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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.
1. To cause something to become physically harder or more solid. In this usage, a noun or pronoun is used between "harden" and "up." The tub of ice cream was melting, so I stuck it in the freezer to harden it up.
2. To become physically harder or more solid. If the ice cream is melting, stick it in the freezer for a bit so it hardens up.
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.
harden something up
to make something hard or strong. Put the meat in the freezer awhile to harden it up before you try to slice it thin. Harden up the ice cream a little in the freezer.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.