There's not a whole lot of hard science backing up most
companion planting suggestions - they're based mainly on
folklore and the educated guesses of experienced gardeners.
Perhaps one of these days a lot of this advice will be
scientifically explained and proven, but some of it is
already withering under the glare of careful observation.
Many of the companion lists suggest that marigolds repel
some garden pests, particularly cabbage worms and Mexican
bean beetles. Research, however, shows that marigolds
actually attract cabbage butterflies, which lay eggs that
hatch into cabbage worms. Furthermore, though it does
appear that marigolds protect beans against Mexican bean
beetles and corn earworms, it seems they also exude
chemicals harmful to bean plants.
While on the subject of marigolds, we should examine the
oft-made assertion that they should be planted among
tomatoes to repel certain pernicious root nematodes. It's
not all that simple. Root nematodes bothersome to tomatoes
are discouraged only by certain French marigolds (Nemagold
and Queen Sophia, for example), and only when the entire
infested area is planted with marigolds for a whole season.
Simply planting a few common marigolds in among the
tomatoes will not do the trick.