So here we are a week and a half before Christmas, and we still have a lot of leaves up in the trees. In a normal year (are there any of those anymore?) most of the leaves would have fallen by now, but there are so many trees that have barely started to drop leaves if they are dropping them at all. So, let’s talk a little bit about why leaves drop in the first place.

To start, let’s discuss something called the abscission zone. Each leaf has an area of the leaf near the stem that is called the abscission zone. You can’t really zee it, but the cells in the abscission zone are small, fibreless cells that are smaller than the surrounding cells.

These cells are affected by changing daylight hours in the fall. During this period of time, several chemical reactions take place in the abscission zone. The cells in the first layer (known as the separation layer) begin to weaken due to the changing chemistry in the leaf. This weakening allows the leaves to fall from the tree due to rain or a breeze.

There is a second layer to the abscission zone known as the protective layer. This layer help protect the tree from the open wound left by the fallen leaf. If you take a look at these leaf scars, you might be able to see some small dots. These dots reveal the end of the veins through which nutrients moved from the main tree to the leaves during the growing season.

One thing that can disrupt this process is an early or sudden freeze. The process of getting the leaves ready to drop from a tree is a gradual process that encourages the bonds to incrementally weaken so the leaf can drop. However, leaves that have not worked their way through this process are flash frozen in time and place when the gradual process is interrupted.

Fall this year has been a bit unusual in that we had fairly warm temperatures for this time of year, but then all of a sudden, we had a few nights with freezing temperatures. This likely disrupted the gradual process. Leaves that remain attached to the tree because of this are called “marcesent” leaves.

With the process interrupted, it’s going to take something stronger to get the leaves to drop. Perhaps a heavy rain, snow, or ice. For those that make it through the winter without detaching, they will get pushed off in the spring by new growth.

Marcesent leaves don’t affect the trees detrimentally, however, it probably means you are going to be dealing with falling leaves for a much longer period of time than we typically expect. Same number of leaves, just dropping over a longer period of time. Good luck!

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org. Photo: Tom Ingram