Tomatoes and eggplants both produce bushy plants with one main stem and many lateral leafing branches. Eggplants are fairly compact, upright plants, while indeterminate tomatoes produce sprawling vines up to 5 feet in length. They are often caged or staked to control their rampant growth and require at least 3 feet of space per plant. If you choose to grow eggplants and tomatoes together, select bush-type, determinate tomato varieties. These plants have a well-behaved, compact growth habit that won't overpower the eggplant.
Both tomatoes and eggplants are susceptible to fusarium and verticillium wilts.These fungal diseases live in the soil and cause leaf yellowing and defoliation. Many tomato plants are resistant to the disease, but eggplants usually are not. By planting the two together, you may increase the risk of infection, especially if you live in a hot, humid climate prone to fungal disease.
The theory of companion planting suggests that some plants share a symbiotic relationship, releasing chemicals or in some other way benefiting other plants. Although growing tomatoes and eggplants together won't harm the plants, the two don't appear to have any special affinity toward each other either. To maximize the benefits of companion planting, grow tomatoes with onions, carrots, parsley, asparagus, marigolds and cucumbers. Avoid planting them with potatoes, fennel, broccoli and cabbage. Plant eggplants with beans and marigolds.
Tomatoes and eggplants both thrive in full sun and moist, rich soils. Plant them in rows in the general garden vicinity, but avoid planting them together if possible. If garden space is limited and you must plant them together, practice disease-reducing strategies. Use soaker hoses and drip irrigation to water them, watering in the morning so leaves dry quickly. Remove garden debris promptly.