A simple definition of companion planting refers to grouping plants with similar physical needs or characteristics together. A much more nuanced approach includes cultivating a diverse plant community and encouraging natural symbiotic relationships. Certain plants, when grown together, provide mutual benefits that increase crop yields or reduce the need for synthetic fertilizers, pesticides, and herbicides.
“Stacking” plants takes advantage of vertical space. Most gardeners plant one crop in one space (think square-foot gardening), but combining multiple crops of varying heights and habits actually makes better use of that area, thereby increasing your yields. The “three sisters” are a perfect example. Corn acts as a trellis for pole beans, while the squash ground cover retains soil moisture and suppresses weeds.
Trees and shrubs are great windbreaks for sensitive planting areas, as long as they’re not blocking access to sunlight. Planting shade-loving crops in the shadow of sun-loving crops keeps both parties happy. Cool season greens can grow throughout the summer if provided with afternoon shade.
Soil fertility and structure
Nitrogen-fixing legumes such as beans, peas, clover, and alfalfa have a special bacterium that pull nitrogen out of the air and turn it into a form plants can use. While these plants release very little of that nitrogen into the soil while actively growing, they become a long-lasting supply if tilled under or left to decompose. Try planting a cover crop of snow peas or clover, and you’ll be greatly rewarded for years to come. Root crops can also be useful in breaking up hard, clay soils for their fibrous-rooted partners.
Repellent - Some plants exude chemicals from their roots, leaves, or flowers that repel certain pests, protecting themselves as well as neighboring plants (lavender, chives, garlic, oregano, and thyme). Use these as a border around desirable crops.
Camouflage - Insects identify food sources by smell and appearance. Heavily-scented species like dill, chives, parsley, catnip, mint, basil, and marigold all make desirable crops harder to locate; while interplanting differing forms and shapes confuses would-be pests.
Trap crop - Certain plants are more attractive to pests, so you can place these in clumps near the crops you’re trying to protect as a lure. Simply spray or destroy the trap crop once it’s become infested.
Magnet for the good guys - Beneficial insects like lady beetles, praying mantises, and parasitic wasps help keep the peace in your garden by consuming pests. Attract them by planting fennel, parsley, carrots, cosmos, sweet alyssum, tansy, and chamomile.
Inversely, there are also plants that should NOT be grown together - whether it’s because they require different moisture needs, attract the same kinds of pests, or exude chemicals that stunt each others’ growth. We’ve tried to make these relationships more clear by compiling into a master chart, but remember, it is just a guide, so feel free to adjust according to your own experience. No two gardens are the same. Click the chart below to print, or get the full handout here.
Did you know?
Successful companion planting naturally increases biodiversity, protecting your garden against depleted soils as well as insect and disease problems. Large swaths of the same plant (monocultures) are much more attractive to pests than interplanted crops. They’re also more vulnerable to disease, as one virus/fungi/bacteria strain has the potential to wipe out an entire field. Healthy ecosystems include a substantial variety of species that can withstand natural pest and disease populations.
Lisa Kronwall, WGC Horticulturist