As the temperature continues to descend, plants are being conditioned to tolerate winter’s chill. The conditioning also works on people, too, but many of us fight it. Sweet gum trees (Liquidambar species) are a bit like us in that respect. Because these trees have large north-south geographic ranges, the seedlings that derive from the lower latitudes often do not respond to the regular seasonal cues that trigger dormancy in northern plants. Some individuals (we have the American L. styraciflua and two Chinese species, L. acalycina and L. formosana) hold their colour well into late November, and some trees do not shed their leaves until the winter solstice.
On the other hand, broadleaved evergreens don’t shed their leaves in autumn at all. While most deciduous leaves are thin and papery, those of evergreens are often waxy and thick to help survive the winter. Many evergreens lose older leaves as new ones emerge in the spring, while others drop a few in summer. But in November, even evergreen plants are shutting down for the cold winter rest. Not so, the eucalypts. These aromatic Australian and Tasmanian trees and shrubs never actually become fully dormant. Eucalypts grow when conditions are right. As soon as the temperature starts to fall, activity slows, but if it warms up, growth starts exactly where it left off.
Finally, some truly deciduous plants only drop some of their leaves in the autumn. This phenomenon, called marcescence, is common in oaks and other plants in the beech family. Marcescent leaves turn brown, but hang onto the branches, often making rasping, rattling noises in the wind. They eventually let go, usually in time for bud break in the spring.
More photos of November in the Garden can be found in our Forums.
Submitted by Douglas Justice, Associate Director, Horticulture & Collections, October 27, 2016