Garden Planning Part 4: What Is Companion Planting?

This is part four of my garden planning guide. For part one all about deciding what to plant, click here. For part two, all about planning your garden layout, click here. For part three, how to start seeds, click here

Companion planting is an age-old practice based on the principle of mutualism, where plants interact positively with each other, promoting growth, health, and pest control. Companion planting is a holistic approach to gardening that recognizes the interconnectedness of plants and their environment. By strategically pairing plants, you can create healthier, more resilient ecosystems that require less intervention and chemical inputs. It's a sustainable and environmentally friendly way to cultivate food and ornamental gardens, rooted in the principles of cooperation and harmony within nature. The goal is to enhance the overall health and productivity of the garden by taking advantage of the interactions between certain plant species.

How companion planting can impact your garden

Companion Planting for Pest Control

Some plants release natural compounds that repel or deter pests, and planting them alongside susceptible crops can help protect them. Marigolds are renowned for their ability to repel a wide range of pests due to the strong scent emitted by their foliage and flowers. This scent acts as a natural deterrent for many common garden pests, including nematodes, aphids, whiteflies, and even certain types of beetles.

Planting marigolds alongside susceptible crops can create a protective barrier, reducing the likelihood of pest infestations without the need for chemical pesticides. Planting basil alongside tomatoes not only improves the flavor of the tomatoes but also deters tomato hornworms and whiteflies. Similarly, planting onions or chives near carrots can help repel carrot flies, while carrots attract beneficial insects like parasitic wasps and hoverflies, which prey on aphids and other pests. Calendula and yarrow attract predatory insects like ladybugs and parasitic wasps, which feed on harmful insects.

Companion Planting for Soil Health

Companion planting plays a significant role in improving and maintaining soil quality. Some plants possess deep and penetrating root systems that can effectively break up compacted soil, allowing for better air and water penetration. Plants like daikon radishes, with their long taproots, are excellent at aerating the soil. These deep-rooted plants help loosen compacted soil layers, creating channels for air, water, and nutrients to reach deeper soil layers, which is beneficial for the overall health and growth of neighboring plants.

Leguminous plants, such as beans, peas, clover, and alfalfa, have a unique ability to form symbiotic relationships with nitrogen-fixing bacteria in their root nodules. These bacteria take atmospheric nitrogen and convert it into a form (ammonia) that plants can utilize for growth. This process, known as nitrogen fixation, helps enrich the soil with this essential nutrient, benefiting not only the legumes themselves but also neighboring plants. By interplanting nitrogen-fixing legumes with other crops, you can reduce the need for synthetic fertilizers and promote healthier soil fertility.

Certain plants have the remarkable ability to absorb and accumulate toxins, heavy metals, and pollutants from the soil through their roots—a process known as phytoremediation. Sunflowers, for example, are well-known for their ability to absorb contaminants such as lead, arsenic, and radioactive isotopes from the soil, effectively detoxifying it. Once absorbed, these contaminants are stored in the plant's tissues or concentrated in specific structures, making it easier to remediate the soil by removing the contaminated plant material. By incorporating phytoremediative plants into companion planting schemes, you can help rehabilitate polluted or contaminated soil, making it safer and healthier for other plants to grow.

In addition to sunflowers, several other plants are considered "dynamic accumulators" because they accumulate specific nutrients or minerals in their tissues, which can then be recycled back into the soil when the plant decomposes. For instance, comfrey is known for its deep roots and high concentrations of nutrients like potassium, calcium, and phosphorus. When comfrey leaves decompose, they release these nutrients back into the soil, enriching it for neighboring plants. Incorporating dynamic accumulators into companion planting designs can help improve soil fertility and structure over time, creating a more resilient and productive garden ecosystem.

Companion Planting to Maximize Growing Space

One of the primary ways companion planting maximizes space is by utilizing vertical space efficiently. Tall plants, such as corn, sunflowers, or pole beans, can serve as natural trellises or support structures for vining plants like cucumbers, squash, or tomatoes. By allowing these vining plants to climb upward, you can take advantage of vertical space that might otherwise remain unused, effectively doubling or even tripling the yield from a given area of land.

Another approach to space optimization involves creating layered planting arrangements akin to natural ecosystems. Tall plants with a dense canopy, such as corn or sunflowers, can provide shade and shelter for lower-growing plants like lettuce, spinach, or herbs. This canopy-and-understory configuration not only makes efficient use of space but also helps conserve soil moisture by reducing evaporation and minimizing weed growth. Additionally, the cooler microclimate created by the shade of taller plants can be beneficial for cool-season crops in warmer climates or during hot summer months.

Companion Planting as a Succession Strategy

Companion planting also enables you to implement succession planting strategies, where fast-maturing or short-season crops are interplanted with longer-maturing or perennial species. For example, lettuce or radishes can be planted between rows of slower-growing crops like tomatoes or peppers. By the time the larger plants reach maturity and require more space, the shorter crops will have been harvested, allowing for continuous production and efficient use of space throughout the growing season.

Pairing plants with compatible growth habits and complementary resource needs allows for denser planting arrangements without competition for resources. For instance, planting shallow-rooted crops like lettuce or herbs alongside deep-rooted crops like garlic or onions enables efficient use of soil nutrients and moisture without overcrowding or stunting growth. Similarly, interplanting nitrogen-fixing legumes like beans or peas with heavy feeders like corn or squash can provide natural fertilization and improve overall soil fertility while maximizing space utilization.

What is the Three Sisters Planting Method?

The "Three Sisters" technique, deeply rooted in Native American agricultural traditions, exemplifies the concept of companion planting based on complementary nutrient needs and symbiotic relationships among plant species. Corn serves as the backbone of the Three Sisters planting method, providing a tall, sturdy structure for the climbing beans to latch onto. As corn grows, it creates a natural trellis for the beans, allowing them to climb upward and access sunlight without the need for additional support structures. In return, the beans, with their twining vines, help stabilize the corn stalks against strong winds and storms.

Beans, such as pole beans or runner beans, are nitrogen-fixing legumes that have a symbiotic relationship with certain soil bacteria. These bacteria, known as rhizobia, form nodules on the beans' roots and convert atmospheric nitrogen into a form that plants can use for growth. By interplanting beans with corn, the beans enrich the soil with nitrogen, a vital nutrient that corn requires for healthy growth and development. In essence, the beans act as natural fertilizers, reducing the need for synthetic nitrogen inputs and promoting soil fertility.

Squash plays a crucial role in the Three Sisters planting system by serving as living mulch. As the squash plants spread their large, dense leaves across the ground, they create a natural barrier that suppresses weed growth, shades the soil, and conserves moisture. This living mulch helps maintain soil moisture levels, reduces water evaporation, and minimizes weed competition, thus improving overall soil health and reducing the need for manual weed control.

The Three Sisters planting method offers synergistic benefits beyond nutrient sharing and weed suppression. The combination of corn, beans, and squash creates a balanced and self-sustaining ecosystem that mimics the diversity and resilience of natural ecosystems. For example, the dense foliage of the squash plants helps to deter pests and reduce pest pressure on the corn and beans, while the deep root systems of all three plants contribute to soil aeration and nutrient cycling. Beyond its practical benefits, the Three Sisters planting technique holds cultural and spiritual significance for many Native American communities. It reflects a deep understanding of the interconnectedness of all living beings and the importance of harmonious relationships with the land. The practice has been passed down through generations as a sacred tradition, embodying principles of reciprocity, sustainability, and reverence for the natural world.

Companion Planting to Increase Yields

Pairing plants that require cross-pollination can increase yields. Bees and other pollinators are attracted to a variety of flowering plants, so including a diverse array of flowers in your garden can ensure good pollination for crops like tomatoes, cucumbers, and peppers. Many plants rely on cross-pollination for successful fruit production. Cross-pollination occurs when pollen from the male reproductive organs (stamens) of one plant is transferred to the female reproductive organs (pistils) of another plant of the same species. This process introduces genetic diversity and promotes robust fruit set and development.

Incorporating a variety of flowering plants into the garden landscape serves as a natural strategy for attracting and supporting pollinators throughout the growing season. Plants such as lavender, borage, bee balm, sunflowers, and cosmos are known for their prolific blooms and attractiveness to bees and other pollinators. By interspersing these flowers among vegetable crops, you can create an inviting environment that encourages pollinator activity and enhances pollination efficiency.

Pairing crops that require cross-pollination with compatible flowering plants ensures that there are sufficient pollinators present when the crops are in bloom. For example, tomatoes, cucumbers, and peppers all benefit from bee pollination. By planting these crops alongside companion plants that bloom concurrently and attract pollinators, you can increase the likelihood of successful pollination and maximize fruit set and yield.

Plants That Don’t Grow Well Together

Not all plant combinations result in positive interactions, and some plants may be incompatible due to competition for resources or allelopathic effects (where one plant releases chemicals that inhibit the growth of others).

Understanding the potential for both positive and negative interactions between plants is crucial when companion planting. While many plant combinations offer synergistic benefits, it's essential to recognize that not all pairings result in positive outcomes. Plants with high nutrient demands can compete with neighboring plants for soil nutrients. This competition can result in nutrient deficiencies for one or both plant species, hindering their growth and productivity.

Certain plants produce allelochemicals—compounds that inhibit the germination, growth, or development of other plants. These chemicals can be released into the soil through root exudates or leaf litter, creating an inhospitable environment for neighboring plants. For example, black walnut trees release juglone, a toxic compound that can inhibit the growth of many plants, including tomatoes, peppers, and potatoes. Planting susceptible crops near black walnut trees can lead to stunted growth, yellowing of leaves, and overall poor performance.

Some plants have aggressive growth habits or spread rapidly through rhizomes, runners, or seeds. When planted alongside less vigorous species, these invasive plants can overrun the garden, outcompeting and displacing other plants. Mint and bamboo are notorious for their invasive tendencies and should be grown in contained areas or avoided altogether in mixed plantings where they may dominate and crowd out other plants.

Certain plant combinations may attract common pests or serve as breeding grounds for disease vectors, increasing the risk of pest infestations and disease outbreaks for nearby plants. Planting susceptible crops in close proximity to each other can create ideal conditions for pests and diseases to spread rapidly. For instance, planting tomatoes next to potatoes can increase the likelihood of both crops being affected by potato blight.

To mitigate the risks of negative interactions between plants, it's essential to consider factors such as plant compatibility, growth habits, nutrient requirements, and susceptibility to pests and diseases when planning companion planting arrangements. Conducting research, observing plant behavior, and experimenting with different combinations can help you identify harmonious pairings and avoid potential conflicts in the garden. Successful companion planting often involves a combination of experimentation, observation, and adapting practices to the specific conditions of the garden.