When should seedlings stay indoors during hardening?
Written by
Julia Anderson
Reviewed by
Prof. Charles Hartman, Ph.D.In order to harden off seedlings, one must know when to stop. Last spring, I lost a tray of seedlings, which were all tomatoes, to a 42°F (6°C) night which is below their minimum temperature of 45°F. Now I pay more attention to the limit for each species: only basil stays inside at 50°F (10°C), while kale can take 35°F (2°C).
Temperature Thresholds
- Tender crops: Delay hardening below 50°F (10°C)
- Cold-tolerant plants: Pause below 35°F (2°C)
- Soil temps: Keep indoors if below 55°F (13°C) for peppers
Weather Risks
- Heavy rain: >1 inch (2.5 cm) per hour risks root rot
- Hail: Any size pellets damage leaves irreparably
- High winds: Gusts >25 mph (40 km/h) snap stems
Monitoring Tools
- Wireless thermometers with frost alerts
- Anemometers to track wind speed drops
- Soil probes for real-time root zone temps
Rehardening Steps
- Restart at 50% of previous outdoor exposure time
- Use 20% shade cloth for 2 days post-delay
- Skip fertilizing until plants stabilize
A $25 wireless thermometer rescued my basil during an April cold snap when the temperature dipped early enough to get me prepared. When alerting me to a temperature of 47°F (8°C), I was able to move the plants indoors 30 minutes before the leaves were damaged. Use tools, while visually checking for shivering seedlings, if they are shivering, you should be too.
Here's what I learned: hardening during drizzle caused the lettuce to collapse. I now wait 48 hours after storm before hardening them. In case of an unexpected freeze, I drape frost blankets over the trays, which are still indoors, as it provides some buffer against the abrupt temperature drops near the windows.
Read the full article: The Complete Guide to Harden Off Seedlings
