Milpa
A polyculture - the species are claimed to help each other, and each claim below names its mechanism.
Mesoamerican. Three Sisters is the northern expression of this system.
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 50 m²538.2 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.
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
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.
fallow cycle restores fertility at field scale
Mechanism: 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: confirmed against 2 sources in the research-based literature.
the 2 sources
Penn State Extension, 'Growing Cover Crops for Nitrogen on Vegetable Farms', extension.psu.edu/growing-cover-crops-for-nitrogen-on-vegetable-farms (read 2026-07-11): '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'; 'Mineralization is the process where organic nitrogen, which is largely not available to plants, is converted by soil microorganisms' into a plant-available form.
Univ. of Wisconsin-Madison Extension (Crops & Soils), 'Late Summer Planting Legumes to Produce Nitrogen Credits for Next Year', cropsandsoils.extension.wisc.edu/articles/late-summer-planting-legumes-to-produce-nitrogen-credits-for-next-year (read 2026-07-11): '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 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
- Alternatives: Sunflower
- What the alternative costs: Sunflower can carry the beans, but it is a heavy nitrogen feeder that competes with the fixer and casts shade, and it needs a ~30-day head start or the vines pull it down.
Fixer
- Canonical: Common Bean
- Alternatives: Runner Bean, Cowpea, Lima Bean
Cover
- Canonical: Butternut
- Alternatives: Winter Squash, Summer Squash
Fruiting understory
Leaf crop
- Canonical: Amaranth
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
- Cucurbits: full load
- Nightshades: partial load
Rules that gate it
A plant's mature spread must fit its allocated area.
Mechanism: Geometry.
Evidence: settled without a trial - arithmetic.
remedy
Reduce plant count, or choose a bush cultivar group.
A guild deposits family history across its whole footprint.
Mechanism: Soilborne inoculum and overwintering pest stages disperse across the bed. A guild dominated by one family forecloses that ground for that family's rotation interval.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
Same soilborne-persistence basis as other rules here (read 2026-07-11): NC State Extension (Verticillium microsclerotia 'can survive in the soil without a host for up to 10 years'; Septoria/early blight 'overwinter on infected tomato debris and solanaceous weed hosts') and UMN/Ohioline (squash bug overwinters 'as adults in sheltered places ... they fly to growing cucurbit plants'; squash vine borer 'overwinters in the soil as a pupa').
Those sources establish that inoculum and pest stages persist in the ground and are not confined to the exact spot a plant occupied; this rule applies that persistence across the whole guild footprint. The whole-footprint attribution is the corpus's rotation geometry (history attaches to ground, not to the individual plant — see the data model), not a claim from an external source.
remedy
Rotate the whole bed the guild occupied, not just the rows that family grew in.