Companion Planting

Like the individuals that form any good (or bad) human relationship, plants also have chemical properties that can create mutual benefit or a toxic relationship when they’re grown together.

Like that mysteriously perfect marriage your friends seem to have, these companion plant arrangements may seem too good to be true, but solid scientific research supports what growers have learned over many generations.

All plants produce a range of waste products that are emitted through their foliage, stems and roots, which can chemically change the surrounding soil environment.  Certain plants -most famously herbs-but also other varieties including African and French marigolds, nasturtiums, and even tomatoes contain strong smelling substances that are emitted by their leaves that overpower the scents emitted by other plants, confusing insect pests.  The time tested practice of planting strongly scented herbs among a vegetable garden will most certainly help avoid and mitigate insect and animal pest problems.

Borage is a tall, aromatic herb, for example, that fits nicely in a tomato patch, and repels tomato horn worms. Basil deters flies that lay eggs on tomato leaves, and is reported to actually improve the flavor of tomatoes as well. The powerful scent of the wormwood plant overwhelms the sense of smell that many mammals rely on for survival, thus discouraging them from browsing through your vegetable garden. Are you losing plants to deer, but not ready to invest in a 7’ deer fence? Strongly scented yarrow, oregano, rosemary, and lavender and mint will discourage them. Puzzled over how to reign in a mole or gopher problem? Castor oil plant will do the trick.

Not only will many aromatic flowering herbs like thyme, mint, sage, rosemary and dill discourage harmful insect pests, but they’ll actually attract beneficial insects like parasitic wasps, preying mantis, and tachinid flies that prey on them and help control their population naturally. Herbs like bee balm, thyme and mint, as well as flowers like daisies, petunias, cosmos, sunflowers, and nasturtiums will also attract pollinating bees into your garden in addition to beneficial predatory insects.

Sometimes a taller companion plant simply offers a little relief in the form of shade to a shorter neighbor, as in the case of beet tops shielding lettuces from a scorching afternoon.

But the benefits of some companion plants go beyond scented foliage. The roots of marigolds, for example, exude a chemical that discourages harmful nematode varieties. If you’ve ever found a healthy plant suddenly wilt and die, only to dig up the roots to find clusters of ball like knots, you’ve seen the damage that soil nematodes can cause.

Sold on companion planting or at least curious enough to give it a shot?  The companion planting guide below lists beneficial plants along with problematic insects that each one deters.

Aster                                     variety of insects
Basil                                      flies, mosquitoes, asparagus beetles
Borage                                  tomato worms
Calendula                           tomato worms, asparagus beetles
Catnip                                   ants, aphids, flea beetles, Japanese beetles
Chives                                  aphids, Japanese beetles, whiteflies, mites
Chrysanthemum              Japanese beetles, Mexican beetles
Coreopsis                            variety of insects
Coriander                            aphids, carrot flies, Colorado potato beetle, mites
Garlic                                    aphids, borers, Japanese beetles, spider mites
Geranium                           cabbage worms
Henbit                                  variety of insects
Horseradish                       potato beetles
Hyssop                                 cabbage moths
Lavender                             moths
Marigold                             nematodes, Mexican bean beetles
Marjoram                            variety of insects
Mint                                      ants, cabbage moths, aphids, flies
Nasturtium                         squash bugs, beetles
Oregano                              cabbage butterflies, cucumber beetles
Parsley                                 variety of beetles
Radish                                  cucumber beetles
Rosemary                            bean beetles, cabbage moths, carrot flies
Rue                                        flies, Japanese beetles
Sage                                      bean beetles, cabbage moths, carrot flies, slugs
Sunflower                           aphids
Tansy                                    ants, fleas, flies, Japanese beetles, moths, squash bugs
Thyme                                  cabbage worms