The myriad charts detailing plant combinations for companion planting are dizzying. There is more pseudo-science and anecdotal information available about companion planting than there is scientific evidence to support the age-old practice. In spite of this solid science stands behind certain aspects of companion planting.
Today scientists, industry professional and a growing number of gardeners favor a more scientific approach to the old time plant pairings. The focus is on growing a community of crops and promoting diversity in the garden. Called intercropping, yesterday’s companion planting is evolving.
Instead of the monoculture of rows and clumps of like plants that make up a conventional vegetable garden, we now create plant communities where plants can mutually benefit one another. Home gardeners are in a good position to take advantage of special interrelationships between plants. Raised beds, square foot gardening, wide rows and small organic gardens are good venues for intercropping because the plants must be planted in close proximity to have an effect on each other. The focus is on repelling pests, attracting beneficial insects and increasing yields.
Plants react to one another in three ways: chemically, physically and biologically.
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Many plants are chemically quite active. The legume family is a good example. Plants can’t use the gas form of nitrogen available from the air. Legumes take in unusable nitrogen from the air and convert it to usable nitrogen. Special nitrogen fixing bacteria in small nodules on their roots fix the gas turning it into the ammonia form of nitrogen which plants absorb. As legumes decompose they additionally enrich the soil with nitrogen they captured from the air, thereby benefiting neighboring plants.
Most plants produce defensive chemicals to help fend off insects and diseases. These chemicals may be poisons, act as feeding deterrents or have fungicidal properties. For example, chemicals in oak leaves retard the development of insects that feed on them. It is well known that black walnut trees release compounds toxic to other plants (a phenomenon called allelopathy). The smell of pyrethrum or thiophene in the leaves of some plants deters insect pests.
Many flying insects locate their host plant by smell. A monoculture (rows of a single crop) makes it easy for pests to find their favorite plants and quickly spread through the crop. However, a densely planted mix of plant species thwarts pests. Flying insects are confused by the smell of highly aromatic plants. Basil, rosemary, lavender and sage along with alliums, onions and garlic planted throughout a garden act to repel many insects.
Physically plants can benefit from close proximity below and above ground. Plants with deeper roots coexist well when planted next to low growers with shallow roots, like spinach, lettuce and coriander. Short shade tolerant crops grow well under tall sun-loving plants that may also provide wind protection or act as a trellis. Certain trees, maple for example, move groundwater from their lower roots to their upper roots where certain shallow rooted plants can grow under otherwise droughty conditions.
Trap cropping is a form of companion planting that uses decoy plants to attract insect pests and intercept them away from nearby desirable crops. It is most effective to surround the area containing a preferred plant so pests encounter trap crop first. Nasturtiums, for example, are known for trapping aphids as well as black, green and white flies.
Plant insect and bird friendly plants nearby a vegetable patch. Birds eat slugs. Hoverflies eat aphids, bees pollinate. Daisy-like flowers attract ladybugs. Borage attracts beneficial insects. Grow a habitat for beneficial predatory and parasitic insects that prey on the bad bugs. A continuous succession of blooms keeps the good bugs around season long.
There are reasons to avoid planting some plants near one another. Some have an allelopathic effect on others. Onions stunt beans and peas. Fennel stunts a number of plants. Certain vegetables should not be planted near each other because they attract the same insect pests. Tomatoes, pepper and dill all draw the tomato hornworm.
Each garden is different. Try combinations of your favorites. Keep in mind which plants may benefit each other.
Reach Debbie Menchek, a Clemson Master Gardener, at firstname.lastname@example.org.