Home gardeners that embark on the “organic” journey often find themselves wondering about companion planting. They read about growing organic vegetables, and inevitably come to this subject. Logically, one starts to think, “Is it okay to plant this in my flower bed, or plant some of these next to some of those? Will one or more of the plants interfere or compete with one another? Will one plant provide benefits to the adjacent plants?” Just receiving simple one word answers doesn’t really teach us very much. In this post, I’ll attempt to explain some of the basic science behind companion planting, and the conundrum between plant companions that repel pests and planting those that attract beneficial insects.
Growing two or more different plants in the same garden or container raises a few concerns. Here’s a list of basic topics gardeners need to consider when companion planting. Symbiotic nitrogen fixation, mutual climate co-operation, trap cropping, biochemical pest suppression, the attraction of beneficial insects, and biodiversity. Holy seed starts Batman, that’s a lot to think about! Let’s begin with how these work individually.
Symbiotic Nitrogen Fixation
Plants cannot survive without nitrogen. Unfortunately, plants can’t just breath nitrogen gas which is abundant in our atmoshpere. Instead, microbiological organisms use the ammonia form of nitrogen to manufacture the proteins and other nitrogen-containing nutrients needed to survive. So, what plants can we use to get this task done? Organic gardeners typically use cover crops as companion plants . Peas, beans and other legumes fix atmospheric nitrogen using small growths on their roots called nodules. Those microbes, and the bacteria in your soil, then act as the plant nitrogen broker. Plants ask the microbes for N, the microbes go get it from the nodules and bring it back for the plant in exchange for exudates the plant produces and emits through its roots. Through this symbiotic practice of nitrogen fixation, you will spend less money on nitrogen fertilizer.
Mutual Climate Co-operation
Mutual climate co-operation refers to planting crops that, when they become full size, complement the needs of plants growing around them. A good example of this is planting tall, sun-loving plants like corn, alongside short, shade-tolerant plants, like beans, that will lend a helping hand to the corn. These combinations will help both plant types produce better yields, and provide more pest control benefits overall. Another superior companion plant is Marigolds. Marigold is a sun lover but will enjoy the shade provided by other garden plants as they get older. Dappled sunlight will extend your Marigold’s flowering cycle. The scent and exudates of those flowers is what keeps bugs away from your eggplants or peppers. A little more on Marigolds later.
Trap cropping refers to the planting of a specific plant. It’s a decoy plant. The goal is to attract pests away from your feature crop . What I call ‘Double Trap’ crops, are those that attract bugs away, plus prevent those bugs from reproducing. For example, Southern Giant Curled Mustard, when planted around a garden perimeter, makes for an effective trap crop in controlling pierce-sucking insects that feed on the leaves. These insects generally leave blotches and holes in leaves. Sound familiar? The mustard plant gets male insects to release pheromones which attract other like kind bugs. In addition, this mustard produces a type of enzyme called homing endonuclease, that can render pierce suckers sterile.
Biochemical Pest Suppression
Some plants exude chemicals that can protect neighboring plants. For example, marigolds release thiopene, an aerial pest suppression that helps to repel many insects and nematodes. Nasturtiums have been shown to have larvacidal effects on leafminers. In addition, some plants emit pheromones that confuse male insects, causing them not to mate. Another form of biochemical suppression is a phenomenon called allelopathy. Allelopathy is the release of chemicals from the root system to discourage insects. Voodoo Lily can produce allelopathic chemicals that deter fungus gnats. These chemicals can have a positive or negative influence on surrounding plants. Those that have a negative influence can be an important part of another plants’ defense, reproduction and growth systems.
Attracting Beneficial Insects
Planting neighboring plants to attract good bugs is traced all the way back to ancient Egypt. This strategy is used when the main plant lacks the ability to attract the good guys on its own. To determine what type of beneficial insects you want to attract, get an idea of what pest insects you are likely to encounter. My personal favorite to attract and keep around is the green lacewing. The lacewing larvae resembles an alligator and they attack and eat everything you want them to. There is a long list of good predators and an equally long list of plants they are attracted to. I promise you’ll always find fern yarrow, dill, coriander (cilantro), and mint planted around my garden. There is a subtle balance however, between the number of companions that are repellants and those that are beneficial attractants. This leads us right into biodiversity.
Many insects and disease organisms prefer specific plant species. For example, horn worms love tomatoes so much that they are commonly called tomato worms. Cabbage loopers love to chow down on not only cabbage, but also broccoli, brussel sprouts, cauliflower, collards, and kale. Now think about how the hornworm views a monocrop field of tomatoes, or looper moths see a monocropping of any of the vegetables mentioned. It’s an all you can eat buffet with 100% of your absolute favorite foods. It is imperative to mix up not only your crop types, but also your repellants and attractors. You reduce the likelihood of a massive infestation by creating a balance of plants that will keep the bad guys away and keep the good guys around.
Let me give you a specific example that you can put to use right away. This example targets all too common garden pests, like spider mites. I prefer to use repellant plants you can eat. Aditionally, consider at least 4 attractants. As an example, I enjoy eating coriander (cilantro), thyme, fennel and dill which are both repellants and attractants. Then I mix in angelica, tansy, chrysanthemums, marigolds and fern yarrow here and there around the garden.
I’m barely scratching the surface of this subject. To learn more about Organic Gardenng, this EDIS link is a good place to begin:
- Mayer, Daniel (2010) “The complete guide to companion planting. Everything you need to know to make your garden successful”. Atlantic Publishing Group ISBN-10: 16202300382.
- Landon, Amanda J. (2008). “The “How” of the Three Sisters: The Origins of Agriculture in Mesoamerica and the Human Niche”. Nebraska Anthropologist. Lincoln, NE: University of Nebraska-Lincoln: 110–124.
- Pleasant, J. (2006). “The science behind the Three Sisters mound system: An agronomic assessment of an indigenous agricultural system in the northeast”. In Staller, J. E.; et al. Histories of maize: Multidisciplinary approaches to the prehistory, linguistics, biogeography, domestication, and evolution of maize. Amsterdam. pp. 529–537
- Holden, Matthew H.; Ellner, Stephen P.; Lee, Doo-Hyung; Nyrop, Jan P.; Sanderson, John P. (2012). “Designing an effective trap cropping strategy: the effects of attraction, retention and plant spatial distribution”. Journal of Applied Ecology. 49 (3): 715–722. doi:10.1111/j.1365-2664.2012.02137.x. ISSN 1365-2664
- “Pacific Northwest Nursery IPM. Flowers, Sweets and a Nice Place to Stay: Courting Beneficials to Your Nursery”. Oregon State University. February 2013
- Dan Pearlman, Glenn Adelson, (1997) “Biodiversity exploring values and priorities in conservation” Blackwell Science Press
- Horticulture Research International, Wellesbourne : “Insects can see clearly now the weeds have gone”. Finch, S. & Collier, R. H. (2003). Biologist, 50 (3), 132-135
- “The Self-Sufficient Gardener Podcast–Episode 24 Companion Planting and Crop Rotation”.2010.
- Pleasant, Barbara (June–July 2011). “Organic pest control what works, what doesn’t”. Mother Earth News (246): 36-41.