While humans, honeybees and ants are social creatures, plants are not. To suggest that trees and shrubs have companions is to misread the theory of plant associations.

Compelling evidence from science-based research supports the notion that plants aggressively seek allies and embattle their enemies, but more subtly than mobile creatures. They employ vegetative and chemical mechanisms to entwine, spray seeds, loom aloft and tunnel below, to out-compete or cooperate for nutrients, water and light. From the microscopic to elephantine, green growing creatures and some fungi, too, exploit beneficial and harmful connections to defend and befriend in their goal to grow and reproduce. Don’t we all?

For example, oaks welcome mycorrhizal fungi to their roots for a mutual exchange of nutrients, while walnut trees exude juglone to prevent competitors’ seedlings from gaining nearby dominance. Alfalfa, cucumbers, oats, rye, sunflowers and tobacco join the walnut, suppressing weed seeds in the soil but only after they all have withered and died—an advantageous legacy over the next generation’s adversaries.

Let us apply plant association principles to the same purpose: to conquer weeds, control disease and discourage pests, to replenish soil nutrients, to provide shade and moisture.

Many gardeners work seasonally to enhance and renew organic matter and minerals to the soil. If this sounds familiar, try the legumes, clover or rye, at summer’s end for a nitrogen boost with added erosion control. Sow no later than Nov. 1 and till-under at spring flowering, before seed sets.

Partner plants that exchange benefits. Site shade-loving, moisture-hogging shrubs, perennials and annuals beneath maple trees, which transport groundwater from their lower to upper roots, where it seeps through the topsoil.

To eliminate root damage to susceptible fruits and vegetables (eggplant, pepper, squash, tomato) terminate harmful nematodes (microscopic roundworms) by preplanting a dense harvest of French and African marigolds or interplant thickly during the growing season.

For insect pests that detect their buffet by fragrance, confuse with more interplanting. Select strong scented herbs and vegetables (chives, garlic, leeks, onions, oregano, rosemary, sage, shallots).

Experiment with a trap-crop, luring targeted insects from their preferred feast, then spray or hand-gather from the green-bait. To divert Japanese beetles from your roses, excite them with basil.

By their chemistry some plants repel insects harmful to others. Basil also deters tomato hornworms, mosquitoes and flies from its culinary “companion.” Catnip ambushes flea beetles and ants on their way to eggplant. Horseradish ensnares potato bugs.

Use polyculture or diversity to reduce damage. If you must have heaps of tomatoes or azaleas or other beloved plants, then choose from varied hybrids or heirlooms; use staged growing habits and unique disease resistance.

Recruit pollinators (honeybees, butterflies) and insect predators (wasps, flies) with angelica, carrot, coriander, dill, parsley, and parsnip with their pungent, flat-topped flowers.

Two things to remember about “companion” planting: Avoid lists with no evidence-based science to support their claims; begin your own rigorously documented observations and tests of traditional folklore. Muster some human companions to replicate your results. Publish on a social network. Your gardening friends will certainly get back to you with their field trial results, supporting or rejecting your data.

TIP OF THE WEEK
Control fire-blighted apple, crabapple, cotoneaster and pyracantha during dry weather. You’ll need bare hands or disposable gloves, twine and a 12-ounce, wide-mouthed jar, half-filled with isopropyl alcohol. Cut 6 inches below the damage, but after each cut; dip the pruners in the alcohol. Burn or bundle the prunings for the landfill. Lubricate the shears when you’re finished.