Minimum Temperatures for Hardened-Off Seedlings in Spring

Seedlings moved outdoors too early without hardening off will stall or die—even above freezing. The key is acclimation, not just temperature.

Hardening Off: The 10-Day Rule

1. Days 1–3: Full shade, protected from wind/rain, 2–3 hours.
2. Days 4–6: Dappled morning sun, increasing exposure daily.
3. Days 7–10: Full sun (for sun-lovers), overnight if above 45°F.

Temperature Tolerances (After Hardening Off)

Greenhouse Minimums

• Unheated greenhouse: 5–10°F warmer than outside at night.
• Vent on sunny days—temps can spike to 100°F+.
• Use a min/max thermometer to track actual conditions.

Your Situation

Losses at 40°F suggest seedlings weren't hardened off. Next batch: start hardening 2 weeks before planned transplant date. Keep a log of species and temps—patterns will emerge.

Respect the transition. Hardened seedlings handle cold; soft ones don't.