Companion planting is an idea as old as gardening itself. When planted together, certain combinations of plants thrive, are healthier and have higher crop yields. These plants, be they herbs, flowers, fruit or vegetables, are said to “like” each other and be good companions. On the flipside, some combinations prove incompatible or make for “bad” companion plants. Seeing as plants don’t have feelings (despite some being known to sulk!), what does this really mean? Numerous lists of companion plants are widely available, many based on folklore and ideas passed on from gardener to gardener. Is there any scientific truth to companion planting?
One of the earliest examples of companion planting was put into practice by the Native Americans, who grew corn, beans and squash together, and called this “three sisters” planting. Grown together, the three crops thrive. The corn provides shelter and a frame for the beans to climb up. The beans, being legumes, take nitrogen from the air and, with the aid of bacteria on their roots, make the nitrogen available in the soil for all three plants to use. The large leaves of the squash plants act as a living mulch, shading the soil, keeping in moisture and preventing weeds from growing. So, this plant association is very successful and these three plants are “good companions”. They have similar growing needs, assist each other’s growth and don’t outcompete each other for nutrients, so all three thrive. This principle is true for all good plant associations and is the basis for companion planting.
Companion planting takes the “SNAP” approach to growing: plants are grown together to provide shelter (S), nectar (N) and alternative (A) food sources, such as pollen (P), to attract beneficial insects for pollination and to prey on insect pests.
Tall plants or those with dense canopies provide shelter for lower growing plants that need shade or protection from wind. Others provide nectar for insects that pollinate surrounding plants, leading to increased flowers, fruit and vegetables. The blue-flowered phacelia and lavender, for example, provide nectar for bees that are valuable pollinators and for hoverflies whose larvae eat aphids.
Some plants can be used to mask other, more desirable plants. The masking plant produces scents that confuse or deter insect pests so they won’t attack the crop plants. For instance, planting garlic near roses may deter aphids from attacking the roses. Many herbs, including tansy and hyssop, produce strong scents that repel insects, so are useful when planted among vegetable crops.
The use of marigolds to repel pests is one of the most accepted principles of companion planting. The African marigold’s distinctive smell repels some insect pests, such as aphids, but attracts hoverflies that eat aphids. In addition, the roots give off a toxic chemical called thiopene, which kills nematodes, minute roundworms that live in the soil and damage many plant species. So, marigolds are good companion plants in the vegetable garden.
Decoy or sacrificial plants can be used to lure insect pests away from crop plants. Nasturtiums are often used as they are attractive to a number of insects so may be attacked in preference to the crop plant. Insects, such as white butterflies, lay their eggs on the nasturtium leaves and the caterpillars eat these leaves instead of those of the crop plant. This works best when the decoy crop surrounds the preferred crop. This technique is also known as “trap cropping”.
Some plants are beneficial to their companions as they improve the general growing conditions. Members of the legume family, such as peas, beans and clover are examples. They make nitrogen, a plant nutrient available in the soil, for surrounding plants to use, so all benefit. This is called symbiotic (mutually beneficial) nitrogen fixation.
Providing plants and habitats in the garden where beneficial predatory insects can thrive will also be useful to all plants. These areas are sometimes called “refugia”. Many species of wasps, spiders, beetles, ladybirds and some flies are good predators of other insect pests, such as aphids, and should be encouraged by planting flowers in the Asteraceae family, such as daisies or cosmos, to attract them to the garden. By ensuring you create habitats that support beneficial insects you not only reduce pest damage to your crops, but also need to use less pesticides, which is better for the environment.
The secret to companion planting is biodiversity. Think old fashioned cottage gardens where vegetables, herbs, fruit and flowers were planted together. By planting a rich mixture of flowering plants and herbs among vegetables and fruit trees, you encourage a range of beneficial insects and birds to visit your garden and they are the best companions for your plants.