Growing Up Together: The Science Behind Companion Planting
Not all traditional companion planting recommendations hold up to modern day science, but some do. Monica Mansfield pored through numerous studies to shed light on what works and what doesn’t in the garden.
If you ask a group of gardeners what they think about companion planting, you’re bound to get a mixed response. Some swear by the traditional practice and follow the recommendations religiously in their own gardens, while others dismiss it as nothing more than “garden woo.” As with most things, the truth lies somewhere in the middle.
Companion planting is the practice of planting crops together that have a mutually beneficial effect on one another. For example, many gardeners plant garlic throughout their garden to deter pests and studies have shown this to be an effective strategy.
A well-known example of companion planting is the Three Sisters garden, which was practiced by some Native American tribes in North America. They would plant corn, beans, and squash together. The beans would fix nitrogen from the air and help feed the corn, which is a heavy nitrogen feeder. The corn would act as a trellis for the beans, and the squash would spread over the ground helping to suppress weeds and deter raccoons from eating the corn.
Many of the traditional recommendations and companion plant lists come from the observations and experiences of past gardeners, mythology, and the occult, but they rarely give the reasoning behind their recommendations. As more scientific studies are performed, we are finding that some of these beneficial plant associations can be proven and explained by science, while others have been disproven and dismissed.
In an effort to distance themselves from the supernatural aspect of traditional companion planting, many scientists prefer not to use the term companion planting at all, instead using terminology such as plant associations, mutual climate cooperation, intercropping, trap cropping, nurse cropping, and symbiotic nitrogen fixation.
Some of these techniques can be used to manage pests in the garden. While it isn’t likely that any of these techniques will rid you of pests completely, they will help you build an ecosystem in your garden that is more favorable to beneficial insects and doesn’t lay out an all-you-can-eat buffet for the harmful ones.
Beneficial Insect Attractors
One of the best ways to cut down on pests in the garden is to encourage beneficial insects to call your garden home by planting their favorite flowers. There are many beneficial insects you can attract, but ladybugs and parasitic wasps are two of the best.
Parasitic wasp larvae eat aphids, beetle larvae, bagworms, cabbage worms, Colorado potato beetle, corn ear worms, cucumber beetles, cutworms, gypsy moth caterpillars, Japanese beetles, leaf-miners, mealybugs, Mexican bean beetles, moth caterpillars, sawfly larvae, scale, squash vine borers, tent caterpillars, tobacco budworm, tomato hornworm, and whiteflies. Plant marigolds, zinnias, yarrow, white clover, cosmos, thyme, rosemary, dill, and lavender to attract them to your garden.
Read also: Beginner's Guide to Beneficial Insects
For the pests that your beneficial insects leave behind, plant trap crops to lure them away from your main crops. Many studies have shown trap crops are an effective way to manage pests in the garden, however, the technique must be managed properly or else the trap crop becomes nothing more than a pest nursery and can do more harm than good.
The key is to find a trap crop that is more appealing than the crop you are trying to protect, plant it at the right time so that pests infest the trap crop before the protected crop is available, and then treat or dispose of the trap before the pests move onto other plants in your garden.
For example, nasturtiums will attract aphids away from other crops. In California, the need to spray cotton fields for Lygus Hahn was almost completely eliminated by using alfalfa trap crops. Snap beans have been successfully used as a trap crop to control Mexican bean beetles in soybeans.
Intercropping is when you plant two or more crops together that have a beneficial effect on one another. The practice can create more biodiversity in your garden which has been proven to effectively manage pests. If a pest finds a crop they like in a monoculture setting, they just hop from plant to plant. However, in a polyculture, where different plants grow together, the variety makes it harder for the pests to find their favorite crops for dinner.
One study’s results suggest that intercropping garlic or undercropping Chinese chives is an effective pest management technique for strawberry crops.
Another study confirmed that alliums effectively suppressed Fusarium wilt in cucumber seedlings. The scientists were trying to understand the mechanism behind the results and, although still not entirely understood, they believe it has to do with the microorganisms in the allium’s rhizosphere.
Gardeners have used marigolds to repel pests in their gardens for generations. Current studies show that this is effective in some cases and not in others. One study intercropped marigolds with tomatoes and found it to be effective against whitefly as long as the marigolds were planted when the tomatoes were very young. However, another study proved them to be ineffective at protecting carrots from the carrot fly. The same study did show, however, that interplanting young onions with carrots helped to manage carrot fly populations, but once the onions started to form bulbs, the benefit was removed.
Although intercropping has been used successfully for centuries and science confirms some of the applications, the technique can have drawbacks. If you plant your crops too closely together, they will compete for nutrients which can have a negative impact on your harvest. Proper spacing is still important in polyculture if you want your plants to thrive.
Biochemical Insect Suppression
Scientists are still trying to understand the mechanisms behind why some plants help to manage pests. Although they don’t have a complete understanding, they are starting to put the pieces together. One hypothesis suggests that some aromatic plants contain volatile oils that interfere with pests feeding, distribution and mating, which results in lower pest populations. Plants such as neem contain chemicals that have been proven to repel pests and treat fungal disease. Pyrethrum is a well-known botanical insecticide that comes from certain daisies. Basil has been shown to successfully repel thrips in tomatoes.
While these studies make a case for companion planting, other studies have proven other pairings to be ineffective. For example, research has shown that marigolds and mint do not repel the onion fly or cabbage root fly as once thought.
Symbiotic Nitrogen Fixation
Legumes are nitrogen fixers, meaning they can convert nitrogen from the atmosphere into food for themselves and can then feed other plants around them. They feed other plants in three main ways: their leaf litter will feed the soil when they die and decompose, their root exudates feed the surrounding soil, and soil microbes will take nitrogen from them and feed nearby plants via mycorrhizal networks.
Some studies have shown that the major benefit of these nitrogen fixers only comes once they die and decompose, returning nitrogen to the soil. Others show that they do indeed supply a substantial amount of nitrogen to surrounding plants, which also seems to be related to higher microbial populations in the soil. To be sure you’re getting the benefits, you can follow heavy nitrogen feeders such as corn and tomatoes with a crop of peas or beans and then let them decompose and feed the soil where they are planted.
Read also: Friends with Benefits
Mutual Climate Cooperation
When designing your garden layout, you always want to remember that in order to be healthy, plants need nutrients, sunlight, water and the proper pH in the soil. Plants that do well in the same pH zone will grow well together. Shade loving plants will grow well next to tall plants that block the sun for part of the day.
Nurse crops can be used to shield smaller plants from harsh conditions while they grow. By planting seeds and starts next to larger plants that will be pulled by the time the small plants grow larger, the nurse plants will protect the babies from harsh sunlight and can act as a windbreak.
Companion planting can be used as a tool to help manage pests in your garden, but should be viewed from a scientific perspective.
Not all traditional companion planting recommendations hold up to modern day science, however, some do and can be an asset to your garden. It’s important to keep this in mind when reading traditional companion planting charts and manuals. Keep a healthy skepticism, read the science, and plan your garden accordingly.