It's a soft science: Hardening off for winter
How long does a plant need to become hardy for winter?
We figure two weeks as a minimum time for a woody plant to
acquire cold hardiness. It is, however, only a guess and it's
influenced as much by science as it is our own (in)tolerance for
fussing over plants.
How long a given plant needs to become hardened to cold is best
answered 'it depends.'
This has been explained by Dr. Curt Peterson of Michigan State
University in this way: "Different species of plants acclimate at
different rates and times, and plants with seed sources of southern
origin have slower acclimation rates. The result is acclimation
later in fall... Sensitive species will probably need a few more
weeks to reach dormancy after the hardy plants have reached
Perhaps you didn't realize it, but you have probably seen
examples of plants that took their hardiness cues quickly. For
- Just a few days of cold is all that's needed to convince kale crops to bolt into flowering
- Also, some azaleas, forsythia, lilacs and other spring bloomers
regularly open a few flowers if a fall warm spell follows as little
as a week of cold weather.
Those plants needed just that week to enter and complete
dormancy. They were then eligible to take a cue from the next
warm-up to count winter "done."
However, most hardy plants outdoors take 2 to 4 weeks to
complete first stage hardening after being prompted by shorter days
in fall. Then they use the next few weeks or a month of
cold-above-freezing to complete second-stage hardening, and will
resist waking for 500 - 1,000 hours or longer.