Companion planting is about the interactions of different plants growing in close proximity. Allelopathy is based on the principle that certain plants can attract or repel insects or provide beneficial support to other plants. It can also work the other way around where one plant can be detrimental to another’s growth. Many invasive plant species interfere with native plants through allelopathy. The leaf litter and root exudates of some Eucalyptus species are allelopathic for certain soil microbes and plant species. The tree of heaven, Ailanthus altissima, produces allelochemicals in its roots that inhibit the growth of many plants.
The science of companion planting is in its infancy: we are only just beginning to discover and to understand how many interesting and useful interactions there are between plants, animals, insects and the environment in which they live.
Today’s garden trend is to compartmentalise the gardens – vegetable patch, rose garden, flower garden, herb garden – but for the best companion effects flowers and herbs need to be cultivated amongst vegetables and fruit. The cultivated area doesn’t need to look like a farmyard; with careful planning the gardened area can be very attractive.
Examples of companion planting include chives growing under roses, with the chives helping the roses to resist black spot and increasing their perfume, and onions that fail to flourish if planted with beans. Nasturtium is reputed to be a food source of some caterpillars which feed on members of the cabbage family: eggs of the pests are preferentially laid on the nasturtium, on which they hatch and feed. The smell of marigolds is claimed to deter aphids from feeding on neighbouring plants: the flowers attract hoverflies in search of nectar, the larvae of which are predators of aphids.
A recent visit to some open gardens in the UK was an eye opener. Although many of the principles of companion planting were present in cottage gardens centuries ago, only recently British gardeners have again embraced the concept of companion planting and natural gardening in a big way. Native plants are thriving in meadow gardens, providing food and shelter for wildlife, at the same time feeding the soil. These ‘corridors’ promote diversity of plant and insect life.
In the Western Cape the survival of our magnificent floral biodiversity is dependent on providing corridors where insects can move safely and freely. There is no reason why our gardens can’t be a corridor, why we can’t help nature along if we plant the right plants that offer food and shelter. Give nature a helping hand; learn about natural principles of insect control, about plants own protection mechanisms, and about the dire consequences of invasive plant species, pesticides, herbicides and insecticides. By doing all the right things you will also be a good companion.