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 it."

Perhaps you didn't realize it, but you have probably seen examples of plants that took their hardiness cues quickly. For instance:

• Just a few days of cold is all that's needed to convince kale crops to bolt into flowering stage.
• 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.