Hardening Off Seedlings

Growing Seeds
Hardening Off

Hardening off is a vital step needed to prevent your plants dying or wilting from sudden changes in conditions. Hardening off seedlings that you started indoors is simply transitioning them gradually from indoor growing to the garden outdoors. It’s easy and is an important step so your seedlings survive in your outdoor garden.


Seedlings need to be gradually introduced to the outdoors in stages over a period of at least a week and preferably two. Hardening off usually takes two to three weeks, but the warmer the initial growing conditions, the longer the hardening off period. Hardy plants acclimatise faster than half-hardy or tender kinds.
To be on the safe side, do not plant out tender plants before the date of the last frost which is usually late spring in the south of England, later in the north and Scotland.
Hardening off simply means getting your seedlings used to outside conditions gradually. Give them a little more exposure to wind, sun, and temperature variations each day, until they are ready to be planted out.



  • The best time to harden plants off is on wet or overcast days under cover of a greenhouse or cold frame. Overcast days don’t stress the plants with high afternoon temperatures and are unlikely to produce hard frosts at night.
  • Once hardened off and planted out, still watch the weather – even if a late frost is forecast, plants can be covered with fleece or bubble-wrap and still survive.
  • At first, limit the amount of direct sunlight they receive – choosing a place which is shaded in the afternoon.
  • Keep a few plants back inside as an insurance policy, just in case an unexpected frost kills the main batch off.