Companion planting creates an environment in the garden that encourages the quality of nearby crops. By placing the right plants together, growth is enhanced, soil is improved and the insect population is controlled. According to the Cornell University Department of Horticulture, the "Three Sisters" method of companion planting is the best historic example of how Native Americans created a system of companion planting using corn, beans and squash. Stalks of corn provided support for climbing beans, which did not compete with the corn--and provided their own nitrogen, while the squash created a dense cover to shade out weeds that would affect the corn and beans. This system provided a balanced diet from a single plot of land.

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Garlic is a companion plant for fruit trees.

Garlic

Garlic used in companion planting repels aphids, caterpillars, mites and Japanese beetles. Planted at the base of peach trees, garlic repels borers and prevents leaf curl. Planted near apple trees, garlic protects against apple scab. Herbs like garlic are considered beneficial nursery plants because they attract good insects by providing shelter, pollen and nectar. Beneficial insects includes lady beetles, lacewings, wasps and parasitic flies.

Comfrey

Comfrey is a beneficial companion plant for the avocado tree and most other fruit trees by serving as a trap crop for slugs. A trap crop pushes insects away from other essential plants with a disagreeable taste or a bad smell. The comfrey plant also accumulates phosphorus, calcium and potassium and helps keep surrounding soil moist and rich.

Chives

Chives help prevent apple scab when grown under apple trees. As a repellent for mites and nematodes, chives--along with garlic--is one of the most popular repellents because of their powerful ability to repel beetles and aphids, according to the Alabama Cooperative Extension System website.

Nasturtiums

When planted under fruit trees, nasturtiums deter squash bugs, white flies, cabbage moths, potato beetles and the striped pumpkin beetle, and act as a trap crop for aphids. When planted in a circle around apple trees, nasturtiums limit wooly aphid damage.

Lavender

Lavender repels fleas, ticks and mice. Planted near and under fruit trees, lavender will deter the coding moth, while attracting beneficial insects such as butterflies.

Tansy

Planted around fruit trees, tansy repels flying insects, Japanese beetles, squash bugs, cucumber beetles, moths and ants. Tansy also concentrates potassium into the soil.

Other Companion Plants

Planted around apple trees, clover has long been used as a companion plant and will attract predators of the woolly aphid. Onions will repel borers, slugs, cutworms and mites. Planting leeks will help improve fruit tree growth. Modern agriculture relies on chemicals and machinery to grow crops and control insects, while home gardeners can reap the benefits of companion planting to attract beneficial insects and avoid the use of insecticides on fruit trees.