Most plants have a natural “sleep” period (called dormancy) during the winter, because actively growing plants can’t stand freezing temperatures for very long. During dormancy, growth stops while plants wait for good growing conditions.
By late summer most plants have begun preparing their annual rest, a process called “hardening off”. This process is triggered by the shortening day length. Many spring-flowering trees and shrubs set their flower buds at this time. As days grow shorter, plants slow and finally stop new growth, stockpile food in their roots and stems, and then go dormant.
Different plants respond differently to temperature extremes. For example, a peach tree’s leaves and branches can stand temperatures 10 degrees below zero, but the flower buds may be damaged, affecting the next season’s flowers and fruit. Spring-flowering bulbs like tulips, daffodils, and hyacinths need several weeks of chilling before they’ll begin growing. Pussy willow and forsythia brought indoors in the fall won’t sprout and flower, but the same branches brought indoors in early spring will provide a wonderful display—because their chilling requirements have been met.
Plants that haven’t hardened off fully can be damaged by early frost or cold temperatures. Most winter injury occurs in late fall and early winter. What can gardeners do to help or harm the hardening off process? Gardeners should be careful to avoid late-summer pruning, or fertilizing after mid-summer. Pruning and feeding in late summer stimulates tender new growth that won’t have time to harden off before winter.
Most cold regions experience one or more warm spells during each winter. If a plant breaks dormancy during winter thaw, the next cold spell quickly kills the new, delicate growth (and sometimes the entire plant). How do plants know when it’s really spring, and it’s safe to begin growing?
Plants have “learned” not to be fooled into thinking it’s spring before it really is, by having specific chilling requirements. This is is the number of hours of temperatures between 32 degrees and 45 degrees before they break dormancy. Plants are somehow able to keep track of the number of hours they are exposed to this very specific range of temperature. This memory is also carried within seeds.
With their hard seed coats protecting the delicate plant embryos within, seeds are miracles of evolution. Many seeds are able to endure long periods of drought, heat, or freezing temperatures—yet are ready to spring to life when favorable conditions arise. Depending on the adaptations they’ve made to their native environments, different plants’ seeds may need darkness, light, warmth, chilling, or even exposure to fire before they’ll germinate!
Here’s one trick to help your roses harden off for winter. Rather than cutting them or “dead-heading” the spent blooms, leave them to mature on the plant and form “hips,” or fruit. The process of developing fruit and maturing seeds encourages roses to harden off properly, reducing “winter kill.”
Steve Boehme is a landscape designer/installer specializing in landscape makeovers. “Let’s Grow” is published weekly; column archives are online at www.goodseedfarm.com.