harden off(redirected from hardening off)
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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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.