Companion planting is the practice of strategically growing certain plants together to create a mutually beneficial environment. By leveraging natural plant interactions, this technique fosters a healthier, more productive garden. Understanding how different species influence one another allows gardeners to enhance crop development, naturally manage pest populations, and improve soil vitality. This time-honored technique creates a diverse ecosystem where plants actively support their neighbors.
How Plants Interact Positively
The mechanisms behind successful companion planting fall into three main ecological categories.
Pest Deterrence
One significant benefit is pest deterrence, often achieved through masking or repulsion. Aromatic herbs release volatile organic compounds (VOCs) that confuse or repel insect pests, making it difficult for them to locate host plants. Certain plants also function as “trap crops,” drawing pests away from the main crop, or they attract beneficial insects like ladybugs and parasitic wasps that prey on garden pests.
Soil Health and Nutrient Provision
A second mechanism is the improvement of soil health and nutrient provision. Legumes, such as beans and peas, form a symbiotic relationship with Rhizobium bacteria in their root nodules. This process converts atmospheric nitrogen into a usable form, enriching the soil for nearby, heavy-feeding crops. Other plants, known as dynamic accumulators, possess deep taproots that draw up micronutrients from deep within the soil, making them available to shallower-rooted plants when the accumulator’s leaves decompose.
Physical Support and Microclimate Modification
The third mode of interaction involves physical support and microclimate modification. Taller, sturdy plants, such as corn or sunflowers, can serve as natural trellises for vining crops like cucumbers or pole beans. Additionally, broad-leafed, low-growing plants act as a living mulch, shading the soil to suppress weeds and maintain a cooler, more consistent soil temperature and moisture level. Varied root depths also reduce competition for water and nutrients by accessing different soil layers.
Essential Positive Pairings
The classic example of a supportive plant community is the “Three Sisters” combination of corn, beans, and squash. The tall corn provides a structure for the climbing beans, while the beans fix nitrogen in the soil to feed the corn and squash. The large leaves of the squash plants shade the ground, suppressing weed growth and conserving moisture.
Tomatoes and Basil are another highly effective pairing. Basil’s strong aroma deters common tomato pests, including the tomato hornworm moth and whiteflies. Marigolds planted near tomatoes are also effective, as their roots release phytochemicals that suppress nematodes in the soil.
Carrots and Onions or chives work well together due to pest confusion. The strong scent of the alliums helps to mask the carrot’s scent, deterring the carrot rust fly. Similarly, the aroma of carrots may confuse the onion fly, providing mutual protection from root-maggot pests.
For brassicas like cabbage and broccoli, Thyme is a useful companion because its oils deter the cabbage worm and cabbage looper. Borage, a blue-flowered herb, is an excellent partner for strawberries and tomatoes because it attracts pollinating bees and is believed to improve the fruit’s flavor.
Plants That Should Not Be Grown Together
While many plants thrive in partnership, some combinations can be detrimental to a garden’s success. This negative interaction is often due to allelopathy, where one plant releases biochemicals that inhibit the growth or germination of another species.
The most commonly cited allelopathic plant is Fennel, which exudes compounds from its roots that stunt the growth of nearly all other vegetables and herbs, so it should be planted in isolation. Black Walnut trees are notorious for releasing a chemical called juglone from their roots, which is highly toxic to many plants, particularly those in the nightshade family like tomatoes, peppers, and potatoes.
Beyond chemical inhibition, some plants compete too fiercely for resources. Planting Potatoes and Tomatoes together is ill-advised because they are both heavy feeders and are susceptible to the same diseases, such as blight. Furthermore, combining plants with similar root structures leads to intense competition for water and nutrients. Beans and most Alliums, like onions, should be separated, as the alliums can inhibit bean growth and negatively affect the flavor profile of the crop.
Designing Your Companion Garden
Successful implementation of companion planting relies on strategic placement and timing, moving beyond simple row planting.
Intercropping and Proximity
Intercropping involves planting different crops in close proximity, often alternating rows or groups, to utilize space efficiently and confuse pests. For example, planting fast-maturing crops like radishes or lettuce between rows of slow-growing plants allows for two harvests before the larger plant fills in. Proximity is a practical consideration, as the beneficial effects, particularly pest-deterring aromatics and nutrient exchange, are strongest over short distances. Companions should be planted close enough for their root zones to interact or for their scents to mingle, often intermingled within the same bed. Mixing small clusters of diverse plants, rather than long, uniform rows, also helps prevent the rapid spread of pests and diseases.
Timing and Space Management
Timing is a significant factor, especially when using a plant for structural support or shade. For the Three Sisters method, the corn must be established a few weeks before the beans so the stalks are strong enough to support the vining growth. Considering the vertical dimension allows for Space Management, where tall plants are intentionally positioned to provide necessary afternoon shade for heat-sensitive plants like lettuce or spinach, optimizing the microclimate within the garden bed.