What is hardening off and why does it matter?
Hardening off is a gradual adjustment of indoor-grown plants to outdoor conditions. Indoors, leaves are tender, with thin protective layers, and they meet no wind. Outdoors, bright sun, wind and temperature swings can burn leaves, break stems and slow growth.
Skipping the step is one of the most common causes of transplant failure. Even store-bought seedlings often need it, especially if they were grown in a greenhouse.
Think of it as training. A few days of mild stress builds thicker leaves and stronger stems, but too much too soon causes damage, so go slowly and watch each tray.
When should I start?
Begin about a week or two before your planned planting date, which you set from your frost dates and the crop's needs. Cool-season crops can start hardening earlier, since they tolerate cooler nights. Warm-season crops should wait for settled weather.
Check the forecast before you begin. If nights are well below what the crop tolerates, delay. Your extension office can tell you the typical planting windows in your area.
If your plants outgrow their pots while waiting for the weather, move them to larger containers and keep hardening on pause until the forecast improves.
- Cool-season crops can start first
- Warm-season crops wait for settled weather
- Check the forecast before each step
- Slow the schedule if leaves look stressed
- Keep a note of start and finish dates
How do I do it step by step?
Start by putting the plants outdoors in a sheltered, shaded spot for a short time on a mild day. Each day, lengthen the time outside and move them into more sun and wind. Bring them in at night at first.
Over the course of a week or more, they should be spending a full day in sun, and later a night outdoors if the weather is mild. Keep them watered, since outdoor conditions dry pots quickly.
If you are busy, a cold frame or a covered, ventilated spot can do the same job with less carrying.
- Short time in shade on day one
- More time and more sun each day
- Bring in on cold or windy nights
- Keep watered through the process
What should I watch for while hardening off?
White or bleached patches on leaves signal sunburn, and wilting signals drying wind or lack of water. Move plants back to shelter and slow down the schedule. Damaged leaves may recover, but severe damage can set plants back for weeks.
Wind can snap stems of tall plants, so shelter them. Pots on the ground can be visited by slugs or animals, so raise them or protect them.
Keep hands off tender leaves while carrying trays, and use a wagon or a tray with a handle to move many plants safely. Dropped trays cause more loss than weather in many gardens.
How do I handle cold nights?
Know the lowest temperature each crop tolerates, and never leave tender plants out when the forecast approaches it. Cool-season crops handle much cooler nights than tomatoes, peppers and squash.
If in doubt, bring them in or cover them. Row cover, cloches and temporary shelters can add protection, but they are not a substitute for watching the forecast.
A min-max thermometer outside tells you what the night actually did, which beats trusting the forecast alone. Write down the lows for the week.
Do all plants need hardening off?
Most transplants do, including vegetables and flowers raised indoors or purchased from a greenhouse. Direct-sown crops do not, since they grow up in the conditions where they will live.
If you buy plants from a store that keeps them outdoors, they may already be hardened. Ask the seller how they were grown.
Different varieties in one tray may harden at different rates, so pull aside any that look stressed and give them a gentler schedule.
What if the weather turns bad after planting?
Protect plants if a cold snap or storm arrives. Cover with row cover or an upturned container, secured against wind, and remove the cover when it warms. Keep a roll of row cover handy during spring.
If a plant is damaged, give it time before replacing it, since some recover. Keep spare seedlings if you have them.
Write down your timeline and what happened. Next spring you will know how many days your plants needed and how soon you can plant.
Key takeaways
- Start a week or two before planting and increase exposure daily.
- Begin in shade, then add sun and wind gradually.
- Bring tender plants in on cold nights.
- Watch for sunburn and wind damage and slow down if you see it.
Sources and further reading
Last reviewed: November 12, 2026. Written by the Homestead & Small Farm Editorial Team. See our editorial policy.