Tomato and basil are a classic companion planting, with basil said to repel pests and diseases. Basil, especially in flower, attracts beneficial insects, including various wasps, which prey on caterpillars like the tomato hornworm. Plant basil on the edges of tomato rows, not between plants, so they get enough sun, or place pots of basil and tomato side by side on a patio. Other good tomato companions: lettuce, chives, garlic, borage, marigold. Do not plant with corn, dill, fennel, peas, potato or cole crops.
Swiss chard (in the beet plant family) is a great companion plant for cabbage family members, including kohlrabi and curly purple kale. The plants also stage a beautiful edible planting with contrasting colors and leaf textures. Other good cabbage family companions: lettuce, carrots, rosemary, oregano, marigold, nasturtium. Do not plant with beans, tomato, pepper or strawberry.