Q: My oak trees still have their leaves this late in winter. Why didn’t they fall off like all the other leaves?

• Colleen from Poland

A: While most deciduous trees drop their leaves in fall after giving us a color show, some oaks don’t follow this seasonal script. Instead, they keep some of their leaves into spring. The name for this phenomenon is marcescence (pronounced “mar-CESS-enss”).

Oak marcescence is more common in young trees, on lower branches and for some types of oaks more than others. And, other trees can have this phenomenon. Besides oaks, the American beech, the hornbeam and witch hazels can keep their leaves during the winter. Many times, there are very few leaves left on during winter, and these species are not as noticeable. We are more likely to see oak trees with their leaves hanging on in winter and into early spring.

In the fall, most deciduous trees, taking clues from the environment, develop a layer of cells between the base of the leaf and the tree called an abscission layer. As the abscission layer develops in the fall, it causes the leaf to separate from the tree and fall to the ground. If this layer doesn’t develop, the leaf remains on the tree. In marcescent trees the abscission layer does not develop until spring.

The exact reason for marcescence is not known. However, through research, some possible explanations have been developed.

The Harvard Arnold Arboretum suggests that keeping the leaves may help protect the buds and the twigs from cold and herbivores. This winter shield could be especially crucial for younger oaks, as they are often damaged by deer in our area where there are large populations of deer. According to Penn State Extension, large deer populations result in no or very few oak tree seedlings to provide oak trees for the future.

The University of Maryland proposes that marcescent leaves may help the tree by adding nutrients to the ground in the new year when they fall off and decompose. Additionally, the newly fallen leaves help conserve soil moisture by shading the ground around the tree.

No matter what the reasons for this phenomenon, it causes no harm and gives us something to look at besides bare branches until spring arrives.

Our clinic will resume regular staffed hours in April. If you have questions now, contact the Mahoning County Extension Office’s clinic at 330-533-5538 or read details at http://go.osu.edu/mahoningclinic.

For more information on marcescence see: http://go.osu.edu/marcescence

Sprague is an Ohio State University Extension Master Gardener Volunteer in Mahoning County.