so. Instead, the most useful plant for deterring cabbage root fly was lambs-quarters (the common weed). The power player against onion fly was a garden variety bedding geranium. In both cases, the protective companion plants distracted egg-laying flies, causing them to waste valuable time looking for the host plants they desired. The lesson here is that diversity, rather than a strange plant combination, is key to a healthy garden. Simply grow plants together that share common preferences for site, soil and season, and space them as closely as you can without crowding them.

Companion Planting to Attract Allies

Herbs and flowers won’t fill up your dinner plate, but having them in your garden can go a long way toward boosting populations of beneficial insects – your garden’s police force. Which insectary plants are best? Look to your own garden for answers by taking notice of plants that are abuzz with tiny insects when they are in bloom. In my garden, flowering Greek oregano and various thymes become beacons to beneficials; other gardeners report great success with sweet alyssum, which often blooms all summer. Keep your eyes open for plants small buzzing insects can’t resist. Some scientists think that when strong insectary plants are in bloom, pest insects within 50 feet (15 meters) are likely to be kept in check.

Companion Planting for Shade and Support

Most of the time we gardeners worry about plants shading each other too much, but in some situations a bit of shade is welcome. For example, you might protect a late sowing of lettuce from summer sun by growing it in the shadows of taller tomatoes. In South America, gardeners have long grown corn on the west side of potatoes. As summer heats up, shade cast by the corn helps keep the potatoes cool and moist.

The ways you can use companion plants to change light patterns in your garden are endless, or you might use companion plants as living supports. For thousands of years, Native Americans grew corn, beans and squash together, as many gardeners still do. Sowing climbing beans around the base of corn plants that are half grown gives you an instant trellis. Tall sunflowers make great trellis plants, too.

As you experiment with companion planting, let your own experiences be your guide. Charts and lists can lead you astray, as Nova Scotia researcher Tara Moreau learned when she tried various companion plants to deter Colorado potato beetles. Two of potatoes’ most highly rated companion plants, horseradish and marigolds, actually increased the number of beetles on the potato plants. This was not an isolated case. In Kentucky, entomologists looking for ways to deter Japanese beetles from roses found that several reputedly effective companion plants made the problem worse.

Don’t let this happen to you. To use companion planting to make your good garden better, trust what you see with your own eyes, and use plenty of good old common sense. If you’ve found a garden combo that works especially well, please share your experience in the comments section below.

- Barbara Pleasant