Beyond folklore: Companion planting for pest control, pollination and more

AMERICAN FALLS — Companion planting is the practice of growing different plant species together for mutual or one-sided benefits. It can also be described as a gardening technique that blends traditional wisdom with emerging scientific research. While folklore often highlights popular companion planting practices, a deeper understanding of its principles can help gardeners achieve better results. Most gardeners have heard of some form of companion planting. This article seeks to clarify the science behind companion planting.

Companion planting can involve two to multiple plant species and introduces additional levels of complexity as more plant species are planted together. The goal of companion pairings is to provide a physical, chemical or biological alteration to the environment.

The scientific research on companion planting is still evolving. Principles from fields like agriculture (intercropping – planting multiple crops in alternating rows) and ecology (studying beneficial associations in natural environments) provide valuable insights. These disciplines highlight the potential of companion planting to enhance biodiversity, nutrient cycling, pest management, physical support and microclimate modification in gardens.

Biodiversity: Creating a a balanced garden

A key goal of companion planting is to enhance biodiversity in the garden. Diverse plant species create a more complex and resilient food web. In simple ecosystems, the decline of one species can disrupt the entire system. However, a garden with many interconnected species is more tolerant of such disruptions.

In a biodiverse garden there are a wider variety of insects, both beneficial and harmful. These insects can in turn attract predators like birds, reptiles and other wildlife, further enriching the ecosystem. To succeed, choose plant pairings that complement rather than compete to ensure all plants thrive without depleting shared resources, unless the plant is benefiting a more valuable crop. The presence of beneficial predators and parasitoids helps control pest populations naturally occurring with reduced need for pesticides.

Beneficial insects: Welcoming the garden’s helpers

Companion planting strategies focus on attracting and supporting beneficial insects that prey on common garden pests. By providing food and shelter for these beneficials gardeners can encourage them to establish and thrive in the garden.

Examples of beneficial plantings include:

• Carrot or parsley family (apiaceae) (e.g., dill, fennel, parsley, and cilantro), which provide nectar and pollen for a wide range of beneficial insects like ladybird beetles, lacewings, hoverflies and parasitic wasps.
• Sweet alyssum, a low-growing flowering plant, attracts hoverfly larvae which are voracious predators of aphids.
• Borage is a favorite of bees and attracts predatory wasps that help control caterpillars and other pests.

It is important to recognize companion planting will reduce pest pressures but not remove the pests from the garden.

Nutrient cycling: Feeding the soil to feed plants

Companion planting can improve nutrient cycling in the soil. This can reduce the pressure of using synthetic fertilizers, promoting a more sustainable and environmentally friendly approach to gardening.

• Legumes such as beans and peas have a symbiotic relationship, with nitrogen-fixing bacteria in their root nodules. These bacteria convert atmospheric nitrogen into a plant’s available form. When the legume plants are incorporated into the soil or left to decompose, they will release this stored nitrogen, benefiting neighboring plants.
• Deep-rooted plants, like tomatoes and melons, draw nutrients and water from deeper in the soil, making them available to shallower-rooted plants.
• Plants with taproots like carrots and radishes help to break up compacted soil, improve the aeration and drainage, which benefit the root systems of all accompanied plants.

Pest management: Deterring unwanted visitors

Companion planting offers repellent, trap and disruptive methods for managing pests in the garden, though scientific effectiveness of some methods is still debated:

• Repellent plants: Certain plants like basil, chives, garlic and marigolds release volatile organic compounds (VOCs) that are believed to repel certain pests. However, research suggests that the effectiveness of repellent plantings in open gardens is often limited, as these VOCs dissipate quickly in the air. The pest and bloom time of flowers is critical for these to be effective, and further research needs to be done to provide that information for most plant species.
• Trap crops: Trap crops are plants that are particularly attractive to pests. These plants sacrifice their health for more valuable plants. Gardeners can lure pests away from the valuable plants and make them more easily managed or removed. Examples include radishes that attract flea beetles, diverting them away from other vegetables; nasturtiums that attract aphids, drawing them sway from susceptible plants; and mustards are highly attractive to many pests that target brassicas, like cabbage and broccoli, as well.
• Disruptive crops: Disruptive crops are interspersed, nonhost plants among target crops that can confuse pests and make it harder to find their preferred target crop. This disruption can reduce pest damage and slow population growth. Research has shown that visual disruption, rather than scent, is the primary mechanism for this effect.

There are numerous guides online to reference for companion planting. Experimenting with these pairings will help determine what works best for your garden, as results can vary. West Virginia University Extension has put together a plant friend or foe table for your reference.

Physical support and microclimate modification: Working together

Companion planting can also provide physical support and modify the microclimate to benefit their neighbors.

A classic example is the “Three Sisters” planting method. This is an indigenous agricultural practice that grows corn, beans, and squash together. The corn stalks provide a natural trellis for the beans to climb, while the squash vines shade the ground — suppressing weeds and conserving moisture. The beans also fix nitrogen in the soil, feeding all the sisters. Tall plants, such as corn or tomatoes, can provide shade for more delicate, shade-loving species — like lettuce, which can bolt (seed prematurely) in hot, sunny conditions. Ground covers and intercropping can help prevent soil erosion from wind and rain, protecting the soil structure and promoting a healthy environment for plant roots and beneficial insects.

Cautions and considerations: Not all plants are friends

While companion planting holds promise, it is important to consider that not all plant combinations are beneficial.

Some plants have negative effects on each other. Allelopathic plants release chemicals into the soil that can inhibit the growth of other plants. For example, beans are known to release allelopathic chemicals that can hinder the growth of onions and other members of the onion (allium) family.

Another consideration about plant pairings is conflicting needs. Plant species that compete for light, water and nutrients can lead to competition and thus a reduction in yields. For example, planting tall, sun-loving tomatoes next to bush beans may result in the beans being shaded out. A more suitable pairing would be to plant a low-growing, shade-loving plant like lettuce.

Conclusion: Embracing the experiment

The scientific research on companion planting is still ongoing. The principles discussed offer a framework for understanding the potential benefits and challenges of this gardening practice.

By considering biodiversity, beneficial insect attraction, nutrient cycling, pest management strategies and the specific needs of different plant species, gardeners can make informed decisions about companion planting in their gardens.

Companion planting at its heart is a form of experimentation and observation. Keep records of your companion planting trials, noting what works well and what does not.

By embracing the spirit of experimentation, you can uncover the secrets of successful companion planting in your own garden.