How to harden off your seedlings
Hardening off is the process of gradually introducing indoor-grown seedlings to outdoor conditions. It reduces shock and helps plants adjust to sunlight, wind, and temperature changes before planting out.
Skipping this step often leads to weak growth or plant loss.
Quick answer: how to harden off seedlings
Harden off seedlings by placing them outside for short periods each day over 7–10 days, gradually increasing exposure to sun and wind, and bringing them back indoors at night until fully acclimatised.
5 step guide to hardening off seedlings
Step 1: Start in a sheltered spot
- Place seedlings outside for 1–2 hours
- Keep in shade and out of wind
- Avoid direct sun initially
Step 2: Increase exposure gradually
- Add a few hours each day
- Slowly introduce more light and airflow
- Keep conditions controlled
Step 3: Introduce sunlight
- Move into partial, then full sun over time
- Watch for signs of stress
- Avoid sudden exposure
Step 4: Extend outdoor time
- Leave plants out for most of the day
- Begin exposing them to normal conditions
- Still bring indoors at night
Step 5: Leave out overnight
- Only once temperatures are stable
- Plants should now be ready for planting out
Best varieties & why (that need hardening off)
- Tomatoes and peppers
- Sensitive to temperature changes
- Cucumbers and courgettes
- Fast growers that need gentle transition
- Flowers like zinnias and cosmos
- Benefit from gradual exposure
Any indoor-grown plant will benefit from hardening off.
Common issues, pests & diseases
Wilting or leaf scorch
- Cause: too much sun too quickly
- Fix: reduce exposure and build up gradually
Stunted growth
- Cause: transplant shock
- Fix: extend hardening off period
Cold damage
- Cause: leaving plants out too early
- Fix: bring indoors at night
Weak seedlings
- Cause: lack of airflow before hardening
- Fix: improve conditions earlier
- See: https://www.theonlinegardencenter.com/how-to-garden/why-are-my-seedlings-leggy
Best fertiliser, soil, lighting & growing conditions
- Light
- Gradually increase exposure to full sun
- Water
- Keep soil evenly moist
- Avoid drying out during transition
- Soil
- Well-draining and stable
- Feeding
- Light feeding once established
Consistency is key during this transition phase.
Time of year
- Typically done in spring, before planting outdoors
- Starts after the risk of severe frost has passed
- Usually follows indoor seed starting by several weeks
Climate and regional notes
UK & Ireland
- Take extra care with fluctuating temperatures
- Late frosts are common
Europe
- Northern regions require slower hardening off
- Southern regions can move faster
USA
- Timing depends on last frost date
- Warmer regions allow earlier planting
Pro tips for better results
- Check weather daily before placing outside
- Use a cold frame if available
- Avoid windy days early on
- Group plants together for protection
- Don’t rush the process
10 frequently asked questions about hardening off
What is hardening off
Gradually exposing indoor plants to outdoor conditions.
How long does it take
Usually 7–10 days.
Can I skip hardening off
Not recommended — plants may suffer shock.
When should I start hardening off
Once seedlings are strong and ready for planting out.
Can I leave plants out overnight immediately
No, only at the end of the process.
What happens if I don’t harden off plants
They may wilt, stop growing, or die.
Do all seedlings need hardening off
Yes, if grown indoors or under cover.
Can I harden off in direct sun
Only gradually — not at the start.
Should I water during hardening off
Yes, keep soil consistently moist.
Can I use a greenhouse instead
Yes, but still gradually introduce outdoor conditions.