Here we are in mid-January and most deciduous trees and shrubs (excluding conifers) have shed their leaves. But long after the last frost and through a couple of modest snow storms, there are still trees around the front range with leaves that are stubbornly hanging on, as you can see from the photos I took in my neighborhood last week.
Marcescence is the retention of dried, dead leaves during the winter. Typically, as woody plants prepare to shed their leaves in the fall, cells at the junction of the twig and the leaf petiole (stem) release enzymes and form an abscission layer, which aids in the separation of the leaf. Marcescent leaves do not develop this thin-walled cell layer and therefore, do not drop readily.
Early severe cold weather can cause marcescence as the development of the abscission layer is halted and the leaves do not release. Front range gardeners will recall an extreme case in November 2014 when an exceedingly mild fall was interrupted by a one day temperature plunge from a high of 58 degrees to a low of 16 degrees. The result was subsequent damage and loss of many hardwood plants the following season and beyond.
Some plants are more apt to hold leaves longer, including several oak species, hazelnuts, American lindens and beech trees. According to Jim Finley of Pennsylvania State University, “Marcescent leaves are often more common with smaller trees or more apparent on lower branches of larger trees, which in forest conditions would be growing beneath taller trees where the reduced sunlight might slow the abscission process.” Lower leaves are therefore exposed to cooler temperatures, resulting in leaf retention. It should be noted that upper leaves can also exhibit marcescence.
Marcescent leaves eventually drop, either due to wind, snow load or the push of new spring growth. Under normal circumstances, marscence does not damage trees.
“Winter Leaves that Hang On”, Jim Finley, Department of Ecosystem Science and Management, Pennsylvania State University.
Written by Linda McDonnell, a Denver County Master Gardener