"Companion Planting is the growing together of all those elements and beings that encourage life and growth; the creation of a microcosm that includes vegetables, fruits, trees, bushes, wheat, flowers, weeds, birds, soil, microorganisms, water, nutrients, insects, toads, spiders, and chickens." - John Jeavons, How to Grow More Vegetables Than You Ever Thought Possible
Carrots, Peas, lettuce, chives, onions, leeks, rosemary, sage, bush and pole beans, brussels sprouts, cabbage , onions, peppers, melons are all suitable companion plants.
Radishes serve as a general aid in repelling many insects and draw aphids, Flea Beetles, and some other pests away from peppers, squash, and cucumbers.
Planting radishes in a bed that hosted a cole crop in the last three years under some circumstances is not advisable. Cole crops are cool weather brassica and mustard family plants. Radish is closely related to these plants and shares many of the same problems. Although they shouldn't go in succession with one another, there are circumstances when there are advantages to inter-cropping them.
The primary problems inflicted on radishes by cole crops is cabbage Root Maggot, If you've had problems with this in the past keep them apart. Beneficial Nematodes are very effective in controlling root maggots. Incorporating wood ash into the soil also helps to prevent the occurrence of root maggots.
Radish can be planted with carrots , the radish will mature very quickly and loosen the soil for the carrot seeds as they germinate. The radishes will already be harvested when the carrots are still in pampers.
This same scenario works with other root crops such as beets , parsnip and even legumes.
Radish and peas can be inter-planted. The radish will deter some of the pests that feed on peas.
"Lettuce grows well with ....carrots, and it has long been considered good to team with radishes." Carrots Love Tomatoes: Secrets of Companion Planting for Successful Gardening
Lettuce interplanted with radish seems to make radishes more tender, radish which matures much quicker than lettuce will help lure early season pests away, flea beetles in particular.
Onion yield is increased when intercropped with Radish. Rapidly maturing early Radish helps prep the soil for onions and there is negligible competition for nutrients.
and Legumes like plants that produce their crop under the soil surface. The radish precedes the legume in germination and in the same scenario with as with carrots, it preps / loosens the soil for the slower maturing legume.
Although early season radishes will not be able to take advantage of the bean or legume nitrogen fixing attributes in the same season, the mature radish will draw pests from the young bean seedlings - a trap crop.
Avoid planting radish with, or in succession with potatoes , as radishes seem to increase the incidence of potato blights such as scab and Black root
Comfrey leaves are recommended to prevent scab.
Please keep in mind that many inter-actions are also percentage based. If an insect that attacks a certain garden plant is repelled by another type of plant - that doesn't mean that absolutely no insects of the genus in question will go near the plants in question - it simply means a much lower percentage than normally would have attacked the plant will show up.
Radishes deter the Cucumber Beetle, planting near plants that are favored by this voracious garden nuisance are advantageous. Cucumber,melons,squash,and pumpkins all benefit from being interplanted with radishes.
Avoid Fennel which is allelopathic to most garden plants. It exudes compounds designed by nature to eliminate competing plants from its immediate area. Dill is one of the few garden crops that can be grown with it.
1. - Intercropping of cabbage, sweet pepper, onion and radish
2. There is some evidence that inter-planting Radish with some varieties of cabbage may result in a decreased cabbage yield, cause unknown - possibly competition or related variables, possibly random variable. See Footnote # 1 -Same Study