Every plant in an ecosystem affects its environment in some way; whether depleting or replenishing the soil of certain nutrients, attracting and/or repelling insects or wildlife, or preventing erosion with their intricate root system. For centuries, farmers have used different plants for different reasons in their gardens--not just for their aesthetic values or because they produce fruits and vegetables. Many farmers choose to use companion planting to help their crops in the belief that planting certain plant species together provides combined benefits, usually to both plants.
Known Beginnings of Companion Planting
In the book, "Roman Farm Management", based on the writings of Varro, a Roman Agronomist who wrote and published at least five books in 37 BC, Varro states that "a grove of thickly planted full grown walnut trees renders sterile all the surrounding land." This is one of the earliest recorded references to the understanding that certain plants cannot be planted near others or that plants can have chemical effects on their environments.
"Three Sisters" Companion Planting
Native Americans are cited often when discussing companion plantings because of their ability to provide for themselves in the North American landscape and the farming knowledge that they passed on to the English and Spanish settlers of the 1500s. The Native Americans believed that it was beneficial to plant the "three sisters"--corn, beans and squash--together on the same hill. The purpose of this is that the three help each other to thrive and grow. The corn helps to support the growing bean vines, the beans provide nitrogen to the soil and the squash vines through the plants along the ground, providing shade and weed protection for the other two.
Benefits of Companion Planting
Companion planting can be beneficial in helping to control the crop damaging insects. Many plants have their own insect repelling chemicals, like the pyrethrins in certain chrysanthemum flowers, and some flowers have a fragrance that is offensive to wildlife. This is why farmers will plant marigolds around the circumference of their gardens to help deter deer from eating their crops. Companion planting can also be a space-saving feature in a garden, allowing more than one type of crop to be grown in each plot; as seen in the Native American system of "Three Sisters" planting.
In an article written by Stacy Nelson on the Marin County Extension website, it is noted that using companion planting to incorporate plants that repel certain insects is not as effective as once thought. The individual plants repel insects but do not provide protection for the surrounding plants of different species. The author of Pests of the Garden and Small Farm, Mary Louise Flint states that using companion planting can be effective when the plantings consist of plant species that attract beneficial insects: "Ladybugs, hover flies, lacewings, spiders, and parasitic wasps are natural enemies of plant damaging insects like aphids, mites, whitefly, scale and thrips."
Attracting Beneficial Insects
Beneficial insects are attracted to gardens that provide plenty of pollen and nectar. Plants such as chrysanthemums, sunflowers, flowers and herbs from the carrot family, coneflower and alyssums flower and produce pollen throughout the growing season and help to attract the natural predators of crop-destroying insects.