A few kinds of seed need light to germinate, and burying them stops them coming up. Sow these on the surface - pressed into firm, moist soil and left uncovered, or barely dusted over - rather than at the depth a larger seed would take.
Promising · suboptimal if ignored → advise
How it works
These seeds carry the light-sensing pigment phytochrome, which must catch light to switch on the hormone cascade that releases the seed to germinate, so a seed buried past a few millimetres sits in the dark with that switch never thrown and stays dormant.
What to do about it
Press light-requiring seed onto firmed, moist soil or medium and leave it uncovered or only barely dusted, then keep the surface damp - a seed sitting on top dries out fast and a fine mist rather than a heavy watering keeps it in place. The plant's page says whether it needs light; a seed that needs darkness is the opposite and is covered as usual.
How much it matters: No germination-rate figure is invented. The size of the effect is the difference between a seed left in the light and the same seed buried in the dark, which the sources describe as coming up versus not; because sowing depth is entirely the gardener's own act, the rule advises rather than blocks.
The evidence
Evidence: confirmed against 1 source in the research-based literature.
the source
North Carolina Extension Gardener Handbook, ch. 13 'Propagation' / Anthony V. LeBude & Frank A. Blazich (NC State Extension, 2022), content.ces.ncsu.edu/extension-gardener-handbook/13-propagation (fetched in-container 2026-09-20, read 2026-09-20): 'When sowing light-requiring seeds, sow them on the soil surface'; the plants it lists as requiring light for germination are 'ageratum, begonia, browallia, impatiens, lettuce, and petunia'; and 'Extremely fine seeds such as carrot, petunia, and snapdragon should not be covered, but simply dusted on the surface of the germinating medium and watered with a fine mist.' The page states the practice, not the phytochrome mechanism.
Where this rule stops
Gated by applies_to.light_to_germinate to the species the corpus records as light-requiring (achillea, dill, sweet alyssum today; the flag is O163 data, so the rule follows the data as it grows), so it surfaces on exactly those plant pages and is cited by the germination guide. It gates nothing and docks no score - depth is the gardener's step. The phytochrome term is textbook plant physiology naming WHY light is the trigger; the extension source gives the practice (surface-sow, do not cover). A light-INHIBITED seed (light_to_germinate false, e.g. basil, which the corpus stores from Iowa State's darkness code) is the opposite case and is covered as normal - this rule does not fire on it.
Plants this fires on
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): promote to verified — maintainer read the source (2026-09-20)
Cite this page
Milpa Gardens. "A few kinds of seed need light to germinate, and burying them stops them coming up. Sow these on the surface - pressed into firm, moist soil and left uncovered, or barely dusted over - rather than at the depth a larger seed would take." Evidence grade: Promising. https://milpa.garden/rules/few-kinds-seed-need-light-germinate-burying/
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.