Nearly every gardening book mentions the Three Sisters: corn, squash, and beans. In North America, from what’s now Canada to Mexico, Native Americans used to plant these three in unison. Together, the three make a “complete protein.” In other words, eating these three vegetables will supply all the essential amino acids in enough abundance to meet a person’s protein needs. In addition, growing them together has some other benefits. The corn supplies a pole the beans can climb. The squash covers the ground, suppressing weed growth. As the story goes, the beans fix nitrogen from the atmosphere and enrich the soil.
As I’ll relate in this article, the actual situation might not be quite so tidy. Although the Three Sisters mythology is long, scientific verification of its claims is often lacking. Still, a few studies have been done, and a recent scientific study came to a remarkable conclusion – the Three Sisters method of cropping has a hidden benefit. Intercropping these three plants results in less damage from some insect pests.
The Sisters
Each of the crops in this system was domesticated in Mesoamerica. Corn was domesticated around 9,000 years ago, squash roughly 10,000 years ago, and beans around 8,000 years ago.
Corn, or maize (Zea mays), is a cereal grain that, by weight, is the most abundant cereal crop in the world. Corn can be used to make a wide variety of human food products, including cornmeal, cornstarch, corn syrup, corn oil, and masa. In Mesoamerica, it’s a staple food in the form of tortillas and tamales.
Ethanol from fermenting corn is used as a fuel additive and as the base for some distilled spirits, such as bourbon. Sweet corn is a favorite crop of gardeners, while most commercial growers grow field corn. The corn grown by Native Americans practicing Three Sisters production methods was field corn.
Our bodies can’t synthesize essential amino acids, and they must be taken in by diet. There are nine of them – histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. All plants produce all of the essential amino acids, but not at the levels required in humans. For example, corn is low in lysine, which is high in pole beans, and the niacin corn contains only becomes available if released by alkali treatment (nixtamalization). On the other hand, squash is low in methionine and pole beans are low in cystine, while both of these amino acids are plentiful in corn.
Squash (Cucurbita pepo) includes winter squashes (including pumpkins) and summer squashes (including zucchini). Summer squashes are harvested early, before their rinds have hardened and seeds have fully matured. Winter squash is harvested when it has completely matured. The seeds of winter squash can also be roasted and eaten. In Three Sisters agriculture, winter squashes are grown.
Common beans (Phaseolus vulgaris) are legumes. They’re grown for their edible seeds and sometimes (as in green beans) for their unripe seed pods. Like most legumes, they form root nodules that harbor rhizobia – nitrogen-fixing bacteria that help reduce the plants’ need for nitrogen from the soil. In the Three Sisters system, pole beans are grown for the dried beans they produce.
All three of the Three Sisters yield foods that can be stored for extended periods after harvest, providing another reason for early Americans to grow them.
The Method
In the Three Sisters method of agriculture, the soil is mounded into hills about a pace (a few feet) apart. Multiple corn seeds, often four, are planted on each hill. These are allowed to sprout. After they do, the same number of bean plants are planted next to the corn seedlings. Next, squash seeds are planted near the edge of the mound, in the spaces between mounds, or as a single squash plant in a nearby mound.
One effect of hilling the soil is that it drains quickly when wet. Corn doesn’t do well if its roots are exposed to standing water too long. The other effect is that the sun warms the soil in the mound more than it warms the soil at ground level.
Eventually, the beans can be trained to the corn stalks, and they will start to climb the plants. This may provide some protection from “lodging,” a common problem in corn where strong winds can blow the stalks over. Anecdotal evidence suggests the “hairy” squash vines might deter raccoons. Likewise, after establishing themselves, the squash leaves cover the hills, shading out weeds. Some sources claim, again anecdotally, that the corn stalks disorient the adult squash vine borers (Melittia cucurbitae), which are common insect pests of squashes. Additional soil is added to build up the mound early in the growing season.
Before synthetic fertilizers, growers enriched the soil in other ways. Each year, the plant material can be collected and incorporated into the mound, improving the soil with organic matter. Adding organic material to a reasonably well-draining soil over many seasons will yield richer soil.
Likewise, there are opportunities to improve the seed stock. Productivity among the mounds can be assessed, and seeds from the most productive mounds can be used to seed the next season’s crops. In this manner, specific cultivars that work better together can be propagated and spread.
Three Sisters cultivation can be considered a form of no-till agriculture. The hills are mounded and weeds removed, but the surrounding area is left untouched. The hills have to be weeded early in the growing season, but the spaces between mounds don’t.
Evaluating the Three Sisters
If you’re a gardener or a small-scale farmer, how does the Three Sisters system stack up as a means of production? First of all, at the most basic level, it works. Some gardeners might wonder if growing these crops so close together would result in one plant suppressing the others’ growth. Most people know, for example, that nothing grows beneath a walnut tree (Juglans sp.) because the roots of a walnut secrete chemicals, called “allelopaths,” that suppress growth. And walnuts are far from the only plant that does this. However, in the case of the Three Sisters, this doesn’t occur. The three plants grow fine next to each other.
So, Three Sisters works at a basic level, but historical information helps us evaluate the method further. When Europeans first encountered the Iroquois and other Native tribes practicing Three Sisters agriculture, they reported being impressed by the productivity of the Natives’ fields. Given that Native Americans from what’s now the northern U.S. down into what’s now Mexico practiced this form of agriculture, it can be inferred that it has some merits.
If you’re a grower interested in historical crop-production methods, growing food crops without synthetic inputs, or using the Three Sisters methodology for cultural reasons, give it a try. You’ll likely be happy with the results.
The Science
You may wonder if the Three Sisters method works better than other methods (including modern monocropping) and whether the many claims about the Three Sisters can withstand scientific scrutiny.
First, one thing commonly claimed about the Three Sisters is just plain wrong: Beans don’t add nitrogen to the soil. Like most legumes, common beans have nodules in their roots that harbor nitrogen-fixing bacteria (rhizobia). These bacteria take gaseous nitrogen (N2) from the air and convert it into ammonium (NH4), a nitrogen source usable to the plant. (Interestingly, the bacteria can live independently but can’t fix nitrogen unless associated with a legume.)
Fixing nitrogen takes energy in the form of adenosine triphosphate. As such, the bacteria produce only what they and the plant need. They don’t make enough nitrogen to add to the soil. Commercial bean farmers still need to add nitrogen to their crops, something they wouldn’t need to do if the rhizobia made enough excess nitrogen to enrich the soil. However, rhizobia reduce the amount of nitrogen required for the beans. And corn is a “heavy feeder,” so this gives the hill a bit of a break when it comes to nitrogen needs.
It would be great if the Three Sisters method was tested against other cultivation methods head-to-head. Unfortunately, as master’s student Rhea Martinez found in 2006, there were no peer-reviewed scientific studies of the Three Sisters, though a few have been conducted since.
Martinez conducted a study that compared Three Sisters assemblies of the plants to monocropped corn, squash, and beans on flat ground. Neither the mounds nor the crops grown on flat land received synthetic nitrogen fertilizer. Her study found that hilling increased the soil temperature in the first two weeks of a growing season and decreased the temperature in the remaining weeks, compared with growing monocultures on flat ground. The higher temperature could help germination, although she didn’t measure that. Martinez’s study found that weed cover was slightly reduced in Three Sisters hills, but not significantly, except in one instance. She found that Three Sisters mounds retained less moisture than crops grown on flat ground. This would benefit a rainy environment but not a dry one.
She looked for differences in insect pests among the crops (including cucumber beetles, corn smut, and corn earworm) but found no difference. However, one of the Three Sisters sites did show significantly less damage from raccoons. There was no difference in soil nitrogen levels between the treatments. Martinez pointed out that this was a single-year study, so the possible benefit of adding plant matter to the hills after each growing season might result in more fertile soil. Plants grown in monoculture yielded the same as those in the Three Sisters configuration, except that the squash yielded fewer per plant in the Three Sisters plots. However, taken as a whole, the Three Sisters arrangement yielded more calories per acre than the monocropped (unfertilized) crops individually.
A more recent study in 2016 focused on the nutritional yields of interplanted Three Sisters cropping versus monocropping. The yield for corn was virtually the same in both growing methods. Pumpkins grown alone yielded more than pumpkins grown in a Three Sisters assembly. Likewise, beans yield far fewer calories when grown with Three Sisters intercropping. However, the researcher found that the Three Sisters method yielded more calories per acre than any of the monocropped plants.
This study intended to compare the Three Sisters system with monocropping under conditions the ancient Native Americans would’ve encountered. As such, no synthetic fertilizer was used on any of the crops.
Late last year (2024), researchers revealed preliminary results from one of the most involved studies on the Three Sisters. In Oaxaca, Mexico, they grew corn, beans, and squash individually, in pairs, and all three together. They carefully counted all the insects visiting the plants and found that plants grown together suffered less pest damage. Part of this was because every plant attracts its insect pests, which in turn attracts the insects that prey on those pests. More diversity in a plot meant a wider diversity of insect predators. They found that part of the protection resulted from chemical signaling between plants. When corn is damaged, it produces chemical defenses, including volatile compounds that attract predators, including parasitic wasps. As it turns out, the chemicals from corn also stimulate the beans to excrete more sugar into their extrafloral nectaries – little glands under their leaves. This sugar attracts ants and wasps, which consume caterpillars feeding on both the corn and the beans. Also, parasitoid wasps that eat this nectar live longer, allowing them to lay their eggs inside more caterpillars.
While pest protection works for the Three Sisters, it won’t work for every combination of plants and insects. However, researchers are hopeful these early results can someday help farmers manage pests with fewer synthetic pesticides.
The Three Sisters was a widely practiced agricultural method prior to European colonization of the U.S.; the Native Americans were able to make it work well. These days, it has the potential to attract organic gardeners and small farmers wishing to grow organic produce.
Chris Colby is a homebrewer, food lover, author of two books, and gardener. He lives in Bastrop, Texas.
Originally published in the May/June 2025 issue of Grit magazine and regularly vetted for accuracy.
