What a milpa is, and why Three Sisters is only part of it
The best-known polyculture in North American gardening is really the northern tip of a much older field system, and the difference comes down to scale.
Photo: Rasbak at Dutch Wikipedia · CC BY-SA 3.0, via Wikimedia Commons.
The famous version
Corn, beans, and squash, planted together in a mound. The corn stands up, the beans climb it, the beans feed the soil, and the squash sprawls across the ground to shade out the weeds. Three Sisters is the one plant team just about every gardener has heard of, and unlike most of what gets called companion planting, it holds up. Every one of those relationships has a real reason behind it, one you can name and check.
It’s also a piece of something bigger. Three Sisters is the northern tip of the Mesoamerican milpa, and a milpa is a field.
The system it came from
The milpa is a whole field system with a fallow cycle, meaning a patch left to rest on purpose. It carries the same three roles and adds more: a fruiting layer of chile or tomatillo underneath, and a leaf crop like amaranth. It’s grown at field scale, and the rest year is part of the design.
The corpus puts a hard floor of 50 square meters540 square feet on it, and the reason is worth quoting:
Milpa is a field system with a fallow cycle, not a bed. Below field scale you are growing Three Sisters and calling it something else.
That keeps the word from drifting. You can grow four species in two square meters20 square feet and it might be a lovely planting, but calling it a milpa would make the word mean nothing.
Why the corn is the part that bites
The part of this system most likely to let a first-time grower down is the corn, and the reason is pollination, meaning how the pollen gets where it needs to go.
Corn needs a block of at least three short rows side by side for good kernel set, and four rows is better.
If you skip this, the planting fails. The planner won’t allow it.
How it works: Maize is wind-pollinated and protandrous (each plant sheds its pollen before its own silks are ready). Tassels shed pollen that must land on the silks of neighboring plants. In a single row much of the pollen blows out of the row, and each silk it misses is one missing kernel.
Evidence: checked against 5 sources in the research-based literature.
Always plant corn in blocks of at least four rows. Corn planted in a single row will have much of its pollen blown out of the row, and will produce ears that have blank areas where kernels did not form.
This crop is wind-pollinated; therefore, plant in blocks of several rows rather than 1 or 2 long rows to ensure full ears.
richmond.ces.ncsu.edu
Glass Gem CornNC State Extension, Richmond County Center
When tassels start dropping pollen, stalks can be gently shaken. This mimics wind and will release the pollen to float down to the silks.
Remedy
Plant corn in a block of several short rows rather than one long row; four rows is best. For a small patch, shake the stalks gently while the tassels shed pollen, or dust a cut tassel over the silks, every few days. Or leave corn out and give the beans a trellis.
Corn is pollinated by the wind. Pollen drops from the tassel at the top and has to land on the silks of the plants around it, and every silk that gets missed is a missing kernel. A single pretty row of corn in a small bed gives you ears with gaps in them, and the gardener figures they did something wrong. They planted the right thing in the wrong shape.
This is also why the footprint gate runs through the whole planner. A plant team that doesn’t fit your bed is shown grayed, with the reason and with the nearest version that does fit. It’s never swapped for something else without telling you.
The nitrogen story is the one everybody repeats: the beans feed the corn. It’s the shakiest of the three relationships, and the corpus grades it that way.
Beans fix nitrogen that feeds the corn growing beside them, that season.
This claim only partly holds up. The planner won’t plan around this claim. What is known instead is below.
What is known: Rhizobia in legume root nodules do fix atmospheric N. Most of it is incorporated into the legume's own tissue. Some does move to a neighboring non-legume the same season via direct root contact and rhizodeposition, but the amount is small (a few percent of the neighbor's N uptake) and does not meet a heavy feeder's demand. The bulk of the benefit arrives the following season, after residues decompose.
Evidence: checked against 3 sources in the research-based literature.
Most of the nitrogen converted by the beans will not be available to the corn and pumpkins the first year; the bean roots have to break down to release nitrogen. Corn is a heavy nitrogen [feeder].
Most of the nitrogen a bean plant pulls from the air only becomes available once its roots and leaves break down, which is next season. Some transfer does happen between living plants, but the measured amounts are small. The beans do improve that ground. They just don’t fertilize this year’s corn in any amount that will change your harvest.
Two of the three relationships among the Three Sisters are strong. This one is a good hunch with real numbers attached, and it’s shown that way instead of being smoothed into the story.
What the squash actually does
Living mulch suppresses weeds and conserves soil moisture.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: A living cover keeps a leaf canopy over the ground. It intercepts light at the soil surface, so weed seedlings are shaded out before they establish, and it slows surface evaporation, so the soil holds more water. In a Three Sisters planting this is the squash's role. Its broad leaves shade the bed.
Evidence: checked against 2 sources in the research-based literature.
This one holds up. The leaves cover the soil as a living mulch, the effect is measurable, and it earns the third sister her spot on her own. Scaring off pests has nothing to do with it.
Growing it at garden scale
If you’ve got a bed and no field, grow the Three Sisters and call it that. Give the corn a block of several short rows. Expect the beans to improve the ground for next year, with little effect on this year’s corn. And let the squash do the job it’s good at.
That’s the version of this system that survives a checked source, and it’s still a really good planting.
