How to Harden Off Seedlings Before Planting
Seedlings raised indoors or in a greenhouse need a gradual transition before they face outdoor sun, wind and changing temperatures. Nebraska and South Dakota Extension both recommend hardening transplants over several days, but their schedules differ. Use the guidance that fits your plants and local conditions rather than treating the two timelines as one exact prescription.
Two Extension schedules
| Source | Overall period | First day | Progression and cautions |
|---|---|---|---|
| Nebraska Extension | 7–10 days | 2–3 hours in a shaded place, sheltered from strong wind | Add a few hours outside and more sun each day. After several days with 10–12 hours outside, leave plants out all day and night for a couple of days. Reduce watering gradually, but do not let plants wilt. |
| South Dakota State University Extension | 7–14 days | 3–4 hours in a protected location during the warmest part of the day | Increase outside time a few hours at a time and increase sun gradually. Bring plants indoors each evening; keep them inside if daytime temperatures are forecast below 45 °F, or if strong wind or rain is expected. |
The first-day exposure differs between the sources: Nebraska says 2–3 hours in shade, while South Dakota says 3–4 hours during the warmest part of the day. Both describe gradual exposure in a protected place and increasing time outdoors. Nebraska also cautions against allowing seedlings to wilt and against fertilizing during the hardening period.
Match the timing to the crop
The University of Idaho guide says warm-season crops such as tomatoes and peppers prefer nights of at least 60 °F before moving outdoors; its planting guidance places these crops about 1–3 weeks after the average last frost. Cool-season examples such as broccoli and onions can be moved out earlier according to the guide's 2–5 week window. The seed-starting calculator shows those source-based windows.
Hardening does not remove a crop's temperature limits. Check the forecast and the crop-specific guidance before transplanting. If frost is expected, see how to protect plants from frost.
Sources
- Nebraska Extension, Hardening Off Transplants.
- South Dakota State University Extension, Harden Your Transplants Prior to Planting Your Garden.
- University of Idaho Extension, Start Seeds Indoors with Success (CIS 1209).