Companion planting is the idea that planting certain herbs and vegetables in close proximity to one another will be mutually beneficial to both. For example, one herb may keep pests away from a certain vegetable and allow both to grow healthy and strong. However, some plants are incompatible when planted next to each other and can harm the other. While scientists and farmers have figured out why some pairings work well and others don't, agriculturalists have yet to fully identify why each of the various pairs of companion plants grow well together.
Tomatoes seem to be loved by gardeners everywhere and have several companion plants that work well with them. Carrots and onions are compatible with tomatoes, as are several herbs and flowers, including parsley, nasturtium and marigolds. Planted near asparagus, tomatoes will protect the green vegetable from the asparagus beetle.
Carrots grow well next to beans, lettuce, onions, peas and radishes. Carrots and leeks planted together will repel pests common to both vegetables.
In addition to the carrot, peas also grow well with radishes, turnips, cucumbers, corn and beans.
See Resources section for a complete list of companion plants.
Herbs can also be used in companion planting. One of the most common herb and vegetable pairs is basil planted near tomatoes. Both plants flourish when paired.
Mint and cabbage grow well together, because the smell of the mint deters the cabbage moth.
Rosemary seems to be the workhorse of the herb companions. It grows well next to cabbage, beans, carrots and sage. Rosemary also deters a number of pests, including the cabbage moth, bean beetle and carrot fly.
Some plants should never be paired or planted in close proximity.
Tomatoes do not grow well next to anything in the brassicae family, including cabbage and brussels sprouts. Fennel can also be harmful to tomatoes.
Dill dislikes being planted near carrots and seems to be the only plant that doesn't work with carrots.
The Irish potato is incompatible with a number of vegetables, including cucumbers, pumpkins and other squash, tomatoes and turnips.