Plants which have been raised in heated conditions need to be ‘hardened ff’ before planting into garden or containers. Just think what it is like when you visit a foreign country with different temperatures to what you are used to. If you have good information, you will go to your holiday country – either with fewer or more clothes.
You will be prepared. No such luck for a young seedling! It has to depend upon you to carefully acclimatize it to its new home. It will need to be hardened off!
Even if you are planting new plants/seedlings out into the garden from the greenhouse in early summer, when it is (should be) nice and warm you will need to harden the plant off. It has spent all of its life in protected conditions. No drying wind, no direct sunshine, no cold nights!
If possible, the hardening off process can start to take place in the
place where your young plant has been living. If it is a greenhouse,
then either turn down the heating bit by bit, or open the vents, or move
the plant to a cooler place. The lower the plant in the greenhouse, the
cooler it will be.
Then just move the plant outside in a sheltered place for a day, and either protect during night time or bring back indoors. As the evenings/nights get warmer then you can start to leave the plant outdoors all of the time. Most plants can be hardened off successfully in a couple of weeks. If you start the process too soon in the year, no amount of hardening off will protect a half hardy annual from frost! Far better to delay planting out in the garden by a few weeks than risk having your plant go into severe shock because of sudden temperature changes.
If you have grown your seedlings in a cold frame – or maybe vegetables under cloche protection, the same applies. Gradually remove the covers over a period of a week or so, and your plants will be better placed to withstand the change if they have been properly hardened off.