Hardening off acclimatises indoor-raised seedlings to outdoor conditions over 7-14 days before transplanting. Without this gradual transition, seedlings suffer transplant shock, wilting, stunted growth, or death. Start by placing plants outside for 2-3 hours in a sheltered spot and increase daily. A cold frame simplifies the process by providing adjustable ventilation. UK last frost dates range from late April in southern England to early June in Scotland, setting the transplanting deadline.
Key takeaways
- Hardening off takes 7-14 days and should start 2 weeks before your planned planting-out date
- Begin with 2-3 hours outside in a sheltered spot and increase exposure daily
- A cold frame is the simplest tool for controlled hardening off
- UK last frost dates range from late April (south) to early June (Scotland)
- Bring seedlings indoors overnight until night temperatures stay above 5C
- Skipping hardening off causes transplant shock, wilting, and potential crop failure
hardening off seedlings transplanting cold frame growing season propagation
Our team produces practical UK gardening guidance from source research, contributor records and established horticultural practice. Editorial standards are overseen by Matt W.
