Milpa
A small field of corn, beans and squash with pepper and amaranth worked in, the fuller Mesoamerican system the Three Sisters comes from.
Why these plants work together
Here is how these plants help each other, with how sure we are of each:
- Corn provides a living trellis
- Squash foliage suppresses weeds and retains soil moisture
- Fallow cycle restores fertility at field scale
- Beans pass a small share of their nitrogen to the corn in-season; most of it arrives next year
Space it needs
At least 50 m²540 ft². If your bed is smaller, the planner grays this plant team out, says why, and offers the nearest adaptation that fits. It never swaps in something else without telling you.
What it looks like planted
- 36 × Corn
- 27 × Common bean
- 1 × Amaranth
- 2 × Butternut
- 1 × Pepper
Between seasons
Nothing is left living in the bed over winter; the whole planting is re-established.
What it claims
Squash foliage suppresses weeds and retains soil moisture
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.
The 2 sources
Mulching for soil and garden health UMN Extension
Studies show that mulch reduces evaporation from the soil surface and helps soil hold water.
Creating a Three Sisters Garden Cornell Cooperative Extension (Warren Co. Master Gardener)
The large, prickly squash leaves shade the soil, preventing weed growth
Fallow cycle restores fertility at field scale
How it works: A legume fixes atmospheric nitrogen into its tissue as it grows. When the crop is killed and its residue decomposes, soil microbes mineralize that organic nitrogen, converting a form plants can't use into one they can, and release it for the next crop. How much depends on how well the legume grew, and it is lower on sandy soils.
Evidence: checked against 2 sources in the research-based literature.
The 2 sources
Growing Cover Crops for Nitrogen on Vegetable Farms Penn State Extension
When cover crops are killed and begin to decompose, nitrogen can be released from the residues back into the soil where it can feed the next crop
Late Summer Planting Legumes to Produce Nitrogen Credits for Next Year Univ. of Wisconsin-Madison Extension (Crops & Soils)
The nitrogen credit depends on the amount of crop growth and when the legume was planted. Nitrogen credits are 50 lb N/a lower on sandy, coarse-textured soils.
Beans pass a small share of their nitrogen to the corn in-season; most of it arrives next year
Evidence: checked against 3 sources in the research-based literature.
The 3 sources
Creating a Three Sisters Garden Univ. of Arkansas Extension
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].
Evidence of considerable C and N transfer from peas to cereals via direct root contact but not via mycorrhiza Hupe et al. 2021, Sci Rep 11:11424
N transfer rates of between 3.1 and 6.0% ... from soybean to maize
Nitrogen uptake and transfer in a soybean/maize intercropping system in the karst region of southwest China
N transferred from soybean to maize account for 2.4%–3.3% of the N uptake of maize.
Who does what, and the swaps
Each job in this planting can be done by more than one plant. The usual choice comes first, and each swap says what it costs.
Support
- Usual choice: Corn
- Swaps: Sunflower
- What a swap costs: Sunflower can carry the beans, but it is a heavy nitrogen feeder that competes with the beans and casts shade, and it needs a ~30-day head start or the vines pull it down.
Fixer
- Usual choice: Pole bean
- Swaps: Runner bean, Cowpea, Pole lima
Cover
- Usual choice: Butternut
- Swaps: Winter squash, Summer squash
Fruiting understory
Leaf crop
- Usual choice: Amaranth
What it costs the ground
Rotation history belongs to the ground itself, so this is what the planner records about this patch after the plant team comes out.
- Grasses: counts as a full season of this family
- Legumes: counts as a full season of this family
- Cucurbits: counts as a full season of this family
- Nightshades: counts as part of a season of this family
Rules that decide where it fits
A plant's mature spread must fit its allocated area.
Well establishedHow it works: Geometry.
Evidence: settled without a trial (arithmetic).
Remedy
Reduce plant count, or choose a bush cultivar group.
For rotation, a plant team's crop families count across its whole bed
PromisingHow it works: Soilborne inoculum and overwintering pest stages disperse across the bed. A plant team dominated by one family forecloses that ground for that family's rotation interval.
Evidence: checked against 2 sources in the research-based literature.
Remedy
Rotate the whole bed the plant team occupied, including rows that family never grew in.
Where this plant team comes from
Mesoamerican. Three Sisters is the northern expression of this system.
Read next
- Crop rotation for the home gardenMove crops by plant family so soil-borne pests and diseases can't settle in. What to rotate, how far apart, and what rotation can and can't do.
- What is a milpa? The fuller system behind the Three SistersThe Three Sisters is the famous three-crop version; the milpa it comes from is a whole interplanted field. What the milpa adds, what it looks like planted, and how to grow a piece of one.
- What the evidence says about companion plantingMost companion-planting chart entries are tradition, and few were ever tested. Here's the short list of pairings that hold up, the famous ones that don't, and how to tell which is which.