Companion planting for pest management in the vegetable garden

Recently, I finished reading a book by Jessica Walliser called ‘Plant Partners’. The book’s focus is on science-based companion planting in the vegetable garden. While many gardeners routinely practice companion planting, the science of why and how it works has not been well understood. Yes, many of us plant marigolds and basil around the garden. But how do these plants interact with other plants in the garden?  

This post will focus on companion planting in the vegetable garden for pest management. We all likely have a pest that we cannot control in the garden. For me, it’s the cabbage worm in Cole crops such as broccoli, cabbage, and kale. As an organic gardener, I routinely use row cover to protect the crops. However, when the cabbage worms get out of control, I apply an organic treatment of Bacillus Thuringiensis (Bt).

Now, with a scientific companion planting approach, there’s another tool in our gardening toolbox. As with most things in gardening, having an intentional approach allows for evaluation and adaptation. What do I mean? Well, as mentioned, I always do companion planting but without the knowledge of ‘why’. Without a ‘why’, it’s hard to judge the success of any given strategy. Let’s take a closer look at companion planting in the vegetable garden.

Companion planting vs. intercropping

Intercropping is another way to describe companion planting. With intercropping, there are multiple strategies that can be deployed. First a definition. Intercropping is the planting of more than one plant within proximity to other crops for an intended biological effect. The benefits of intercropping include the following:

• Intercropping may use resources of light, water, and nutrients more efficiently than a single crop
• Crop mixtures can have lower densities of insect pests
• Intercropping allows for alternative uses of cover crops
• Healthier soil can be built with intercropping
• Intercropping attracts and supports pollinators
• A more biodiverse garden is created with intercropping

Besides benefits, here are the strategies that can be deployed with intercropping.

• Mixed intercropping is when plants are blended within a garden bed
• Row intercropping is when distinct plants are planted in alternating rows with each other
• Relay intercropping is when a second crop is blended together with an existing crop right before harvesting the initial crop
• Trap cropping is a form of intercropping where a second crop is used to attract insects that might otherwise attack the main crop. In other words, the trap crop is more attractive to the insect than the main crop.

The above sounds a lot like companion planting. Well, companion planting is really the same thing as mixed intercropping. Most home gardens use this form of companion planting. I will, therefore, focus on mixed intercropping in the home garden.

The Science

Plants are rather amazing when you think of the ways they have adapted to persevere. Mostly, plants protect themselves via chemical compounds produced by the plant itself. Furthermore, plants protect themselves by emitting a volatile chemical into the air. This volatile chemical can either warn neighboring plants about insects or attract predatory insects to feed on leaf-eating pests.

Lastly, plants can modify the chemistry of the soil through the production of root exudates. These root exudates can make conditions either better or worse for other nearby plants. Certain plants are known as allelopathic plants. Allelopathic plants release chemicals that can inhibit the growth of other plants (such as weeds) or act to repel insects. For example, sweet basil releases oils into the soil and these oils act as an insect repellent to both basil and the plants around basil.

Therefore, knowing about and understanding the impact of the chemicals released by plants informs our choices with companion planting. For example, it’s commonly understood that the odor/fragrance emitted by marigolds is a deterrent to insects. However, there is no science to back up this thinking. Yes, marigolds are a deterrent to pests by not for the reasons we initially thought.  

Let’s look at the science of broccoli interplanted with marigolds. Marigolds mask the cues that pests use to find broccoli plants. Thus, marigolds act as a pest deterrent for broccoli.

To better understand this phenomenon, we need to understand how insects find plants. There are three ways that insects find plants.

• First, by a chemical emitted by the plant.
• Second, by a visual profile of the plant.
• Third, by a combination of both chemical and visual clues.

Mixed intercropping allows us to interrupt these visual and chemical pathways.

Companions and Targets

In this section, we will look at several combinations of mixed intercropping that can have a benefit on the target crop being protected. Scientific studies have found these causal relationships. However, you may have a different experience in your home garden. Therefore, it’s important to observe and keep notes on any impacts.

Repellent impact

1. Plant basil with tomatoes to repel both thrips and tomato hornworms.
2. Plants in the Allium genus (Onions) have a repellent property against a variety of insects including moths, aphids, and mites.
3. Protect Brassica crops (broccoli, cabbage, kale) by the repellent effect of sage, rosemary, thyme, mint, chamomile, and nasturtium. Effectiveness will vary and is dependent upon the insect and plant involved.
4. Intercropping tomatoes with cabbage has been shown to be a repellent to the diamondback moth.

 A further note about marigolds. French marigolds have been shown to alter the chemical profile of the target they are protecting. They do this via the interaction of underground roots. Marigolds produce root exudates that are then absorbed by the roots of the neighboring plants. Thus, providing insect protection. How amazing!

Camouflage impact

Some companion plants act as a visual barrier. For example, growing dill or corn can act as an insect barrier. Because of their height, they visually camouflage the target plant and impede insect movement. Sunflowers can also act as a visual barrier.

Natural enemy impact

Another method of managing pest populations is to plant flowers that bring natural enemies into the garden. In this case, the flowers act as biological control. Creating greater diversity in the garden will naturally create greater varieties of prey. Companion flowers act as both a food source (pollen and nectar) and shelter to natural enemies.

One study showed that when dill was interplanted with eggplant, there were lower populations of the Colorado potato beetle. In this case, the flowering dill plants encouraged more natural enemies of the Colorado potato beetle.

Additional companion planting suggestions

• Plant sage or thyme (companion) with Brussel sprouts to reduce diamondback moths.
• Partner broccoli and cabbage with thyme (companion) to reduce the egg-laying behavior of cabbageworm butterflies
• Intercrop green onions (companion) with Chinese cabbage to lower flea beetle populations
• Green onions (companion) planted with collards reduced aphid populations
• Plant Collards (companion) several feet away from cabbage to lure the diamondback moths to the collards as they prefer collards over cabbage.
• Use blue Hubbard squash as a trap crop for squash bugs and/or squash vine borers. The blue Hubbard should be planted 3-4 weeks before the squash and locate several feet away from the squash.
• Plant Nasturtiums (companion) with zucchini to control squash bugs. Plant side by side.
• Radish and Pak Choi (companion) can be used to lure flea beetles from various vegetables including tomato, eggplant, peppers, cabbage, broccoli, and kale. Interplant the radish or Pak choi next to the main crop and sow a few weeks in advance.
• Interplant calendula (companion) with Cole crops (broccoli, cabbage, kale, collards) to reduce aphid numbers.
• Growing sweet alyssum around carrot plants can deter pests that lay eggs in the soil.

In summary

Understanding the science behind companion planting in the vegetable garden is an amazing area of study. Because most studies are conducted on large farms, adaptation to the home garden may take some trial and error. What we know for sure is that having diversity in any garden is better than a monoculture (single species of plant). This year, I am thinking differently and will focus on mixed interplanting in each of my vegetable beds. Give it a try and remember to record your results!

Here are a few posts that may be of further interest.

• Managing garden pests with an integrated approach
• Install a solar-powered fence to keep groundhogs out of the garden
• Using cover crops in the home garden

References

Plant Partners by Jessica Walliser

Companion Planting and Insect Pest Control

Joyce E. Parker(1) , William E. Snyder(2) , George C. Hamilton(1) and Cesar Rodriguez‐Saona(1)

 (1) Department of Entomology, Rutgers University, New Brunswick, NJ, USA

(2) Department of Entomology, Washington State University, Pullman, WA, USA