A seedling raised indoors is set out gradually, not all at once: moved straight from a windowsill or grow-light into full sun and wind it suffers sunscald, windburn, or transplant shock, because its soft indoor growth has not hardened to outdoor conditions.
Promising · suboptimal if ignored → advise
How it works
Indoor-raised seedlings grow soft - a thin cuticle and waxy layer, unlignified stems, and high tissue water - because the light, wind, and temperature swings that toughen a plant are absent indoors. Exposed abruptly to direct sun the thin leaves scald, in wind the soft stems break or desiccate, and the whole plant stalls from the shock. Hardening off - a week or two of increasing daily outdoor exposure - thickens the cuticle and waxy layers, develops lignin in the cell walls, builds carbohydrate reserves, and lowers freeze-prone cell water, so the plant can take the move.
What to do about it
Begin about one to two weeks before setting out. Put the seedlings outside in a sheltered, part-shade spot during the mild middle of the day, a few hours at first, and lengthen the time and the sun each day, bringing them in at night; keep them off exposed spots on windy days and when nights fall below about 45 F, and ease off watering (without letting them wilt) to slow the soft growth. Then transplant.
How much it matters: No figure is stated or invented. The stress is worst for a tender, fast-grown seedling moved straight into full sun and wind and least for one eased out over one to two weeks; because timing and exposure are the gardener's to control and vary by crop and weather, the rule advises rather than warns.
The evidence
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
UMD Extension, 'Hardening Off Vegetable Seedlings for the Home Garden', extension.umd.edu/resource/hardening-vegetable-seedlings-home-garden (read 2026-08-21): 'Hardening is the process of exposing transplants (seedlings) gradually to outdoor conditions. It enables your transplants to withstand the changes in environmental conditions they will face when planted outside in the garden'; 'Begin hardening transplants 1-2 weeks prior to setting out plants in your garden'; 'place them outside in a shaded, protected spot on warm days, bringing them in at night. Each day, increase the amount of sunlight the transplants receive'; 'Don''t put tender seedlings outdoors on windy days or when temperatures are below 45 F'; 'Reduce the frequency of watering to slow plant growth, but don''t allow plants to wilt.'
Illinois Extension, 'Starting a Garden: Hardening Off Indoor Seedlings' (Good Growing), extension.illinois.edu/blogs/good-growing/2020-04-06-starting-garden-hardening-indoor-seedlings (read 2026-08-21): 'Plant stems can snap due to strong winds, or leaves can become sunburned from being exposed to direct sunlight'; 'Hardening off reduces the growth rate and thickens the cuticle and waxy layers'; 'It also stimulates root development, increases the amount of carbohydrates in tissues for food reserves, reduces the amount of freeze prone water in cells, and develops lignin in the cell walls'; 'gradually reduce how often you water; however, don''t allow the plants to wilt.'
Where this rule stops
A STANDING SET-OUT ADVISORY (trigger.kind always, universal like the watering rules), so it surfaces on the species pages and can be cited by the seed-starting guides. It sits after another rule here (start indoors) and before another rule here's frost floor and another rule here's transplant depth: those schedule and place the transplant; this one conditions it. It gates nothing - the app cannot know whether a plant was raised indoors - and never docks a score.
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.
- verified: maintainer read the sources 2026-08-21 (invariant 4)
- harden off indoor seedlings before setting out (Promising advise, unverified)
Cite this page
Milpa Gardens. "A seedling raised indoors is set out gradually, not all at once: moved straight from a windowsill or grow-light into full sun and wind it suffers sunscald, windburn, or transplant shock, because its soft indoor growth has not hardened to outdoor conditions." Evidence grade: Promising. https://milpa.garden/rules/seedling-raised-indoors-set-out-gradually-not/
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.