For centuries, gardeners have observed the same crops growing with vastly different levels of health and vigor, even when grown under very similar conditions. Many of these observations, have been replicated by other gardeners, giving them some measure of credence and respectability in the scientific community.
Potatoes are considered unfriendly to turnips, tomatoes, winter squash, including pumpkins, cucumbers and English peas. In a small garden, grow potatoes in a large plastic or galvanized metal trash can. That way you can separate your crop of potatoes from these vegetables.
Various members of the onion family are known to deter garden pests, due to their pungent fragrance. Chives, garlic, green onions and leeks are particularly effective in deterring rabbits from browsing in your garden, as well as repelling harmful flies and beetles that eat desirable plants. Do not plants onions near beans, peas or corn, as they will retard the growth of these vegetables. Also keep onions away from the cabbage family, which includes cabbage, cauliflower, broccoli, mustard greens, kale and Brussels sprouts.
Sometimes when two plants are planted together, one of them will benefit and the other will struggle. When members of the cabbage family are planted near tomatoes, they suffer and do not grow well, although tomatoes remain unaffected. However, when tomatoes are grown near potatoes or corn, none of them do well. Strive to plant potatoes and corn as far as possible from your tomatoes, to the extent your garden allows.
Cucumbers are generally not affected by other garden vegetables, although they do not do well when planted near strongly scented herbs. Cucumbers also have the ability to discourage the growth of weeds the following season in the garden bed where they are grown.
Sunflowers exude a substance from their roots that discourage other plants from growing nearby. The benefits carry over to the following year, when fewer weeds will grow in the ground in which sunflowers grow the previous year. However, beans and potatoes are highly susceptible to the roots of sunflowers, and should not be planted near them.