Gardeners growing vegetables find that companion planting provides many benefits, one of which is protection from garden pests. A major enemy of the carrot is the carrot fly, whereas the leek suffers from the onion fly and leek moth. Yet when leek and carrot live together in companionship, the strong and strangely different smell of the partner plant repels the insects so much that they do not even attempt to lay their eggs on the neighbor plant. They take off speedily to get away from the smell. This is why mixed plantings give better insect control than mono-culture, where many plants of the same type are planted together in row after row. Even when plants are affected by plant diseases, a mixed plant culture can usually alleviate the situation.

It is important to remember that not all "protective" botanicals act quickly. For example, marigolds, to be effective in nematode control, should be grown over at least one full season, and more is better, for their effect is cumulative. One should also realize that certain companion plants will diminish each others natural repelling ability as they grow together.  The effects of plants on one another are important outside the vegetable garden, among trees and shrubs as well as grains, grasses, and field crops.

Wild plants also play a vital part in the plant community. Some are accumulator plants - those that have the ability to collect trace minerals from the soil. They actually can store in their tissues up to several hundred times the amount contained in an equal amount of soil. These plants, many of which are considered weeds, are useful as compost, green manure, or mulch. Some are "deep diggers," sending their roots deep into the ground to penetrate hardpan and helping to condition the soil, and some have value as protectors of garden plants.