Fellow blogger Jodi Torpey asked an excellent question about the speed with which her ash tree dropped its leaves this weekend. Reading her post on this in her blog, I couldn’t help but comment to her that my black walnut had done the same thing – dropped all of its leaves in a great, big circle all around its base.
Oddly, my burr oak still holds its leaves despite the frigid temps, something Jodi has noticed going on all over the area; some trees holding onto their canopies, others shedding them fast. Her question “is it normal for some trees to experience sudden leaf drop after subfreezing temps?” had me diving into the research to find the answer.
Deciduous trees lose their leaves in fall by ‘abscission’.
Taking their cue from changes in temperature, length of daylight, and natural aging, trees form an abscission layer between leaf and branch with cells that get larger and harder, shutting off water flow to the leaf (also shutting off chlorophyll, which is part of the reason leaves change color in fall).
Eventually the leaf is completely separated from the tree and falls gently to earth. Every species of tree will do this on its own schedule; some early, others late.
When temperatures drop below freezing, the abscission layer hardens more rapidly, cutting off the leaf’s connection to the tree. Any weight, such as snow or movement from wind, will make these leave drop from the tree. Some of our trees were at the right stage in their fall abscission to lose every leaf to the freeze all at once.
The good news is that this shouldn’t harm them. Yes, left on their own more of the potassium and phosphorous in the leaves would have been absorbed back into the plant, but the trees can take a small loss of these nutrients if they were healthy.
A different result from this cold snap will have the opposite effect: when we get a hard freeze in early October, some trees haven’t had time to develop the abscission layer. On these trees the leaves freeze and remain attached to the branches.
Eventually wind and snow will force those leaves to drop, but in the meantime there is a danger of branch breaking if we get a wet, heavy snow, where the added weight held by the clinging leaves can be a problem. Keep an eye on your branches when we get those wet snows, and be prepared to prune should damage occur.
I knew I came to the right place for an answer! Thanks for the great explanation and the reassurance about my tree. It is disappointing though, because in a normal fall the ash tree turns a brilliant yellow and is a beautiful addition to the landscape.
I’m so disappointed too – with the much wetter summer I was looking forward to a breathtaking fall display, since trees color up better with a bit of moisture. But nooooo…nature had other plans for us.
I too lost all my green ash leaves this past weekend. The temperature hit 28 degrees and we had 5 inches of snow. Within hours my green ash tree lost all of its leaves as well as my neighbors trees. Great information, I’m sure lots of people were wondering the same thing.
Thanks. I’m really disappointed that we went from a tantalizing bit of fall color – just enough to get our hopes up – to bare branches. But on the bright side, I just went on a tree walk and found a bunch of insects frozen in place, a critter cryogenic display!
Great information! Thank you for such a clear explanation of a complex process. I noticed that the hackberry trees also lost all of their leaves. The catalpa leaves froze, but are still hanging on and look quite dreary!
Now that most of the leaves are on the ground, my question is: are there certain leaves which should not be composted for use in next year’s vegetable garden?
Hi Chan,
Good question – Black Walnut leaves should be avoided due to a chemical called juglone, which inhibits other plants from growing. Although the roots of the black walnut contain the most juglone, leaves have some too. Composting will reduce the amount, but some garden plants – like those tomatoes you love – are very sensitive to it.
Excellent information, Carol. I was wondering this morning why we have some trees in the neighborhood that have brown leaves clinging to them, while others are naked.
Hello, very interesting this article.
I´ve also had this experience, and it´s also due that when a frezze comes, the leafs freezze and when the sun melts the ice, the leafs falls, and I´ve noticed this, as the leafs are still on the tree when it´s still shady and once the sun heats the leafs, they fall inmediatly.
Adam
from Catalunya (spain)