Introduction to the Rules of Companion Planting

Although many a table has been drawn up to indicate which plants prefer which neighbors, there are a few good rules of thumb for deciding where to plant which crops.

Group Shallow-Rooted Plants with Deep-Rooted Plants

In general, shallow-rooted plants will compete for moisture. Companion plants should complement each other, not fight over soil and nutrients. Thus, shallow-rooted plants are often grouped with deeper-rooted companions. The famous pairing of basil and tomato is a good example of this.

In some cases, plants with taproots are used to break up the soil for other deep-rooted plants. For example, carrots and tomatoes are frequently planted together, to the benefit of the tomatoes (although not the carrots).

Do Not Group Plants Which Are Prone to the Same Diseases or Pests

Plants can infect each other. Potatoes and tomatoes, which are both prone to blight, should always be planted away from each other for fear of spreading the infection from one species to another. If one species does become infected, a completely separate crop (ie, neither tomatoes nor potatoes) which is not affected by that disease should be planted in that area of garden the following year.

In general it is wise to avoid planting two species which attract the same pests side by side—this tends to provide an all-you-can-eat buffet for the creatures. The exception is the use of trap crops, which are designed the lure the bugs away from the more important crop. This is one reason for growing climbing beans up sunflowers—aphids, which would ordinarily attack beans, instead feed off the sunflowers. The tough flowers suffer very little damage, and the beans remain unscathed. Nasturtiums are another trap crop, often grown with sunflowers.

Group Plants Which Attract Beneficial Insects

The flip side to the above rule is to let plants woo beneficial insects for each other. Lavender is good planted next to most veggies, as it attracts bees which then help with pollination. Dill, fennel and sunflowers all attract pest-eating ladybugs; parsley and thyme attract tachinid flies, which act as parasites to a number of garden pests.

Use the Structure of Plants to Your Advantage

While there are often good chemical reasons behind companion planting, the physical structure of plants is an important reason for grouping some species together. Native Americans planted a triumvirate of corn, squash and beans—the tall cornstalks provided a climbing pole for the beans, which in turn fixed nitrogen in the soil for the corn; while the low-lying squash acted as a ground cover, choking the weeds out.

Climbing or tall plants can screen shade-loving species from a hot sun, while roots such as carrots or parsnip make efficient use of spaces between shallow-rooted plants.

Don't Be Afraid to Mix Veggies with Flowers

The back yard/front yard, edible/ornamental distinction is purely artificial; any natural environment features the two happily mingling. Many of the best companion planting combinations feature flowers and vegetables; in fact, Roses Love Garlic is the name of a popular companion planting manual! A mixture of herbs, flowers, fruits and vegetables makes for a beautiful and productive garden—in fact, you may even find your non-edibles are edibles after all. Lavender, marigolds and nasturtiums are companion planting staples, and all have edible parts and species.

Sunflowers can act as a trap crop and support

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