Do Eggplants Grow With Onions?

Climate and Timing

• Eggplants and onions both grow in U.S. Department of Agriculture plant hardiness zones 4 to 9. Eggplants can also thrive in USDA zone 10, a hotter environment, while onions can grow in USDA zone 3, a colder environment. But if you want to grow both vegetables, you need to live in USDA zones 4 to 9. Besides location, bear in mind that onions need about 110 days to grow to maturity, so put them in the ground as early as possible. Wait to put in the eggplant seedlings until you’re two weeks past the threat of frost. That way, the eggplants and onions will come close to maturing together.

Sun and Water

• Eggplants and onions have similar sun and water requirements. Place them in a bed where neither gets waterlogged and in an area that receives full sun, meaning six to eight hours of light daily. Use humus-rich soil. Water both vegetables every day, first thing in the morning, unless it rains. Young seedlings need a little more water than mature plants. If you see areas where water pools, endeavor to create a channel to drain that off, thereby avoiding root rot for both plants.

Soil pH and Fertilization

• Soil pH can be tricky with these two vegetables, as can fertilization, which is why they are neutral in the way of companion planting. Eggplants prefer a pH between 5.5 and 6.0, while onions thrive best in a pH of 6.2 to 6.8. A good middle ground for growing them successfully together would be a pH of 6.0 or 6.1. In terms of fertilization, onions are less needy, requiring treatment only in spring and summer. Any treatments that take place later than early summer cause problems with drying bulbs after harvest. Eggplants need ongoing fertilization about every three weeks until they are mature. With this in mind, it’s best to put these two vegetables on opposite sides of a bed so that they share the space but you can control the fertilization applications easily.

Space Constraints

• Onions have vertical growth, while eggplants often sprawl horizontally. That means the eggplants could potentially take over the onions’ growing space. To avoid space issues, use a support system for your eggplants, such as a tomato cage or a stake and some string. This has the additional benefit of keeping the eggplant fruits up off the ground, where various insects such as cutworms like to feast.