Sweet-Potato Milpa
A polyculture - the species are claimed to help each other, and each claim below names its mechanism.
A derived composition (docs/PROPOSAL-35-combinations.md #1, adopted 2026-08-09): the Three Sisters role structure with sweet potato in the cover slot — a milpa for a gardener who does not want squash. Not a documented traditional planting.
What binds this planting
The corpus backs this planting with 2 graded mechanisms its members share:
- Corn requires a block of at least 4 rows (roughly 12 plants) for adequate kernel set.
- Living mulch suppresses weeds and conserves soil moisture.
Space it needs
At least 4 m²43.1 ft². A bed smaller than that does not get this guild quietly substituted for something else — the planner greys it out, says why, and offers the nearest adaptation that does fit.
If your bed is smaller: Sweet-Potato Milpa (Small Bed) (1.5 m²16.1 ft²).
What it looks like planted
- 36 × corn
- 27 × common bean
- 4 × sweet potato
Between seasons
Nothing is left living in the bed over winter; the whole planting is re-established.
What it claims
sweet-potato foliage suppresses weeds and retains soil moisture
Mechanism: 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: confirmed against 2 sources in the research-based literature.
the 2 sources
UMN Extension, 'Mulching for soil and garden health', extension.umn.edu/managing-soil-and-nutrients/mulching-soil-and-garden-health (read 2026-07-11): mulch 'Suppresses weeds'; 'Studies show that mulch reduces evaporation from the soil surface and helps soil hold water.'
Cornell Cooperative Extension (Warren Co. Master Gardener), 'Creating a Three Sisters Garden', warren.cce.cornell.edu/gardening-landscape/warren-county-master-gardener-articles/creating-a-three-sisters-garden (read 2026-07-11): 'The large, prickly squash leaves shade the soil, preventing weed growth' — the living-mulch role in Three Sisters.
beans fix nitrogen available to corn in-season
Evidence: confirmed against 3 sources in the research-based literature.
the 3 sources
Univ. of Arkansas Extension, 'Creating a Three Sisters Garden', uaex.uada.edu (read 2026-07-11): '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].'
Hupe et al. 2021, Sci Rep 11:11424, 'Evidence of considerable C and N transfer from peas to cereals via direct root contact but not via mycorrhiza', pmc.ncbi.nlm.nih.gov/articles/PMC8169652 (read 2026-07-11): reports 'a mean N transfer rate of 8.4%, with a range of 2.7 to 22.5%, ... from N2-fixing legumes to non-N2-fixing mycorrhizal cereals' and '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', pmc.ncbi.nlm.nih.gov/articles/PMC5648690 (read 2026-07-11): 'N transferred from soybean to maize account for 2.4%–3.3% of the N uptake of maize.'
Roles
A role is filled by whatever satisfies its predicate, not by a fixed species list. The canonical filler is the default; the alternatives are priced.
Support
- Canonical: Corn
Fixer
- Canonical: Common Bean
- Alternatives: Runner Bean, Cowpea, Lima Bean
Cover
- Canonical: Sweet Potato
What it costs the ground
Rotation history attaches to the ground, not the bed, so this is what the planner remembers about this patch after the guild comes out.
- Grasses: full load
- Legumes: full load
- Morning-Glory Family: full load
Rules that gate it
Corn requires a block of at least 4 rows (roughly 12 plants) for adequate kernel set.
Mechanism: Maize is wind-pollinated and protandrous. Tassels shed pollen that must land on silks of neighbouring plants. Below roughly 12 plants in a block of at least 4 rows, insufficient pollen reaches enough silks; each unfertilized silk is one missing kernel.
Evidence: confirmed against 4 sources in the research-based literature.
the 4 sources
UMN Extension, 'Growing sweet corn in home gardens', extension.umn.edu/vegetables/growing-sweet-corn (read 2026-07-11): '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.'
NC State Extension, 'Organic Sweet Corn Production' (Horticulture Information Leaflets), content.ces.ncsu.edu/organic-sweet-corn-production (read 2026-07-11): 'Corn is wind-pollinated and should be planted in blocks of at least 4 rows for good pollination to occur.'
UMD Extension, 'Growing Sweet Corn in a Home Garden', extension.umd.edu/resource/growing-sweet-corn-home-garden (read 2026-07-11): 'Plant in blocks of at least three to four short rows, rather than one or two long rows, to ensure good pollination; minimum of three rows side by side (preferably four rows).'
Clemson HGIC 1308, 'Sweet Corn' (updated 2023-01-23), hgic.clemson.edu/factsheet/sweet-corn (read 2026-07-11): 'This crop is wind-pollinated; therefore, plant in blocks of several rows rather than 1 or 2 long rows to ensure full ears.'
remedy
Plant in a block of at least 4 rows, or omit corn and substitute a trellis.