How to harden off seedlings?
Written by
Nguyen Minh
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.Hardening off is the process of acclimatizing seedlings to the outdoors to minimize shock and sunburn. It gradually prepares seedlings to withstand wind, sun, and temperature fluctuations. I learned this the hard way when un-hardened tomatoes succumbed to scorching in a few hours after planting. We lost weeks of growth.
Preparation Phase
- Water seedlings thoroughly before first outdoor exposure
- Choose calm overcast days for initial sessions
- Use temporary shade structures if needed
Monitoring & Adjustment
- Check for wilting or leaf bleaching hourly
- Reduce exposure if seedlings show stress signs
- Extend schedule during cold snaps or heatwaves
Begin, when seedlings develop 2-3 what you call true leaves. Start with morning sessions, sheltered from the elements, and gradually increase exposure. After 7 days, my peppers were in full sun using this methodology, while plants that rushed into the sun were burnt and needed to recuperate.
Be aware of the signals of critical stress while hardening. Patches of white bleached leaves indicate too much sun, while leaves that curl can indicate wind damage. An option I use is to keep my plants in a spray bottle as I move stressed plants into a shaded area.
Adjust for the weather conditions carefully. If the temperature exceeds 85 degrees, shorten the sessions to reduce heat exposure and provide more shade. If the temperature is below 50 degrees, use cloches for frost protection. According to my hardening journal, I have found the ideal temperatures to be between 60°F and 75°F as well.
Steering clear of common hardening errors takes vigilance: Avoid leaving seedlings out in direct sunlight unsupervised. Do not attempt to harden the seedlings on rainy days. You will slow the seedlings down by skipping a day. The most effective way to build the toughness of plants is to expose them to the elements every day.
Complete hardening occurs when plants can withstand full outdoor conditions without stress; transplant them within 48 hours after this point. My hardened seedlings establish 50% faster with minimal wilting compared to unhardened ones.
Read the full article: When to Transplant Seedlings: Ultimate Guide
