University of Wisconsin - Extension

You may have noticed that trees such as oaks, elms, maples, callery pears, willows, and crabapples have kept their leaves this year. Why this year compared to other years?

According to Laura G. Jull, University of Wisconsin-Extension Woody Ornamental Specialist, species such as oaks and beech normally retain their inner leaves toward the center part of the trees, especially when the trees are young. Some oak species such as shingle, pin, and scarlet oaks may retain most of their brown leaves into winter before they eventually fall in the spring.

Other species of trees do not normally retain their leaves so late into winter. Why are they this year?

"The answer is not as clear as you might think," Jull said.

Last year's spring was very early. Researchers at UW-Madison hypothesize that the early spring messed up the internal phenology or internal clock in some plants. In addition, this past fall was relatively warm and wet, which extended the growing season later into fall, preventing plants from going dormant on time. In addition, leaves on some species of trees either did or did not develop good fall color before the severe cold occurred.

December was a very cold, snowy month. According to Jull, the leaves remaining on trees died without forming the proper abscission layer at the base of the petioles, which is what normally occurs during cooler temperatures and reduced day-length.

"Most of our woody species rely on the gradual lowering of temperatures in addition to reduction in daylength to initiate dormancy," Jull said. "If temperatures remain warm, even though the daylength is reduced, some species, especially exotics, may continue to grow late into the fall."

What should property owners do? The leaves will eventually fall from the tree in spring as new growth is initiated.

"It remains to be seen if there will be any lasting damage to the trees from holding onto their leaves into winter," Jull said. "If the trees did not have a sufficient opportunity to translocate the sugars and soluble nitrogen in the leaves back to the root systems, the trees may suffer the following spring as their leaves unfold."

"In spring, the trees will shed the leaves that they were retaining, which will then become a litter problem. However, the leaves can be raked and composted for later use in the garden."