Hardening Off Transplants
Hardening off means gradually moving indoor-grown seedlings outside over 7 to 14 days. You start with a few hours in a shaded, sheltered spot, add more sun and time each day, and finish with a day or two outdoors around the clock before planting. Extension services broadly agree on this timeline. They also agree that tender seedlings should stay inside when it is below about 45°F, and that hardening does not make warm-season crops frost-tolerant.
Why seedlings need it
A seedling grown under lights has never dealt with wind, direct sun, dry air or a cold night. Illinois Extension lists what goes wrong when the move is too abrupt: wind damage to stems, plants lodging (falling over), and sunburned leaves.
Hardening off causes real physical changes. Nebraska Extension describes them: growth slows, the waxy layer on leaves thickens, cell walls build up more lignin, cells hold less water that could freeze, carbohydrates increase and root growth speeds up. The plant ends up tougher and a bit smaller, which is the goal.
A day-by-day schedule
This schedule combines guidance from SDSU Extension, Nebraska Extension and Illinois Extension. Treat the days as a guide and adjust for weather: a cold or windy day means the plants stay in or go back a step.
| Days | Time outside | Location |
|---|---|---|
| 1–2 | 2–3 hours, during the warmest part of the day | Full shade, out of the wind |
| 3–4 | 4–6 hours | Dappled shade, some morning sun |
| 5–7 | 6–10 hours | Increasing direct sun |
| 8–10 | 10–12 hours | Full sun, in the conditions the plants will be planted into |
| Final 1–2 days | 24 hours, left out overnight | Full sun, if nights are warm enough |
The anchors come from the sources. Illinois suggests starting with 2–3 hours, "typically between 12-5 PM." SDSU suggests three to four hours on the first day. Both Nebraska and Illinois work up to 10–12 hours a day before leaving plants out for 24 hours "for a couple days." The middle rows are just a steady progression between those anchors. No source tests those exact hours. Total length ranges from 7–10 days (Nebraska) to 7–14 days (SDSU) to about two weeks (Illinois, University of Minnesota, Utah State). University of Maryland Extension says one to two weeks.
Temperature limits
The cutoffs are where the sources differ most, partly because cool-season and warm-season crops tolerate very different conditions.
| Guideline | Source |
|---|---|
| Keep tender seedlings in below 45°F and on windy days | University of Maryland, SDSU |
| Do not expose warm-season seedlings to temperatures below 45–50°F | Utah State |
| Cucumbers and melons can stay out overnight only if it won't fall below 50°F | University of Maryland |
| Warm-season plants need warm nights, at least 60°F | Nebraska |
| Hardened cool-season plants tolerate the 40s and light frosts | Nebraska |
A practical reading: cool-season transplants (cabbage, broccoli, lettuce, onions) can harden off in chilly weather and handle nights in the 40s once they're tough. Warm-season transplants (tomatoes, peppers, eggplant, cucurbits) should come inside when nights drop below about 50°F. Wait for warmer nights before planting them out.
Hardening is not frost-proofing. Nebraska Extension states that warm-season plants cannot survive freezing temperatures, even after hardening. Maryland says hardened tomatoes can handle a light frost with minimal damage, but that is a narrow margin. Plant tender crops after a frost date chosen at a risk level you're comfortable with. See Your Last Frost Date Is a Probability.
Water and fertilizer during hardening
Every source reviewed here gives the same watering advice: water less often, but don't let plants wilt. SDSU says to "reduce the frequency of watering without allowing the seedlings to wilt." Seedlings dry out much faster outdoors in sun and wind than under lights, so check them daily. Small cells may need water more than once on a warm, breezy day.
On fertilizer, Illinois Extension says to avoid fertilizing before or during hardening off. SDSU suggests skipping it unless plants show a visible deficiency. The goal is to slow soft, fast growth, not push it.
Can you overdo it?
Yes, for some crops. University of Maryland Extension warns that excessive hardening can stunt head development in cauliflower. It also says severe hardening can stop cucumber and melon growth. Hardening off should toughen plants, not starve or chill them for weeks.
There is also a common shortcut that doesn't work. SDSU advises against parking seedlings in an unlit garage, which causes stretching and yellowing. A shaded porch or the north side of the house is a better choice.
Shortcuts that work
- A cold frame. Maryland calls a cold frame "an excellent environment for hardening off transplants." Open the lid more each day, then leave it off. See Cold Frames, Row Covers and Low Tunnels.
- A cloudy transplant day. SDSU recommends planting on a cloudy day to reduce transplant stress, then watering daily until roots establish.
On transplant day
Check the soil temperature at about 4 inches before planting warm-season crops. Soil Temperature for Germinating Vegetable Seeds covers how. Water the plants well before planting and again afterward. Keep a cover nearby for the first couple of weeks, since SDSU notes even hardened plants may need protection during later frost advisories.
Hardening off is the last step of starting seeds indoors. If your seedlings are tall and weak before they even go outside, the problem is usually light. See Grow Lights for Seedlings. For the full sequence from frost date to garden, see Seed Starting and Season Extension.