Stubborn trees add color to the gray of winter

From an early age we learn the difference between deciduous and evergreen trees, but trees that fall somewhere in the middle stand out as botanical oddballs. Marcescence, the phenomenon in which a deciduous tree retains its leaves over winter, boldly defies what we are taught about trees in a way that is notable even without understanding it.

The physiological reason that some trees hang onto their leaves over winter is the disruption of the same process that causes Ginkgo trees to drop all their leaves at once. In autumn, a layer of cells forms at the base of each leaf stem preparing it for leaf fall. In a “normal” deciduous tree, those cells are formed gradually, but in Gingko trees it happens all at once and the leaves drop in one dramatic event during a cold spell. On the opposite end of the spectrum, trees that have marcescence never complete that process and some leaves don’t fall until they are pushed out by new buds in spring.

Although scientists know how it is that some trees hold their leaves, there is some discussion over the evolutionary advantage of marcescence. One common theory is that it provides protection from deer and cold, drying winds on tender branch tips and new leaf buds as they form.

Marcescence is more common in younger trees and it tends to last longer on lower branches, perhaps lending credence to the protection theory.

From a human perspective, persistent leaves offer some benefits in a yard, the most obvious of which is on full display right now. While winter offers an opportunity to appreciate the structure of deciduous trees, those that retain their leaves also offer texture, color and interest that is sorely needed in the grayest months of the year. They also provide a wind break.

Fastidious gardeners might have a beef with such trees, as the leaves will fall in spring long after they have cleaned other fallen leaves from the garden. And late ice storms can cause problems for trees still holding leaves and lead to broken branches.

Some types of trees are more prone to marcescence, including hornbeams (Carpinus), American beech (Fagus grandiflora) and some oaks, particularly pin oak (Quercus palustris) and bur oak (Quercus macrocarpa). In my garden, an English hornbeam (Carpinus betulus) hedge has held its leaves well, making it more effective as a screen even in winter. The native hornbeams (Carpinus caroliniana) hold their leaves longer than other deciduous trees, but have shown bare branches for well over a month now.

Marcescence is a quirk of nature that is out of mind for much of the year but adds another dimension to tree appreciation and selection. Think of it as the stubborn streak in trees, then enjoy what it brings to the winter garden.