Growing certain vegetable plants together to promote growth and prevent insect damage and disease is a practice called companion planting. Companion planting also aids in soil health. As different plants require different nutrients, the practice of companion planting prevents soil from nutrient depletion. Each plant gets what it needs without having to compete. This makes for stronger, more disease resistant plants.
Native Americans developed the companion planting system known as the "three sisters." In this system, three plants--corn, beans and summer squash--are planted together in groups. Corn and squash take nitrogen out of the soil, while beans fix nitrogen in the soil. The beans pull nitrogen from the air and fix it in the soil, so the corn and the squash use the nitrogen.
The beans are planted 2 to 3 inches away from the corn. The corn stalks provide support for the beans, as a trellis would. Plant the squash close to the corn and bean plants. As the squash grows, its leaves provide shade for the corn plants' roots and keep the soil cool.
Onions, Carrots and Lettuce
Onions and other members of the allium family, such as leeks and garlic, give off a strong odor that deters pests. Carrot root flies do not like this odor and so planting onions among your carrots will protect the root vegetables. Since onions also deter slugs and other greens-eating insects, include lettuce and other greens with your carrots and onions.
Radishes With (Most) Everything But Spinach
Radishes and squash are a natural pairing; squash borers detest the smell of this root vegetable. Radishes also repel cucumber beetles and corn borers, so plant them with cucumbers and corn. Radishes attract the leafminer insect, which will chew up all your spinach leaves. Planting radishes a short distance from spinach keeps your spinach safe, as the leafminers would rather chew radish leaves. The root of the radish, the part you harvest, remains undamaged.
Radishes deter pests that attack melons, pole beans and bush beans as well as carrots and peas. Planting this quick-growing vegetable throughout your garden may reduce pest damage to almost all your vegetables.
Hot peppers, such as the jalapeno pepper, contain capsaicin. This chemical compound aids in preventing root rot and some fungal diseases. Plant hot peppers near squash, eggplant and tomato plants, as these vegetables are susceptible to powdery mildew and root rot, both of which may be controlled by the presence of hot pepper plants.