How do the “three sister” plants work together?

The three sisters planting – corn, beans, and squash – is a perfect example of companion planting!

Imagine three sisters in a family. In an ideal world, they would be great companions, cooperating and supporting each other. Each sister would bring along a unique contribution to the trio—perhaps one sister is the leader, another sister is the giver, and the third sister is the protector. This type of cooperative synergy is behind the approach of companion planting. In companion planting, ecologically complementary crops are grown alongside one another. Native tribes in North America, most notably the Iroquois, utilized the three sisters approach.

In the three sisters companion planting, each plant provides a service. Corn provides a strong structure for beans. Beans provide nitrogen to corn and squash. Squash’s leaves provide shade to the soil, and their rough edges deter pests. Source: SSSA compiled photos SVF

The three sister plants—corn, beans, and squash­—is a trio that provides ecological and nutritional balance. The leader in the three sisters is corn, with its tall and sturdy stalks. However, corn has high nutrient requirements, and can deplete the soil of nutrients if left unchecked.

Planting legumes, such as beans, alongside the corn is one way to help minimize nutrient depletion. Legumes are givers in the three sisters combination, as they enhance the availability of a key nutrient in the soil: nitrogen. They help make nitrogen available in the soil by cultivating beneficial bacteria. These bacteria pull nitrogen from the air and convert it to a form that plants can use—a process that is called nitrogen fixation. Pole bean legumes work best in the three sisters trio, as they benefit from being able to climb the corn stalks as they mature and grow. Beans are typically planted a couple weeks after corn so that corn stalks develop before the beans begin to climb.

Finally, the protector in the three sisters combination is squash. Squash’s broad leaves provide a blanket of living mulch: a barrier against weeds, hot sun, and high temperatures. The prickly hairs on the squash also help reduce predation in the garden from rodents and other pests.

In addition to growing well together, the corn-bean-squash trio is also nutritionally complementary, with a mix of sugar, protein, and fiber.

Nodules on roots of legume plants have microbes in them that pull nitrogen from the air, and help convert it to a usable form for plants – a process called nitrogen fixation. Credit: Sarah Lyons

There are other good companions. In natural ecosystems, the relationship between nitrogen fixing bacteria and their plants hosts such as legumes provides the foundation for energy flow and production. Without the nitrogen fixing bacteria, plants would not be able to access enough nitrogen needed to fuel photosynthesis and growth. The nitrogen fixing bacteria benefit from the relationship with their host plants as well. They get access to an energy source of carbon from plant roots that they live on.  As the nitrogen-rich plants and microbes die, they release nitrogen back into the soil. There, it can be taken up by other organisms, and used as a nutrient again.

There are a variety of plant combinations that work well together in addition to the three sisters trio. One strategy to balance nutrient levels in the soil is to pair nitrogen fixing plants with plants that harbor mycorrhizal fungi. Similar to nitrogen-fixing bacteria, mycorrhizal fungi infect plant roots in a beneficial way.  However, instead of targeting nitrogen, these fungi are good at mining nutrients such as calcium, potassium, and phosphorus in the soil, and making them available to plants. In exchange for these nutrients, the fungi get a place to live and an energy source from their host plant. There is a wide range of plants that have mycorrhizal fungi that can be paired with plants that have nitrogen-fixing bacteria. For example, bush beans (with nitrogen-fixing bacteria) can be complemented by mycorrhizal radish and non-mycorrhizal spinach. In this combination, spinach can benefit from the shade provided by the bush beans. The radishes in this mix enhance nutrient availability, and minimize insect damage to spinach by offering a leafy alternative for pests to munch on.

Companion planting is a great way to promote crop productivity by harnessing natural synergies in the soil/plant system. Ultimately, this strategy can make for healthier ecosystems by reducing the need for synthetic fertilizers and pesticides.

Answered by Farrah Fatemi, St. Michael’s College, Vermont

To receive notices about future blogs, be sure to subscribe to Soils Matter by clicking on the Follow button on the upper right! Explore more on our webpage About Soils. There you will find more information about Soil Basics, Community Gardens, Green Infrastructure, Green Roofs, Soil Contaminants, materials for Teachers and more.