Companion plants grow well together. In the vegetable garden, companions can be interplanted to make a plot more productive by sharing space over time, where a fast-growing vegetable is harvested to make room for a slower growing variety. Vegetable companions may also share light requirements with a taller sun-loving plant that provides shade for a shorter type that prefers cooler growing conditions. Other companion plants have complementary nutritional needs.
The Cornell University Department of Horticulture calls the "Three Sisters"--beans, corn and squash--the best-known historical example of companion planting. Native Americans traditionally grew these three crops interplanted, producing a well-balanced meal and a well-balanced vegetable garden. Corn has heavy nutritional requirements, but squash is not a heavy competitor for the same nutrients. Beans help fix nitrogen in the soil. Squash acts as a ground cover, shading out more competitive weeds, and the corn stalks act as a trellis for the climbing pole beans.
Shade and Light Companions
Spinach, lettuce and other salad greens prefer cool growing conditions, and they tend to go bitter in the heat of the summer sun. These greens grow quickly and are generally shallow-rooted, so they can be interplanted with any of a number of taller crops to take advantage of the shade while also serving as a living mulch to keep down weeds. Tomatoes, eggplants and peppers all make good shade companions for salad greens. Greens can also be tucked under the fronds of asparagus plants once they have sprouted for the summer season. Beets and turnips can also tolerate a fair amount of shade. They are grown for their young tops, or to the "baby" stage of roots just 2 inches across. Beets and turnips can be grown between other taller plants like tomatoes, then harvested just in time to make room for the tomato plants to reach their full size and to bear fruit.
Insect Control Companions
According to the Cornell University Department of Horticulture, many insect pests find their food by smell, so interplanting strong-smelling culinary herbs with vegetable crops can often deter or confuse destructive insects. Brigham Young University specifically suggests using borage with tomatoes to deter tomato worms, and rosemary and sage with cabbages to repel cabbage moths. The university also suggests using sage and onion with carrots to deter carrot flies. Marigolds, basil and flowering artemesia are also general insect deterrents in the vegetable garden. According to the Penn State Cooperative Extension, bush beans work well interplanted with potatoes. The two crops serve to confuse each other's traditional insect pests.