harden off
To systematically expose a young plant to outdoor conditions so that it adapts well when moved outside permanently. A noun or pronoun can be used between "harden" and "off." That delicate plant won't survive outside unless you harden it off first.
harden something off
to accustom a young plant to normal weather so it can be moved from a protected environment to the out-of-doors. We put the plants by the open window to harden them off. We hardened off the plants.