Heat bolters that tolerate part sun, such as lettuce and spinach, are better off in the computed summer shadow of a taller neighbour than in open sun.
Promising · suboptimal if ignored → advise
How it works
Leaf temperature. Shade from a taller crop lowers the leaf and soil temperature of the greens beneath it, and high summer temperature is the trigger that sends these species to seed and turns them bitter.
What to do about it
Put a tall, shade-casting crop such as sweet corn or a staked tomato on the sun side of your greens so its shadow reaches them by midsummer. A 30 to 50 percent shade cloth does the same job where no tall crop fits.
The evidence
Evidence: confirmed against 5 sources in the research-based literature.
the 5 sources
University of Maryland Extension, 'Growing Lettuce in a Home Garden', extension.umd.edu/resource/growing-lettuce-home-garden (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Increasing day-length and high summer temperatures usually cause seedstalk formation (bolting) and bitter flavor'; 'Plant long-season lettuces so that crops such as sweet corn, staked tomatoes, pole beans, or deciduous trees will shade the lettuce during the hottest part of the day'; 'Plant successive lettuce crops in spring and use tall crops and trellises to protect the late spring lettuce plantings from direct afternoon sun'; 'Tolerates partial shade (4-6 hours of direct light/day) but grows well with full sun in spring and fall.'
NC State Extension Gardener Handbook, chapter 16 'Vegetable Gardening', content.ces.ncsu.edu/extension-gardener-handbook/16-vegetable-gardening (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Shade-tolerant species, such as lettuce, spinach, and celery, can be planted in the shadow of taller crops, like beans or squash.'
Penn State Extension, 'Heat Proofing Your Vegetable Garden', extension.psu.edu/heat-proofing-your-vegetable-garden (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'A good investment if you want to grow lettuce and other leafy greens all through the summer is a shade cloth'; 'For most vegetables, a shade cloth of 30% to 50% is recommended.' Of fall brassica seedlings, not lettuce: 'They are best planted along a shade barrier or under taller vegetables to protect them from direct hot sun, especially the hot afternoon sun.'
Clemson HGIC, 'Lettuce', hgic.clemson.edu/factsheet/lettuce/ (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'High summer temperatures usually cause seed stalk formation (bolting) and bitter flavor'; 'If this crop is exposed to high temperature or moisture stress, the lettuce will usually have a bitter flavor and will also bolt.'
Clemson HGIC, 'Spinach', hgic.clemson.edu/factsheet/spinach/ (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Plant spinach in early spring to avoid high temperatures and long days, which cause the plants to bolt, or form flower stalks.'
Change history
Every change to this rule is one commit. This is what the project's version history records — the date and the reason — not a summary of it.
- corpus: another rule here shadow shelter - a part-sun heat bolter in a taller neighbour's computed shadow is sheltered; another rule here stops counting it a victim (O133 step 11)
Cite this page
Milpa Gardens. "Heat bolters that tolerate part sun, such as lettuce and spinach, are better off in the computed summer shadow of a taller neighbour than in open sun." Evidence grade: Promising. https://milpa.garden/rules/heat-bolters-that-tolerate-part-sun-such/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.