One of the key elements of organic growing is diversity, as opposed to monoculture. Companion planting takes this one step further, by growing different plants adjacent to each other for a purpose. This can benefit one, or both plants. For instance, companion planting can used to mask the smell of plants, which is used by pests to locate their food. Carrots and onions grown together reduces the incidence of both onion and carrot fly. Some plants cannot abide growing adjacent to others, for example peas and onions. The pea rhizobia are sensitive to root exudates from the onions. Knowing this beforehand prevents problems.

You are unlikely to solve all your problems with companion planting. It is far from an exact science, and results from some relationships often differ from garden to garden. One of the problems is that different varieties of vegetables behave differently, as well as variation due to soil type. The maturity of plants affects their aroma and this may be a significant factor in increasing their insect repellent nature. Some plants attract predatory insects. Flowering umbelliferae (carrots, parsnips, parsley etc.) are particularly attractive to hover flies which also consume aphids. The greater the variety of herbs used, generally the fewer insect problems reported by gardeners.