Sweet-potato milpa
Corn and climbing beans with sweet potato as the ground cover, for a gardener who wants the Three Sisters without squash.
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
- Sweet-potato foliage suppresses weeds and retains soil moisture
- Beans pass a small share of their nitrogen to the corn in-season; most of it arrives next year
Space it needs
At least 4 m²43 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.
If your bed is smaller: Sweet-potato milpa (small bed) (1.5 m²16 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
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
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
Fixer
- Usual choice: Pole bean
- Swaps: Runner bean, Cowpea, Pole lima
Cover
- Usual choice: Sweet potato
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
- Morning-glory family: counts as a full season of this family
Rules that decide where it fits
Corn needs a block of at least three short rows side by side for good kernel set, and four rows is better.
Well establishedHow 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.
The 5 sources
Growing sweet corn in home gardens UMN Extension
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.
Organic Sweet Corn Production NC State Extension
Corn is wind-pollinated and should be planted in blocks of at least 4 rows for good pollination to occur.
Growing Sweet Corn in a Home Garden UMD Extension
Plant in blocks of at least three to four short rows, rather than one or two long rows, to ensure good pollination and full kernel development.
Sweet Corn Clemson HGIC 1308
This crop is wind-pollinated; therefore, plant in blocks of several rows rather than 1 or 2 long rows to ensure full ears.
Glass Gem Corn NC 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.
Where this plant team comes from
A new combination: the Three Sisters layout with sweet potato in the cover slot, a milpa for a gardener who does not want squash. Not a documented traditional planting.