Eggplants (Solanum melongena) are not exactly fuss-free. They require plenty of sun, supplemental feeding and regular watering. Eggplants also get their share of pests. Although eggplants potentially grow as perennials in U.S. Department of Agriculture plant hardiness zones 9 to 12, most people treat the heat-loving vegetables as annuals. To keep these high-maintenance Mediterranean favorites happy, grow them near other plants that either have the same needs, or which enhance the growing conditions for eggplants.
Eggplants thrive on nitrogen, often requiring extra fertilizer at the height of their growing season. Planting eggplants in the same beds as annual legumes benefits the eggplants because peas (Pisum sativum) and beans (Phaseolus vulgaris) pull nitrogen from the atmosphere, then fix it into the surrounding soil. If you grow pole peas or pole beans, set the eggplants in front of the trellised legumes so the eggplants won't be be shaded. If you grow bush peas and beans, alternate rows of the eggplants with beans or peas, or alternate the plants within rows.
Bush green beans also repel one of the eggplant's pests, the Colorado potato beetle. The perennial herb, French tarragon (Artemisia dracunculus "Sativa"), which grows in USDA zones 5 through 8, helps protect eggplants from a broad range of pests. To deter garden moths from damaging eggplants, grow thyme (Thymus vulgaris), a herb that's perennial in USDA zones 5 through 9. Mexican marigold (Tagetes minuta), an annual flower, repels beetles from the eggplant patch. Marigolds are toxic to beans, so choose only beans or marigolds as an eggplant companion -- not both.
A member of the nightshade family, the eggplant grows well with hot and sweet peppers (Capsicum annuum), which are also nightshade plants. Peppers and eggplants have the same growing needs -- full sun, regular watering and rich soil -- while also contending with the same pests and diseases. Grow them in alternate rows, or stagger them within rows. Some people also interplant eggplants with tomatoes, which are nightshade family members with similar growing needs. If you do so,you'll need to take tomatoes' height into account and make sure the tomato plants don't shade the eggplants.
In some cases, the best garden bed companions are the ones whose spacing and sun needs allow them to be tucked between bushy eggplants. Spinach (Spinacia oleracea) and eggplant are often listed as having a positive effect on on another. Whether there is a scientific reason for this is unclear, but certainly spinach, a cool-weather crop, appreciates the shade cast by eggplant during the dog days of summer. Earlier in the season, younger eggplants won't be tall enough to block sun when the spinach needs it most.