Following is a partial list of garden plants with their "good" and "bad" neighbors from http://www.frenchgardening.com/:
Garlic
Good neighbors--strawberries, cucumbers, raspberries, lilies, carrots, tree fruits, roses, beets, tomatoes, tulips
Bad neighbors--peas, cabbage, pole beans
Asparagus
Good neighbors--cucumbers, lettuce, parsley, tomatoes
Bad neighbors--garlic, onions
Beets
Good neighbors--bush beans,dill, cucumbers, garlic, cabbages, coriander, cumin, lettuces, summer squash, onions
Bad neighbors--potatoes, leeks, corn, spinach
Carrots
Good neighbors--Dill, peas, garlic, leeks, chard, radishes, rosemary, sage, chives, lettuces, salsify, tomatoes, chicory, onions
Bad neighbors--None
Celery
Good neighbors--Bush beans, cucumbers, chamomile, cabbage family, leeks, tomatoes
Bad neighbors--potatoes, lettuces, corn
Cabbage family
Good neighbors--artemisias, beans, dill, endive, peas, chamomile, potatoes, lettuce, coriander, cumin, leeks, chard, mint, beets, celery, spinach, tomatoes
Bad neighbors--strawberries, muistard, garlic, onions
Cucumbers
Good neighbors--Beans, dill, peas, fennel, catnip, cabbage, lettuce, coriander, cumin, leeks, corn, beets, celery, onions
Bad neighbors--radishes, tomatoes
Squash
Good neighbors--nasturtium, corn, beets, pole beans, onions
Bad neighborsnone
Spinach
Good neighbors--Strawberries, potatoes, cabbage, radishes, celery, pole beans, tomatoes
Bad neighbors--none
Beans
Good neighbors--Winter savory, strawberries, cucumbers, potatoes, cabbage, lettuce, beets, celery, tomatoes
Bad neighbors--Peas, fennel, garlic, leeksn, onions
Cutting lettuces
Good neighbors--dill, fennel, cabbage, radishes, beets, salsify, asparagus, tomatoes
Bad neighbors--none
Heading lettuces
Good neighbors--Beans, dill, peas, strawberries, fennel, cucumbers, chervil, cabbages, cress, leeks, corn, carottes, mint, radishes, beets, salsify, asparagus, tomatoes, chicory, onions
Bad neighbors--Parsley, celery
Corn
Good neighbors--Beans, cucumbers, potatoes, lettuces, squash, melons, tomatoes
Bad neighbors--Beets, celery
Onions
Good neighbors--Winter savory, dill, strawberries, cucumbers, chamomile, lettuce, carrots, beets, salsify, chicory
Bad neighbors--Beans, peas, cabbages
Leeks
Good neighbors--endives, strawberries, chamomile, cabbages, lettuce, carrots, salsify, celery, tomatoes
Bad neighbors--Beans, peas, beets
Peas
Good neighbors--dill, fennel, cucumbers, cabbages, lettuce, corn, carrots, radishes, squash
Bad neighbors--Beans, potatoes, leeks, tomatoes, onions
Potatoes
Good neighbors--Favas, chamomile, nasturtium, cabbages, cumin, corn, horseradish, mint, spinach, marigolds
Bad neighbors--Peas, squash, beets, celery, tomatoes
Tomatoes
Good neighbors--beans, nasturtium, garlic, cabbages, lettuce, leeks, corn, carrots, parsley, lettuce, radishes, beets, celery, spinach, chicory
Bad neighbors--peas, fennel, potatoes
Garlic
Good neighbors--strawberries, cucumbers, raspberries, lilies, carrots, tree fruits, roses, beets, tomatoes, tulips
Bad neighbors--peas, cabbage, pole beans
Asparagus
Good neighbors--cucumbers, lettuce, parsley, tomatoes
Bad neighbors--garlic, onions
Beets
Good neighbors--bush beans,dill, cucumbers, garlic, cabbages, coriander, cumin, lettuces, summer squash, onions
Bad neighbors--potatoes, leeks, corn, spinach
Carrots
Good neighbors--Dill, peas, garlic, leeks, chard, radishes, rosemary, sage, chives, lettuces, salsify, tomatoes, chicory, onions
Bad neighbors--None
Celery
Good neighbors--Bush beans, cucumbers, chamomile, cabbage family, leeks, tomatoes
Bad neighbors--potatoes, lettuces, corn
Cabbage family
Good neighbors--artemisias, beans, dill, endive, peas, chamomile, potatoes, lettuce, coriander, cumin, leeks, chard, mint, beets, celery, spinach, tomatoes
Bad neighbors--strawberries, muistard, garlic, onions
Cucumbers
Good neighbors--Beans, dill, peas, fennel, catnip, cabbage, lettuce, coriander, cumin, leeks, corn, beets, celery, onions
Bad neighbors--radishes, tomatoes
Squash
Good neighbors--nasturtium, corn, beets, pole beans, onions
Bad neighborsnone
Spinach
Good neighbors--Strawberries, potatoes, cabbage, radishes, celery, pole beans, tomatoes
Bad neighbors--none
Beans
Good neighbors--Winter savory, strawberries, cucumbers, potatoes, cabbage, lettuce, beets, celery, tomatoes
Bad neighbors--Peas, fennel, garlic, leeksn, onions
Cutting lettuces
Good neighbors--dill, fennel, cabbage, radishes, beets, salsify, asparagus, tomatoes
Bad neighbors--none
Heading lettuces
Good neighbors--Beans, dill, peas, strawberries, fennel, cucumbers, chervil, cabbages, cress, leeks, corn, carottes, mint, radishes, beets, salsify, asparagus, tomatoes, chicory, onions
Bad neighbors--Parsley, celery
Corn
Good neighbors--Beans, cucumbers, potatoes, lettuces, squash, melons, tomatoes
Bad neighbors--Beets, celery
Onions
Good neighbors--Winter savory, dill, strawberries, cucumbers, chamomile, lettuce, carrots, beets, salsify, chicory
Bad neighbors--Beans, peas, cabbages
Leeks
Good neighbors--endives, strawberries, chamomile, cabbages, lettuce, carrots, salsify, celery, tomatoes
Bad neighbors--Beans, peas, beets
Peas
Good neighbors--dill, fennel, cucumbers, cabbages, lettuce, corn, carrots, radishes, squash
Bad neighbors--Beans, potatoes, leeks, tomatoes, onions
Potatoes
Good neighbors--Favas, chamomile, nasturtium, cabbages, cumin, corn, horseradish, mint, spinach, marigolds
Bad neighbors--Peas, squash, beets, celery, tomatoes
Tomatoes
Good neighbors--beans, nasturtium, garlic, cabbages, lettuce, leeks, corn, carrots, parsley, lettuce, radishes, beets, celery, spinach, chicory
Bad neighbors--peas, fennel, potatoes
For more companion groupings, see http://www.gardensablaze.com/.
