The Three Sisters Legacy: The Science Behind Companion Planting

Clare Brandt
18 Min Read

There’s nothing quite as frustrating as having some know-it-all tell you why your cucumbers aren’t getting bigger than pickles, or why your tomatoes look like tomatillos.  But when that reason seems like some old wives tale that has no obvious logical basis in science, well, that’s just annoying.

I’m a scientist at heart, and I spent the better part of three years sitting in lectures and labs learning all about horticulture and landscape design.  I know which plants attract the same pests and which will suck the ground dry of water or nutrients.  I know how to identify and treat verticillium wilt, and what to do with a slug/caterpillar/aphid infestation.

But now, here I was being told by an older neighbor—someone who I’ll be the first to admit has more than five times my experience at growing veggies, even if he is a know-it-all—that I shouldn’t have planted my tomatoes anywhere near the sunflowers standing sentry against the back fence.

“Why?” I ask, which is a big mistake because forty-five minutes later I’m none the wiser and my head is swimming with age-old adages.  A Google search doesn’t help either, on the contrary: I’m left to believe that sunflowers would make a rather good support for my tomatoes, but not my beans.  Plus, now I have a long list of veggies, herbs, and flowering plants (and a few trees) that I should or shouldn’t plant together.

Are these all just old wives’ tales, beliefs passed down through the oral tradition from my gritty, old, farmer neighbor and great Aunt Mabel to me?  Or is there some scientific fact hidden here somewhere?  Many scientists, like me, have questioned these adages over the years and have uncovered some truths and some pure hokum.

There is no question that plants influence one another.  Both Varro (a Roman agriculturist) and Pliny the Elder (a naturalist, also Roman) are credited with noting, around two thousand years ago, that nothing likes to grow around the root zone of a Walnut tree.  Walnuts and some other members of the genus Juglans produce an allelopathic toxin—a chemical that inhibits the growth of other plants—called juglone.  But through scientific research, we’ve also discovered that only some plants are susceptible, while others are unaffected by the chemical.

Even further back in history, while domesticating corn, beans, and squash, Native Americans discovered that these three crops grow better when planted together.  According to Iroquois legend, corn, beans, and squash are three inseparable sisters.  The Iroquois believe each of these three crops are precious gifts from the Great Spirit and are watched over by one of three sisters’ spirits, called the De-o-ha-ko.  Iroquois ceremonies to honor the De-o-ha-ko mark the planting season and the first harvest.

Even without the scientific awareness to understand why these companions thrived, the tribes passed on the knowledge—through stories and annual rituals—that corn, beans, and squash should always be planted together.  And this makes sound environmental sense: The beans and corn have a symbiotic relationship in which the corn provides a support for the beans to grow up.  In return, the beans provide extra support for the corn in strong winds.  The squash adds to this partnership by providing ground cover to both conserve water and repress weeds.  In addition, although corn is a hungry feeder, beans (as all legumes do) take nitrogen from the air rather than the soil during the growing season, and so don’t compete for nutrients.

Whether the Native Americans tried to grow their beans up sunflowers, which also were domesticated and grown by tribes more than four thousand years ago, we’ll never know, but according to my Google list, that’s not a combination that would achieve great success.

Sunflowers, notably Helianthus annuus species (annual sunflowers) produce an allelopathic phytotoxin that inhibits seedling germination and seedling growth in some plants.  Scientific studies have shown that extract from sunflowers are effective in suppressing the germination and growth of certain weeds, namely littleseed canarygrass (Phalaris minor), lambsquarters (Chenopodium albun), lesser swinecress (Coronopis didymus), toothed dock (Rumex dentatus), and burclover (Medicago polymorpha).  Unfortunately, it doesn’t seem there is anything but anecdotal evidence (and about 10,000 Google hits) to indicate that sunflowers stunt the growth of plants like beans and potatoes.

So where did this list of companion plants come from, and how reliable is it, anyway?

Linda Chalker-Scott, Ph.D., extension horticulturist and associate professor at Washington State University, writes in her article “The Myth of Companion Plantings”, that, “the problem with using the phrase ‘companion plants’ is that it is broadly used to describe plant interactions in the realms of science, pseudoscience, and the occult… claims that companion plants can be determined by ‘sensitive crystallization’ of their extracts (i.e. to discover which plants ‘love’ each other), or through the study of a plant’s rhythm, its vibration, its music, and its note.”

Robert Beyfuss and Marvin Pritts from the Cornell University Department of Horticulture agree that “companion planting is based upon some very bad science,” in particular the sensitive crystallization method, which was created by Dr. Ehrenfried E. Pfeiffer in the 1930s.

Dr. Pfeiffer used chromatography—a method of separating things into smaller chemical components and displaying them in a visual way—to make chromatograms of different plant combinations.  He concluded that those forming bright or clear chromatograms were beneficial, while combinations that made cloudy or dull chromatograms were antagonistic.  Beyfuss and Pritts note that “the notion that carrots love tomatoes but beans dislike fennel is based upon an analytical laboratory procedure and not on direct observation of the plants in nature.  No legitimate scientist believes that this method can determine compatibility among plant species.”

So, if we can’t rely on the observations of Romans, Old Farmer’s companion plant lists, or our great aunt Mabel, what can we rely on to help our garden and veggie plot thrive?

Scientists agree that there are benefits to planting and maintaining diversity, especially with crops.  There’s a certain amount of security in being diverse:  After all, losing an entire season is far, far worse than losing just one crop.  In addition, interspersing crops within the same area, rather than growing in blocks or rows of the same crop has been shown to confuse—if not deter—pests.  Several species in one area can seemingly disrupt the ability of many herbivorous insects to use visual and olfactory (smell) cues to find their host plants.

Similarly, scientists have begun to study the idea of trap cropping, a method where a plant that is known to be attractive to a certain pest is planted nearby the main crop.  The theory is that the pest enjoys the “trap” plant and leaves the main crop alone.  However, Professors Anthony M. Shelton and Brian A. Nault from the Department of Entomology at Cornell University found that in a commercial application, using collards as a trap crop to control the diamondback moth in a field of cabbage “was unsuccessful because it neither reduced the number of larvae on cabbage nor concentrated the insects on collards.”  They did, however, have some success in a controlled environment tempting the moth away from cabbage and broccoli using garden yellowrocket  (Barbarea vulgaris), a common biennial weed.

It’s also been scientifically proven that some plants produce chemicals that seem to repel herbivorous pests.  Common tansy (Tanacetum vulgare) planted alongside potatoes is said to repel the Colorado potato beetle, although tansy itself is listed as a noxious weed in many states.  This ability to repel could be due to the presence of the chemical thujone, which is also found in arborvitae (Thuja spp.), mugwort (Artemisia vulgaris), oregano (Oregano spp.), common sage (Salvia spp.) and wormwood (Artemisia absinthium).

Marigolds (Tagetes spp.) contain varying amounts of thiophene in their roots and leaves.  This chemical gives the marigold the distinctive smell we all know, and some love.  Apparently carrot root fly and aphids hate them, but not as much as do the soil parasites, nematodes.  Scientists are currently working on perfecting anti-nematode chemicals based on marigold extracts.  But does it work to plant marigolds near your veggies?  Authors Thomas C. Fuller and Elizabeth May McClintock in their book Poisonous Plants of California write: “Different species of Tagetes have been demonstrated to contain substances in the roots that are toxic to some but not all species of root-knot nematodes and root-legion nematodes.  Marigold plants do not actually secrete substances into the soil to kill nematodes; they are killed only in the roots of the plants.”   So it seems like a good planting of marigolds might catch and kill some of your nematodes, and probably would be worth trying if you had a problem with them.

Scientifically significant amounts of thiophene also are found in common yarrow (Achillea millefolium), wolf’s bane (Arnica montana), yarrow/wormwood (Artemisia spp.), bachelor buttons (Centaurea cyanus), pyrethrum daisy (Chrysanthemum cinerariaefolium), sunflower (Helianthus annuus) and tansy (Tanacetum vulgare).  However, contrary to popular companion planting websites, there is none found in pot marigolds, which are a completely different species (Calendula spp.) to Tagetes, putting to rest the age-old argument about which is better, French marigolds or pot marigolds, at deterring nematodes.

One chemical that is found in pot marigolds, however, and in some chrysanthemums (mainly Chrysanthemum cinerariifolium and C. coccineum) is pyrethrum. Synthetic pyrethroid insecticides are based on the naturally-occurring pyrethrins, and are extremely toxic to most insects.  Of course, synthesized pyrethrins are a lot more concentrated than a plant, but that doesn’t mean pests—especially aphids, cabbage worms, leafhoppers, harlequin bugs, pickleworms, spider mites, and others—aren’t deterred by a slight whiff of a chemical that’s toxic, just that you might need quite a few to have any effect.

In addition to using companion plants that may produce a small amount of natural pesticide, what about trying to attract the right kind of insects and other pest-predators?  This seems like the best companion planting method so far.  The trick is to provide the perfect environment for beneficial bugs, especially predatory species—like ladybugs, lacewings, hover flies, mantids, robber flies, spiders, and predatory mites—and parasitic species including tachinid flies, and trichogramma and ichneumonid wasps. Plants that are notoriously good at attracting beneficial species are nasturtium (Tropaeolum majus), yarrow (Achillea millefolium), lovage (Levisticum officinale), fennel (Foeniculum vulgare), dill (Anethum graveolens), chamomile (Matricaria recutita), caraway (Carum carvi), and borage (Borago officinalis), but generally any plants that produce copious nectar and pollen are a good way to encourage beneficial critters into your man-made environment.

And while we’re considering environments, a good companion plant is one that also provides the perfect environment for another.  Tall, sun-loving plants provide shelter for low-growing shade-lovers, or trellis support, or a wind barrier; and the best companions make the best use of your growing space, providing maximum yield per square foot.

Frequently, companion plants are noted to have the ability to improve not only the yield, but also the flavor of the crops surrounding them.  This is up to your taste buds to decide, but one thing is for sure, some aphorisms (like plant basil with tomatoes) may be true for a reason other than increasing fruit yield: they taste great when eaten together.  As a side benefit, this one seems to be true in the garden too. Michael K. Bomford, while at West Virginia University, found in his doctoral dissertation that actually “tomato plants grown with basil produced more fruits per plant.”

So maybe companion planting has more to do with our stomachs than with pure science.  The Native Americans knew the Three Sisters provided everything they’d need to supplement the food they hunted and gathered:  The beans provide the essential amino acids, riboflavin, and niacin; the squash provides vitamins A and C, and vegetable fat from their seeds; and the corn nearly anything else you need to get you through a long, hard winter.  The added benefit being that each of the Three Sisters provided a harvest that would keep and sustain them for months.

Scientifically speaking, the Native Americans got it spot on:  They cultivated a group of crops that used the space they had to the best advantage.  They planted tall, sun-loving corn that provided the perfect companion to shade-tolerant squash.  The beans provide support, while using the corn as a trellis.  It’s possible that they practiced a little environmental pest control since the diverse canopy also is thought to disorient the adult squash vine borer.  In turn, the presence of the prickly vines is said to discourage raccoons from ravaging the sweet corn and beans growing around it.  The diversity of the species guaranteed that they wouldn’t lose an entire season’s worth of food due to poor conditions since beans can thrive in a wet, cool summer even though corn or squash will not.  Plus, they planted things that they could store, and tasted good together.

It seems that, for the most part, oral history and old-wives’ tales are doing what they do best, teaching us some common sense things, like if we have a problem with aphids, we should plant something that attracts ladybugs.  But with anything passed from one generation to the next, the real proof of the pudding is to test and trial; continue what works and ignore what does not.  If you have great success growing your tomatoes next to sunflowers, then good for you.  And if you can’t get your cucumbers bigger than a pickle, you could always try planting them with dill, and if that doesn’t work, just make some dill pickles.  Yummy.


Clare Brandt is a freelance writer based in Colorado.  In researching this article, one of the things she was most excited to see listed on the internet was basil’s ability to repel mosquitoes.  However, although she’s now succeeded in growing basil to near tree-like stature, she’s still plagued with the contemptible insect and therefore assumes that mosquitoes haven’t checked the internet lately.

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