Hardening Off: Preparing Seedlings for the Outdoors

Sweet peas being hardened off

Hardening off is simply the process of gently introducing young plants to life outdoors before planting them out. Seedlings raised indoors or under cover have had a very sheltered start. They are not yet used to wind, temperature changes, or strong sunlight, so moving them straight outside can cause stress or shock.

The key is to introduce these changes slowly and one at a time. Give plants a day or two at each stage so they can adjust before moving on.

Steps for hardening off

Move out of the house
If seedlings were started indoors, begin by moving them into a heated greenhouse or propagator, ideally with a heat mat.

Reduce bottom heat
After a few days, remove the heat mat but keep the plants protected in the greenhouse.

Introduce outdoor air
Start taking trays outside during the day, placing them in a sheltered spot out of strong wind and direct midday sun.

Increase exposure
After several days, leave trays outside day and night, as long as there is no risk of frost. Seedlings in trays are especially vulnerable because their roots have very little protection.

Once plants have adjusted to outdoor conditions, they are ready to be planted out into their final position.