By Carrie Reeves

MISSISSIPPI STATE -- Companion planting to keep harmful insects out of gardens is an old practice gleaned from years of experimentation and folklore passed down through generations.

Gardeners use this practice to camouflage, confuse and repel pests and disease by planting vegetables, flowers and herbs that compliment each other.

"Companion planting is popular among organic gardeners," said Rick Snyder, horticulturist with Mississippi State University's Extension Service. "Some gardeners grow plants such as marigolds or garlic to repel certain insects."

Marigolds, nasturtiums and many herbs have aromatic foliage that is not attractive to bugs. Other plants, such as those in the mint family, have biting juices to repel insects.

"In small organic gardens, use of companion plants can eliminate the need for chemical pesticides," Snyder said. "Most companion plants are those with volatile odors."

Some companion plants trap harmful insects. Nostrum, radish and eggplants tend to trap insects. After being trapped, the insect cannot move and feed, resulting in death.

Companion plants can increase garden yields and flower production. Mixing flowers and herbs with vegetable plantings creates more flowers and better vegetables. The flowers increase nectar production, attracting more predator insects to protect the vegetables and increasing flower yields.

"Herbs, such as chives, are good companions for roses," Snyder said. "Chives tend to repel aphids, but the chives also repel rust flies in carrots."

Some companion plants attract helpful insects. Plants such as alfalfa, carrots and goldenrod attract helpful insects like assassin bugs, damsel bugs, big-eyed bugs, ladybugs and lacewings. These predatory insects eat insects that are harmful to plants.

Gardeners who have experimented with companion planting have found that certain plants have good companions and bad companions. For example, good companions for tomatoes are asparagus, lima beans, cabbage and basil, and bad companions are broccoli, cauliflower, potatoes, rosemary, carrots, chives, dill, onions and parsley. For vegetables like bush beans and peas, gardeners should avoid leeks, onions, garlic and shallot, but carrots and cucumbers are good companions.

Snyder recommended avoiding several other combinations of plants. Do not plant garlic around anything except beans. Tomatoes do not thrive around peppers or potatoes, and avoid planting eggplants with potatoes.

Aromatic herbs are good for preventing insects. Thyme deters the cabbageworm, mint deters the cabbage moth and basil deters flies, mosquitoes and tomato hornworms.

Snyder recommended that gardeners interplant these companion herbs with vegetables or flowers for the best results.

Companion planting is not a proven science. The basis for this type of gardening comes from folklore and experimentation. Organic gardeners have spent years trying different types of plants and herbs near other plants and vegetables.

"Although companion planting has not been proven is scientific studies, many gardeners have improved their gardens using these methods," Snyder said