Companion planting is defined as the planting of certain crops in close proximity to each other to aid or boost crop yields and productivity. It is a type of polyculture whereby multiple crops planted adjacently or in close proximity can be used to promote or enhance the growth of all of the plants. The practice of companion planting has proven that some types of plants do in fact grow better when they are near other specific plant species.

Friendly types of plantings have been in use for thousands of years. First thought to be practiced by the ancient Egyptians, companion planting was practiced before herbicides and fertilizers changed our well-seasoned views about natural and effective plant propagation.

Native Americans had the most well-known system of companion planting—the Three Sisters method of planting corn, beans and squash was used to augment each plant's growth while supplying beneficial nitrogen, support and shade. Corn was planted first and allowed to grow several inches high. The corn stalk was then used as a support to hold up the beans. Climbing beans are legumes that fix nitrogen and feed nitrogen-consuming plants like corn. Lastly, squash was planted between the rows of beans and corn. The squash, being shade tolerant in the shadow of the corn, also protects the soil surface as a ground cover. Moisture conservation is achieved along with weed inhibition and reduced heat stress to all three crops.

Besides the favorable aspects of companion planting, it must be appreciated that some plants cannot be effectively planted close to each other. It is known that some plants do indeed deter insects, but they can also hinder, contaminate or slow the growth rate of other nearby plants.

Hindering growth of companion plants is called negative allelopathy. Interfering with the growth of a neighboring plant can be problematic for the grower. However, knowing that certain plants also exude objectionable compounds that hamper root growth or hinder germination is advantageous. The negative allelopathic effects of corn gluten meal in controlling the germination of weed seeds is now widely used in gardens to control weeds.

The outcome of companion planting in modern gardens can go beyond protecting plants—it can reduce the needs for modern control methods. The opportunity to reduce the use of pesticides, herbicides and fertilizers lowers production costs while increasing yield. Maximizing yield and fruit quality is time and again the primary goal of many gardeners.

The contemporary use of monoculture, or single crop growth, has discouraged the use of companion plantings, but this narrow field of view has changed in recent times. Growers today acknowledge the limits and potential harmful effects of modern pesticides. They are becoming keenly aware that co-plantings will enhance a backyard garden plot in a more naturally effective manner while protecting the environment, and in turn they are fostering ever-increasing harvests.

In some cases, companion plants are used as bait to trap unwanted insects while protecting a primary crop, often referred to as trap cropping. Growers use the pest-attractive, sacrificial crops to protect a main cash crop from infestations. For example, marigolds are often planted with tomatoes to fend off harmful, root-feeding nematodes or tomato hornworms. Hubbard squash is commonly used as a trap crop for other cucurbit-type vine crops to attract striped cucumber beetles.

The beetles vector bacterial wilt or mosaic virus and can cause greatly reduced yields. Companion plants can also be planted to provide a nursery area and habitat for beneficial insects. These advantageous, incubated predator insects then move on to wreak havoc on the unsuspecting, harmful pests that feed on the primary crops.

Pest-attractive plants can be used in concentrated zones in which pesticides may be used only sparingly. Insect traps and pheromone lures can also be used amongst these plantings to further reduce harmful insect populations. Hunting the harmful pests on the sacrificial crops can allow for a more targeted and efficient use of chemical controls.

Planting flowers amongst the main crops can persuade pollinators to visit gardens. Honey bees, butterflies and hummingbirds will come to forage the blossoms. Vegetables like squash and cucumbers that need pollination greatly benefit in these types of plantings. Placing herbs and flowers within your vegetable patch is one of simplest ways to create this natural kind of companion planting.

Cover crops can also be used as in-season companion plants. They are grown to reduce pathogens and insects or to supply nutrients. Giant vegetable growers have discovered in recent years that companion plants help boost weight gains. One of the main factors that improved fruit size is inoculating the sacrificial plants with mycorrhizal fungi. The main plant roots are colonized much faster by these beneficial fungi because the primary plant's roots grow into the sacrificial plant's roots and soil areas that have already been colonized by the fungi.

Early season companion crops can also be returned back into the soil to increase organic matter and nutrient storage for future plant growth. The fresh organic matter then becomes food for the myriad of various and beneficial microbes in the rhizosphere. These stored nutrients will not leach away, become insoluble or get tied up in the soil. They are available when plants are actively growing and serve to boost productivity. Plants such as buckwheat sequester calcium and phosphorus.

The companion cover crop can be plowed down into the soil ahead of the main plant's growth. This supplies the developing crop with a readily available source of root-building phosphorous and fruit-bulging calcium. Nutrients are vitally important to developing plants and in-season cover crops such as legumes can supply nitrogen. Beans planted around nitrogen consumers such as cabbages and corn greatly benefit.

Nature will always endure as it fosters living communities that stand the test of time. Observation and common knowledge indicate that many vegetables and herbs have natural compounds in their roots, flowers and leaves. These compounds may deter or fend off harmful pests and draw beneficial insects. Some companion plants assist other plant varieties to thrive and grow by providing shade, fungi, bacteria or conservation of water and soil moisture.

The essential growth requirements of a thriving garden plot can be influenced by companion plantings. Essentially, companion planting helps harmonize the garden’s flora and fauna, allowing the grower to use and maximize the skills that modern controls and nature provide.