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Ask a Master Gardener: When will the leaves stop falling?

by Peter Sutter | November 27, 2022 at 4:00 a.m.

The week before last was a little too much like winter for me. I think that first snowfall was a little early, of course I always think snow is early no matter when it comes. I hope you were a little more prepared for the freezing weather than I was. I did have my usual Thanksgiving cabbage (in the fridge) ready for the coleslaw. I hope you had something from your garden for your Thanksgiving feast.

One thing I don't need more of is falling leaves. I have an oak tree on the north side of our breezeway, and leaves seem to blow in all winter long with the last of the leaves hanging on until spring.

Michele Warmund, University of Missouri Division of Plant Sciences, had an excellent article explaining what is going on. If you have one of those trees and are wondering if the leaves will ever stop falling so you can finish raking, here is some interesting reading.

"The time of leaf drop depends on the cells that make up the abscission or separation zone at the base of the leaf petiole (stalk from the leaf that is attached to the twig). Shortening day length, frost or drought can trigger formation of the abscission layer.

"In most trees and shrubs, an abscission layer, composed of cells with weak walls, undergoes biochemical changes that cause the cell walls in this zone to dissolve, leaving the leaf dangling until a gust of wind breaks the connection with the tree. As the abscission layer is maturing, a different group of cells at the base of the petiole forms another protective layer of cells, containing compounds called suberin and lignin, to seal the wound after leaf fall. This corky layer protects the tree from disease infection and water loss.

"Unfortunately, not all leaves fall at once. Some trees, such as Quercus (pin, red, and white oaks), as well as Fagus (beech) and Carpinus (hornbeam) retain many of their dried leaves until late winter and then drop them intermittently. This tendency for late retention of leaves, dropping them intermittently during winter, with a final leaf drop just before bud burst in the spring is called marcescence. In these types of trees, the abscission zone fails to develop fully during autumn, allowing the leaves to stay on the tree.

"The reason why some trees keep their withered leaves into winter is unknown. Some ecologists have speculated that marcescence provides unpalatable leaves as a deterrent to deer browsing on buds or twigs during winter, protecting the trees from damage. Others have suggested that leaf retention is a means of trapping snow, resulting in increased soil moisture at the base of the tree after it melts. However, when this occurs, trees are at risk for limb breakage due to the weight of a heavy, wet snow.

"It is known that marcescence is often a juvenile trait that can be lost as some trees age. In a recent study conducted at the University of Missouri, Quercus rubra (northern red oak) trees that had multiple flushes of growth during the growing season, retained their leaves later in the year than those with few flushes. Also, leaves from the last flush on trees having the marcescent trait tended to have higher levels of chlorophyll, as well as slightly higher rates of photosynthesis than leaves from non-marcescent trees (those with early leaf drop).

"While scientists are learning more about the genetics of marcescence, keep the rake handy as you will be using it multiple times over the next five months if you or your neighbors have marcescent trees."

Peter Sutter is a life-long gardening enthusiast and a participant in the MU Extension's Master Gardener program. Gardening questions can be sent to [email protected]

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