Companion planting – a little help from their friends

Almost every home gardener has heard claims of aromatic plant species repelling insects and that some plant associations are more compatible than others.

By using certain plant-insect relationships, it may be possible to minimize their chances of attack by insect pests.

Some believe that less pest damage will occur in gardens containing a variety of plant types because the multitudes of stimuli produced by mixtures of plants may cause the insect to become disoriented, and disrupt its feeding and breeding cycles.

From a scientific viewpoint, not very much is known about these interactions, although the underlying premise is sound from an ecological perspective.

What is “companion planting”?

Companion planting refers to the interplanting of two or more plant species in close proximity. These may be vegetables, annual flowers, herbs or perennials.

Companion planting attempts to mimic the diversification found in nature, which for the most part, creates a balance of insect and plant populations.

It is commonly thought that there are at least five ways in which one plant can influence a neighboring plant.

• It can attract insect pests away from their target,
• it can repel animal or insect pests,
• it can interfere with the growth of an adjacent plant by out-competiting it for light, nutrients or water,
• it may attract beneficial insects which can control pest insects,
• and it can improve the health (and some say flavor) of nearby plants.

The most familiar concept in companion planting is the use of aromatic plants such as herbs and marigolds. These are interplanted with a specific crop in an effort to offer it some protection from insect pests. For instance:

• summer savory when interplanted with beans is said to deter Mexican bean beetles.

• Hyssop, thyme and members of the mint family reportedly discourage the white cabbage butterfly from laying eggs on broccoli, cabbage and other brassicas.

• Calendulas are thought to repel asparagus beetles and tomato hornworms.

• Horseradish is reputed to be an effective deterrent to Colorado potato beetles

• and probably everyone has heard that certain marigold species can reduce nematode populations. There is even a marigold cultivar called ‘Nemagone’.

Vegetables can also be interplanted with each other.

Planting potatoes next to squash hills is supposed to eliminate squash bugs.

Tomatoes in your asparagus bed are said to foil asparagus beetles and members of the onion family are thought to discourage the carrot rust fly.

Also, during the heat of summer, vegetables that like cooler temperatures, such as lettuce, could be planted in the shade of taller corn or pole beans.

What is “alleleopathy”?

Scientists have also known for years that some plants have alleleopathic abilities. Alleleopathy is the process by which a plant produces certain chemicals that can affect, usually in a negative way, the growth of another plant.

The classic example of this is the black walnut tree that produces the compound juglone. Few plants can grow under a black walnut tree and it is thought that the juglone is largely responsible.

These same types of compounds, however, may render your plants less palatable to hungry insects or animal pests. For example, while no plant is 100 percent immune to deer feeding, they usually stay away from aromatic, silver-leaved plants such as artemesias.

Another facet of companion planting involves the use of plants that attract beneficial insects.

Many of our flowering native wildflowers provide food and shelter for beneficial insects such as:

• parasitic wasps,
• predatory beetles,
• flies
• and mites

Low growing ground covers, for example, provide a home for ground beetles and spiders that feed on slugs, aphids and caterpillars.

Other plants such as fennel, coriander, dill, anise, caraway, lovage and daisies serve as host plants for beneficial insects such as parasitic wasps. These tiny creatures can effectively control the larval stages of various insect pests.

Trial and error

Although scientifically developed guidelines for companion plantings are lacking, this does not mean it is not worth a try. The key to successful companion planting, as with so many other aspects of gardening, appears to be experimentation.

Try pairing some plants and see what the results are.

Also, now that you know more about how plants affect each other, when you are looking into why a plant is failing miserably in a certain area, consider its neighbors, as well as cultural and pest problems.

If you have more questions on companion planting or on other home or garden topics, call toll-free, at (877) 486-6271, and visit our Web site at www.ladybug.uconn.edu Or contact your local Cooperative Extension Center.

Posted June 20, 2010