I always look forward to your column. My Japanese Red Maple tree and my Japanese Threadleaf Maple tree both still have not dropped their leaves and it is January. What is the cause? Shirley
I have also noticed the brown leaves on the younger and the older Japanese maples in our area. Not to worry, the failure to drop leaves is weather related and not a serious threat to the health of the tree.
What I suspect happened is that the very cold weather we had in early-mid November froze the specialized cells where the leaf stem meets the branch and stopped the normal leaf dropping process.
The usual process of 'leaf drop' occurs as the autumn days shorten and temperatures gradually cool. The leaves stop manufacturing chlorophyll (the green pigment that allows plants to absorb sunlight and turn it into food that can be stored for winter dormancy) and exposes the hidden pigments that give Japanese maples their bright red, orange or yellow fall foliage. At the point where the leaf stem meets the twig or branch is a collection of specialized cells that form the "abscission layer". Simultaneously, this layer begins to block the veins that move water into the leaf and food into the tree. Once the leaf is completely blocked, the layer becomes dry and brittle and the leaf detaches from the tree.
Unexpected cold weather below 32 degrees can stop this process. Consequently, the leaves do not change into bright fall colors and go directly from green to brown and the leaves stay attached to the tree.
I recommend that you do not remove the brown leaves by hand. The base of the leaf stem is acting as a protective plug and the removal of the leaf would expose a hole in the branch which would permit water and extremes of temperature to enter the interior of the tree.
In the spring when the new leaves emerge the brown leaves will drop on their own.
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About Linda Lillie
Linda K. Lillie is the President of Sprigs & Twigs, Inc, the premier
landscape design and maintenance, tree care, lawn care, stonework, and carpentry
service provider in southeastern Connecticut since 1997. She is a graduate of
Connecticut College in Botany, a Connecticut Master Gardener and a national
award winning landscape designer for her landscape design and landscape installation work.