Companion plants, they are not what you think!

Companion plants! Great, what a good idea. When you first hear the term and think about the concept it sounds great but there is a lot to not like about it. The term “companion plants”  implies that these plants are partners and they “enjoy” each other’s company.  The term is an anthropomorphism or overlaying of human qualities on non-human organisms.  A more appropriate term may be plant associates or plant associations, a term taken from plant ecology, which has more basis for use.

Plants naturally grow together in groups which are called plant communities. These plants evolved under certain climate, soil, and environmental conditions that allow them to live together in the same place. Coastal sage scrub, oak woodland, and juniper pinyon woodland are some common plant communities where I reside in Ventura County. All of the plants growing in these communities receive winter rainfall and summer drought (Mediterranean climate) to which they are adapted to grow in. Plants growing here either resist drought through specific plant adaptations such as reflective leaf surfaces (white sage), abundant trichomes (sycamore), microphylly (buckwheat), succulent water storage (agaves, yucca and other lily family bulb forming plants), and C-4 metabolism (grasses). Some plants avoid drought by growing in the rainy season, setting seed and then remaining dormant during hot dry weather. Plants can grow in this climate because they have the adaptations to do so.

Plants compete for resources and while doing so may provide a place for other plants to grow. Trees have an advantage over grasses because they can grow above, catching the sun and shade the grasses out. But shade may also provide a place for shade adapted plants to grow. Plants surviving challenges in a specific environment may end up growing together. Woody plants also provide perching birds a place to defecate and spread seeds. This is why unexpected things may grow under other plants. Shade may even be necessary for development of proper form. We have noticed in studying western hackberry (Celtis reticulata) that the tree has no apical control and will not develop into a tree shape when grown in full sun. When grown in shade apical control is present and the plant grows a straight trunk. Birds commonly eat hackberry fruit and likely disseminate it under the canopies of other plants. I don’t think the hackberry minds growing as a blob but its “companion” plants cause it to change form due to changes in light intensity.

Celtis reticulata growing in full sun has no apical control and sprawls as a giant bush yet in nature when it germinates in shade, it develops a straight trunk with fewer branches.

Some plants live very closely with others. Mistletoe is a great example. Leafy mistletoe is a hemiparasite deriving its energy from sunlight of its own leaves but utilizing water and photosynthate from its host. Similarly there are free living plants such as Indian paint brush (a member of the Orobanchaceae) that are also hemiparasitic using their roots to extract benefit from neighboring plants. Holoparasites are true parasites deriving all their nourishment from their hosts, e.g., Dodder (Cuscuta spp.). Dwarf mistletoe is also holoparasitic as it largely lacks chlorophyll. These plants are always found on or near their hosts but it is hard to call them true “companions.” The plants clearly associate with each other and in some cases are detrimental as one of the plants stands to gain nothing from the interaction.

This Indian paintbrush is a hemiparasite. It can be free living or associate with other plants and use their photosynthate.

One popular example of “companion planting” is The Three Sisters (TTS) polyculture of corn, squash and beans. This agricultural system is said to be synergistic. Corn provides support for beans and shades the squash, and beans provide nitrogen fixed from the air for the other two members of the system. The system was “practiced” by indigenous Americans all across the continent. Soils, rainfall and climate are quite diverse across the United States, and I am sure that TTS agriculture had mixed success. It is an interesting thought that the human diet can be satisfied by these crops and likely the combination was more about ensuring sustained calories and nutrients for those who grew them. In one published study there was no increase in production when comparing TTS to mono-cultures of the component parts, nor was N increased in soil. This makes sense since it’s not available until the plant dies giving up its nitrogen to the next crop which is the basis of legume cover cropping. Continued use of the TTS system is a zero sum game as corn and squash will rapidly use all the nitrogen from the previous year’s legume crop.

Mutualism is the concept that interactions between two organisms benefits both. There are many examples of plants that have a mutual relationship with insects, birds, fungi and bacteria. I found no examples of plants that have mutual relationships with other plants, e.g., “companion plants”, common to the scientific literature. I thought this was unusual so I called a friend who is a plant ecologist and asked her the question. At first she was enthusiastic and pointed to non-plant-plant relationships. As I redirected her to only plant-plant interactions we could not identify anything. My suspicion is I have missed something important or we will discover one day that there are plants evolved to help one another but for now, it evades me.

There is no doubt that one plant can help another but it’s incidental and not a sign of a mutual relationship. Most plants evolved to grow in communities because the growing conditions are suited to all. Knowledgeable gardeners and landscape architects will group plants that grow well together. This is only common sense.
Understanding how plants grow in nature informs gardeners about adaptations and this in turn elevates the practice of horticulture.

In this image agaves grow at the base of an Alligator juniper, very companionable. But is there a benefit for either plant?
Agaves like to grow next to rocks. My son’s theory is that both rocks and trees protect the agave from being eaten by javelina. Or perhaps there’s just more moisture under the stones?


Martinez, R.T. 2008. An evaluations of the productivity of the native American ‘Three Sisters’ agriculture system in northern Wisconsin. M.S. Thesis. University of Wisconsin-Stevens Point, College of Natural Resources.

Marsh, E. 2023. The Three Sisters of Indigenous American Agriculture. National Agricultural Library (USDA).

Published by

Jim Downer

Dr. Downer has 34 years of experience as a horticulture and plant pathology Advisor with the University of California Cooperative Extension in Ventura County. Dr. Downer’s academic training is from California Polytechnic Univ., Pomona, (BSc. horticulture & botany, 1981; MSc. Biology, 1983;. In 1998 he earned a Ph.D. in plant pathology, from University of California, Riverside. Dr. Downer’s research is focused on mulch, soil microbiology and disease suppression in mulched soils, diseases of shade trees and cultural practices to maintain landscape plants. Dr. Downer is a member of the American Society of Horticultural Science, the American Phytopathological Society, The International Soc. of Arboriculture, and the Western Chapter of the ISA, and the International Society for Horticultural Science. Dr. Downer is an Adjunct professor at California Polytechnic University in Pomona. Dr. Downer serves on the Board of the John Britton Fund for tree Research as the chair of the research advisory committee, and currently chairs the regional conference committee for WCISA. Dr. Downer has a love of shade trees, Shinrin roku (forest bathing/walking) tree work, wood working, horses, gardening, horticulture and the study of plants and their biology.

10 thoughts on “Companion plants, they are not what you think!”

  1. This is a great post. Thanks so much for clarifying terminology and providing a lucid discussion. I’ve often had questions about much of the “companion planting” concept as promulgated by gardeners in vegetable gardening. I suppose that some plants really are antagonistic to others, but I wonder how solid many of the benefits asserted are.

  2. I found this on a leading seed company’s website:

    “Companion Planting: As a legume, mature fenugreek plants will begin fixing atmospheric nitrogen in the soil around the time flowering begins. This makes it a useful companion for nitrogen loving vegetables like corn, cucumbers, and Brassicas. For the same reason, avoid planting beside Alliums.“

    Firstly, is the first sentence true? Is there current research you may know of that backs this? I was under the impression that nitrogen becomes available in the soil only once the plant dies and soil bacteria break down the nitrogen-packed nodules on the plants’ roots. What is known at this stage?

    Secondly, any idea why they say that last sentence about “avoid planting beside alliums”?

  3. I love your blog, books, and videos! Thanks for promoting science in gardening!!

    I have no relevant expertise and have only recently developed an interest in gardening science.

    I’m a bit confused by this post.

    From your reference:

    I found a link to this study:

    Which concludes:

    “The Three Sisters yields more energy (12.25 x 106 kcal/ha) and more protein (349 kg/ha) than any of the crop monocultures or mixtures of monocultures planted to the same area.”

    This seems contradictory to your claims:

    “In one published study there was no increase in production when comparing TTS to mono-cultures of the component parts, nor was N increased in soil. This makes sense since it’s not available until the plant dies giving up its nitrogen to the next crop which is the basis of legume cover cropping. Continued use of the TTS system is a zero sum game as corn and squash will rapidly use all the nitrogen from the previous year’s legume crop.”

    What study are you citing here? And is the ethnobiology article a good source?


    1. Hi Bryan –
      The ethnobotany paper is not a good source of research-based information. The author apparently relies on one paper to draw their conclusions: Mt.Pleasant, J., and R. F. Burt. 2010. Estimating Productivity of Traditional Iroquoian Cropping Systems from Field Experiments and Historical Literature. Journal of Ethnobiology 30:52–79. DOI: Moreover, the data they present are just numbers. There’s no statistical analysis to show whether these are signficantly different.

      If you look this paper up, you will find decidely mixed results and a poor experimental design. There are virtual no statistical analyses except for Table 1. I can’t copy and paste that table but here’s what the text says:
      “Effects of mound spacing on yields of corn, beans, and pumpkin in Three Sisters and monoculture cropping systems. Tompkins County, NY 1993 and 1994. Values within a single column followed by same letter are not significantly different at 5% level of significance. Corn yields in 1994 ranged from 26 to 40 bu/acre (1390 to 2109 kg/ha). Among the six treatments with corn, only corn in Three Sisters at 48-in spacing yielded lower than corn grown in either polyculture or monoculture at the other spacings. Bean and pumpkin again yielded higher in monoculture than in the Three Sisters, and both bean and pumpkin yielded more in 48-in mounds compared to 30-in mounds (Table 1).”

      So how the author of the study you linked got “The Three Sisters yields more energy (12.25 x 106 kcal/ha) and more protein (349 kg/ha) than any of the crop monocultures or mixtures of monocultures planted to the same area.” from the study they cite is beyond me. Bottom line: look at research that’s published in a relevant source (ethnobotany is not crop production), that has experimental data and statistical analyses, and has been peer-reviewed.

  4. I have recently encountered the book, Plant Partners, Science Based Companion Planting Strategies, by Jessica Walliser. Is this good or bad science for home gardeners? I ask this because many facts used in this book were derived from agriculture trials. Thank in advance for your kind reply.

    1. Well, it’s a mixed bag. Much of what’s in the book is research based, but it can’t be translated on what we do in our gardens. We aren’t planting oats and alfalfa. Most of the “plant pairings” you see in companion plant recommendations are not based on actual research but are extrapolated from agricultural work. That’s not a legitimate extrapolation as conditions are completley different.

  5. Students of my vegetable gardening classes, as well as other garden design classes always seem to ask something like “is it okay to plant x-plant with y-plant” which has to do with this idea of companion planting and “carrots love tomatoes.” My standard answer is that there are many other considerations – height, watering/light needs, color, convenience of harvest, personal preference, for example – that come before any of the traditions of planting one plant with another. Adding a somewhat mythical idea of companion planting ends up being one layer too many. Once upon a time everyone had more time to observe plants and to keep track of their most successful practices, which might actually have had to do with unseen factors such as drainage, soil type or specific cultivars, but ended up being passed down as “best practices.” So I just suggest that students who are interested take the time to observe their own gardens and find out what works best where, taking into account each year’s weather – especially with our totally unpredictable weather in St. Louis. And to try everything at least twice before giving up on it since there are so many factors outside of our control when we are gardening, whether that’s food crops or just plants we love to watch grow. Which last point, in my opinion, is the main point about gardening – enjoyment!

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