Plants helping plants

A bumblebee climbs onto a sunflower at Worden Farms in Punta Gorda on Thursday. The Wordens use companion planting, the practice of planting certain crops in proximity to others to assist each other in nutrient uptake, pest control, pollination and other factors, to increase crop productivity. Sunflowers are planted near crops, such as squash, to bring bees and other pollinators. They also attract predatory beneficial insects that assist in biological pest control. (10.20.2011)(STAFF PHOTO/ELAINE LITHERLAND)

Published: Friday, October 28, 2011 at 1:00 a.m.
Last Modified: Thursday, October 27, 2011 at 1:21 p.m.

While we gardeners and casual observers are looking above ground for the obvious fruits, flowers, disease and insects, plants are subtly and actively changing the chemistry of the soil and influencing the growth of microorganisms. They are competing with one another for space, and some are even poisoning the offspring of their competitors. When planted together, some plants become good neighbors, improving the growing environment to benefit their community. And they wage natural chemical warfare on insects.

Folklore and legend are full of references to the practice of growing two or more plants together for the benefit of at least one of them. But the actual study and practice of companion planting is relatively new, partially sparked by present-day interest in organic gardening.

Scientists have not spent much time looking at these relationships among plants and their community, according to an ecogardening fact sheet written by Robert Beyfuss and Marvin Pitts for Cornell University published in 1994 and revised last year.

That means that dedicated home gardeners and farmers are just as likely to discover new combinations and benefits of companion planting as are scientists and horticulturists.

"A key to success," says Sarasota County Extension Agent Robert Kluson, "is experimentation and observation of plant-to-plant interactions in your garden."

Talking with others about your findings is equally important because it is through those shared observations that results are validated. In fact, the aforementioned sources cite experimentation, historical observation and even folklore.

The Iroquois legend of the "Three Sisters" illustrates one of the earliest examples of companion planting. Believing that corn, beans and squash were special gifts to their people from the Great Spirit, Native American Indians planted them together. The corn provided support for the beans to grow toward the sun. The nitrogen-fixing beans enriched the soil for the corn and squash. The shade-tolerant squash spread its big leaves under the corn, keeping the soil moist and preventing weeds from growing.

Following are more some principles and practices of companion planting that can be used by home gardeners. And there is a list of compatible and incompatible plants in Kluson's organic gardening presentation at

Guided by these principles and lists, be open to the likelihood that you will discover new plant companions and practices that work for your own plant community.

Improving a growing environment

All plants, as they decompose, return nutrients to the soil. Some, like alfalfa, clover and legumes, take unusable nitrogen from the air, convert it to a form that can be absorbed by other plants and store it in root nodules. When these decompose, they release sufficient nitrogen that little or no fertilization is required. So even though they aren't growing at the same time, nitrogen-fixing cover crops are beneficial companions for other plants.

If you have ever been frustrated that some plants simply do not thrive in a particular location despite perfect growing conditions, take a look at the plants nearby. Some plants produce substances that are toxic to other plants. This phenomenon can be put to advantageous use. Preplanted sunflowers, cucumbers, oats, alfalfa, rye and tobacco have been found to inhibit weed growth. The relatively new study of this phenomenon is allelopathy.

Plants that protect others from strong sunlight, wind and possibly cold are known as nurse crops. Corn provides protection for its own cover crop, clover, which in turn provides nitrogen for the corn as it decomposes. Tall vegetable plants protect tender leaf lettuce.

Grasses planted between rows of perennial crops keep the soil cooler, prevent erosion, improve water penetration, discourage weeds and provide habitat for beneficial insects.

Trees provide not only shade, but nutrients and moisture for plants that grow in their shadows. Acid-loving azaleas thrive under oak and pine trees, which provide shade and moisture retention. Fallen oak leaves and pine needles also retain moisture and add acid to the soil as they decompose.

Controlling insects

According to the Cornell fact sheet, only about 1 percent of insects are garden pests. That means that indiscriminate use of pesticides, while eliminating pests, does more harm than good on several levels.

Companion planting can minimize insect damage.

Most plants produce chemicals that help them repel insects. Because insects use scent to help them locate food, strong-scented plants and herbs may be intermingled with others to confuse and repel pests.

"We use a general strategy of high crop diversity in our field plantings to reduce insect pests and plant diseases," says Eva Worden of Worden Farms. "Crops in the onion family and many of the herbs tend to repel a variety of insect pests, so we like to plant those crops preventively near other crops that are more prone to insect pests."

Other plants with strong scents that have been used by home gardeners include chives, catnip, horehound, basil, tansy and mint. You can build on these suggestions. Try planting your own favorite herb among vegetable plants and then observe which insects it repels.

With the renewed interest in organic and natural pest control, some practices recorded in folklore have been revived. Your grandmother or her mother may have planted chives at the base of roses to repel aphids. Other tried-and-true combinations include basil planted among tomatoes to repel hornworms, nasturtiums grown near squash to ward off squash bugs and tomatoes planted among asparagus to drive away asparagus beetles. Marigolds, mint, thyme and chamomile have been observed to repel cabbage moths.

Radish plants distract cucumber beetles and low-growing thyme or lavender may repel slugs. Tansy and pennyroyal repel ants and fleas.

French and African marigolds produce a substance that is toxic to some nematodes.

Trap cropping

Trap cropping is the practice of diverting insects away from your food crop by planting another more pest-attractive plant in close proximity. For example, a border of collard greens planted around cabbage will intercept the diamond back cabbage moth before it gets to the cabbage. Once an infestation has settled on a trap crop, it can be isolated and the pests can be eliminated with an organic insecticide without treating the crop you are trying to protect.

The Cornell fact sheet suggests using nasturtiums to trap aphids and flea beetles and using eggplant to trap potato bugs. If you have ever ruffled the leaves of marigold plants, you may have disturbed a cloud of whiteflies. That means they are staying away from other plants like tomatoes. As it turns out, marigolds are good trap plants for a number of pests, including nematodes.

Other pests and the plants that repel them:

--Ants: onions, tansy, mint family

--Aphids: chives, garlic, marigolds, mint family, dried and crushed chrysanthemum flowers, coriander, onions, oregano

--Cabbage moth: rosemary, sage, thyme

--Cabbage worm: tomato, celery

--Carrot flies: leeks, sage, rosemary

--Colorado potato beetle: marigolds, nasturtium, flax

--Cucumber beetle: nasturtium, radish, tansy

--Flea beetle: catnip, garlic

--Flies: basil, tansy

--Leafhoppers: dried and crushed chrysanthemum leaves

--Mosquitoes: basil, garlic, geranium (citrosa)

--Onion flies: garlic

nTicks: garlic

--Tomato worms: borage

Kluson points out that trap crops don't have to be replanted as often as other crops. Marigolds, for example, may continue to distract harmful insects and provide nematode control for several seasons.

You might also try creating a pool of beneficial insects by starting their favorite plants before planting your food crop. That way, the predatory and parasitic beneficial insects will be ready to attack when the pests do show up on the food crop.

Because the ratio of beneficial to harmful insects is so great, it makes sense to use the parasitic and predatory instincts of "friendlies" to control the others. Beneficial wasps and flies are attracted to the large flat-topped clusters of small flowers on dill, parsley, carrot, coriander and parsnip. Other plants and the beneficial insects they attract include:

Alfalfa: assassin bugs, big-eyed bugs, damsel bugs, lady beetles, parasitic wasps

Carrot: assassin bugs, big-eyed bugs, lacewings, parasitic wasps, yellow jacket wasps

Goldenrod: assassin bugs, big-eyed bugs, lady beetles, parasitic wasps

Morning glory: lady beetles, syrphid flies

"Good bugs" perform another crucial function: pollination.

At Worden Farms, sunflowers are planted within crops that require pollination to bring in bees and other pollinators. "The sunflowers also attract predatory beneficial insects that assist in biological pest control," says Worden. Butterfly gardens, suggests Kluson, besides providing visual pleasure, are excellent sources of pollinators.

Spatial arrangements

The way plants are arranged in a growing space can have an effect on insect populations and soil enhancement.

Monoculture, or the planting of a single plant in a large area or long row, provides an easy target for harmful insects and increases the possibility of spreading disease. Intercropping is one solution that is particularly well-suited to the home garden. Two or more crops may be intermingled in rows or partial rows or with no distinctive row arrangement.

Realm of possibilities

There is much to learn from the secret lives of plants that can be integrated into companion planting.

Do not discount the value of native plants and weeds as companion plants. Kluson points out that these seem to fare very well with no interference from outsiders. They have likely evolved their own kind of companionship. Careful observation may reveal which weeds attract harmful insects and which ones attract predators as well as those that seem to prevent other weeds from growing in their space.

All of which can may be of use in home gardens.


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