What are critical mistakes during hardening off?
Written by
Julia Anderson
Reviewed by
Prof. Martin Thorne, Ph.D.Steering clear of critical errors guarantees the survival of seedlings when hardening them off. The most common mistakes made are exposing seedlings to noon sun, neglecting to harden off seedlings during frosts, and being inconsistent in routines. Damage caused is often serious, such as leaf scorch or even root rot. I have made many of these mistakes through the years and have learned that prevention is easier than cure.
Sun Exposure Errors
- Never expose seedlings to midday sun during first 3 days
- Use 40% shade cloth between 10 AM-4 PM
- Transition gradually: start with 30 min morning light
Temperature Management
- Bring plants indoors below 40°F for tender varieties
- Install wireless thermometers with frost alerts
- Water soil before cold nights for thermal buffering
Watering Discipline
- Check top inch of soil before watering
- Water roots only keeping leaves dry
- Allow soil partial drying between waterings
Inconsistent routines hinder the acclimation of plants. To hit the wake-up calls needed for morning hardening sessions, set reminders on your phone to ensure you're well-prepared. Maintain that schedule even on weekends. Missing even one of these days forces the whole process to be restarted. Grouping different plants by stages of hardening can facilitate this efficiency. I get all the materials prepared the night before.
Faster hardening creates excess mortality. Do not compress hardening times below the recommended species minimum time. Hardy plants should have a minimum of 7 days of hardening, and tender varieties should have a minimum of 10-14 days. Extending hardening time, especially with cold spells, can mitigate plant dieback. Patience ensures stronger plants.
Recognizing early signs of trouble is important. White leaf patches mean sunburn. Purple stems mean cold stress. Wilting without hot temperatures means root rot. Solve problems promptly to save plants. I look at seedlings twice daily during hardening.
Read the full article: The Complete Guide to Hardening Off Seedlings
