Hardening Off: Preparing Seedlings for the Outdoors
Sweet peas being hardened off
Hardening off is the process of gradually acclimatising seedlings to outdoor conditions before planting them out. This step is essential to prevent shock and transplant stress, as seedlings that have been raised indoors or under protection are not yet accustomed to wind, fluctuating temperatures, and direct sunlight.
Introduce these stresses gradually and one at a time, leaving at least a day or two between each stage to allow the plants to adjust.
Steps for Hardening Off
Transition from indoors – If seedlings were started in the house, first move them to a heated greenhouse or propagator with a heat mat.
Reduce bottom heat – After a few days, take them off the heat mat but keep them in the greenhouse.
Introduce outdoor conditions – Move the trays outside during the day to a sheltered spot, away from strong wind and direct midday sun.
Extend outdoor exposure – After several days, leave trays outside day and night, ensuring there are no frosts, as seedlings in trays have particularly vulnerable roots.
Plant out in their final position once they are fully acclimatised.