Companion planting is an ancient practice designed to promote healthy plant growth and reduce pest infestation and disease. Companion planting also addresses which plants won't grow well together because they stunt each other's growth or make each other more susceptible to pests and disease. Today, companion planting serves as one tool in the organic gardener's arsenal to grow healthy crops without reliance on chemical fertilizers and pesticides. Not surprisingly, given the popularity of tomatoes in backyard gardens, much research has been done and much has been learned about their preferred companion plants.
Write a list of the vegetables and herbs you want to grow in addition to tomatoes. Some plants that you already plan to grow and know that you'll use may confer benefits on tomatoes or vice versa.
List any particular disease, pest or soil problems that you have had in years past. Where companion planting solutions exist, these should be the first combinations that you try. List any problems that you've had with your tomatoes, as well as other vegetables that you plan to plant this year. Other vegetables won't just benefit tomatoes; tomatoes will benefit other vegetables too.
Determine if any of the problems you've identified can be controlled with companion planting. For example, basil and onions help to control tomato hornworms, while tomatoes may prevent cabbageworms, flea beetles and asparagus beetles. Planting garlic between tomatoes controls red spider mites that harm tomatoes.
Mark any plants that grow well with tomatoes and similarly denote any plants that you should avoid growing near tomatoes.
Draw your garden plan, using this information to inform plant placement. Give priority to companion plantings that will correct past disease and pest problems, then add other companion plants as you have room. Consider other growing needs as you plan. For example, tomatoes grow tall, so don't leave smaller, sun-loving plants in the dark. Tomatoes are also heavy feeders, extracting large amounts of nutrients, so if you plan to plant them with other heavy feeders, provide plenty of compost and other supplements.