A seedling raised indoors is set out gradually, not all at once
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 A person has checked it against its sources. If you skip this, it still grows, just less well. The planner suggests it.
How it works
Indoor-raised seedlings grow soft, with 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. 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 only advises.
The evidence
Evidence: checked against 2 sources in the research-based literature.
The 2 sources
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
Plant stems can snap due to strong winds, or leaves can become sunburned from being exposed to direct sunlight
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How this rule has changed
Every change is dated, newest first. The full record, with each reason, is in the data download.
- Wording made clearer
- A person read the sources and confirmed it
- Added to the rules
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/
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- The rule names its mechanism and is settled, by sources a person has read and checked or by plain arithmetic and geometry.
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