Radishes make great companions for radish, beet, bush beans, pole beans, carrots, chervil, cucumber, lettuce, melons, nasturtium, parsnip, peas, spinach, and squash.
Radishes can help to deter cucumber beetles and rust flies.
They can help protect squash from squash borers.
Radishes can also help corn by fighting corn borers, but you must let the radishes go to seed.
Radishes also lure leafminers away from spinach plants. The leafminers will damage radish leaves, but this does not stop the radish roots from growing.
Radishes also make great container plants. So if you only have room for some flower boxes, you can still grow radishes.