TREE LEAVES WON'T FALL
My mountain ash leaves and one of my apple tree's leaves still haven't dropped, despite being very brown and dry and going through several high-wind days late last fall and through the winter. I've noticed the same in many other yards in town, particularly those with mountain ash and fruit trees. Why are the leaves still hanging on this late in the winter? Does this indicate any potential problem with the tree's health? For fruit trees, does this indicate a potential problem for flower and fruit production this coming season?
When it comes to leaves falling (or not) in autumn, sometimes we can point to a single weather event, such as a quick, hard freeze, while other times we cannot. Leaf senescence and abscission is a result of a complex of issues, including plant hormones, sometimes juvenility, nutrition, drought ,and weather.
First, let’s talk about leaf abscission. There is a zone of abscission at the base of each leaf’s petiole. During abscission in a “normal” year it is believed that the balance of the plant hormones from each side of the zone stimulates this layer of cells to divide and to dissolve, causing the leaf to be shed cleanly. A protective layer of cork forms on the stem side of the layer.
My first hard freeze in 2011 was September 19, and it put an end to my vegetable garden. Even so, the leaves on the little maple I was given in memory of Bob stayed bright green well into the ensuing Indian summer, while neighboring green ash trees dropped their leaves with a single “foof,” Charlie Brown style. Why the difference? Abscission can be species dependent. Some species’ leaves die in the autumn, but the cells in the abscission layers do not. The process of abscission occurs the following spring, and is similar to the process that takes place in other species in the fall. These are termed marcescent species. But what about non-marcescent species that retain their leaves into the winter? Plant scientists agree it’s complex, and there is no one good reason for it.
The leaves of some species of trees will wilt, die, and be retained as a result of drought while other species shed their leaves as a result of drought, even during the growing season. Trees with leaves that are dropped, or that die prematurely, can indeed be stressed. In the natural scheme of things, woody plants reabsorb nutrients from the leaves as they gradually senesce, so there is a possibility of nitrogen deficiency come spring.
By the way, there are other reasons trees don’t shed their leaves. If your apple trees have retained leaves on individual branches, inspect the buds and bark to be sure fireblight isn’t the culprit.
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