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Here's why leaves are still on trees in the Treasure Valley right now

Do deciduous trees still have their leaves in your neighborhood? A Boise State professor explains how trees are impacted by a warm fall and an intense cold snap.

BOISE, Idaho — You may have seen some trees in your neighborhood still have leaves on their branches. Typically, deciduous trees – trees that lose their leaves every year – will lose their leaves in the fall. However, that is not what we are seeing this year.

Both a warm fall and an intense cold snap hinder the process of a tree losing its leaves. This year, we saw both of those things. We had a warm October and then a quick flip to a cold November.

A Boise State University professor, Dr. Marcelo Serpe, studies how plant growth is affected by environmental factors. He said temperatures can affect plant hormones. 

"During the fall, if you have a gradual decrease in temperatures, it causes changes in the hormones present in the plants," Dr. Serpe said. "At the base of the junction of the leaf and stem, there's several layers of cells that is called the abscission layer, or separation layer."

Dr. Serpe added initially, all the cells are glued together. Then, the hormones change the cells in this separation later and the leaves drop. Again, there needs to be a gradual cooling in the fall for this to happen.

He also said if the leaves stay intact from winter into spring, the new growth will eventually force the old leaves off. While that part is not particularly harmful to plants, it can be a detriment to have leaves all through winter.

"When you have more leaves, you can have more accumulation of ice and snow, so then the branches will be heavier," Dr. Serpe said. "So, that can result in breaking some of the branches. Especially, if you have a house close to one of these trees that can be a problem."

Some plants are especially susceptible to damage when there are quick temperature changes. When temperatures go down gradually, it allows plants to build up hardiness and adapt to the cold temperatures. Young parts of the plants are especially susceptible.

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