leser column

Aspens with stuck leaves due to hard freeze.

You might be wondering why so many of our trees failed to produce colorful fall leaves and why so many trees have ugly, brown, dead leaves still attached?

In late October, we had a strong winter storm blow in with freezing conditions producing low temperatures in the single digits and daily highs in the 20s. This lasted for several days. A freeze occurs when temperatures fall to or below the freezing mark (32 degrees fahrenheit). A hard freeze is when temperatures fall below 28 degrees fahrenheit for a long enough time to seriously damage or kill seasonal vegetation. This is what happened in our area.

The result of this killing freeze became quite evident. Leaves on trees that were already turning color were stopped dead in their tracks. Those trees that had not started their leaf color change were killed outright, not able to display their normal fall color.

So what does this all mean you might ask? First, you need to know what is taking place in the fall with your deciduous trees under normal weather conditions. After the leaves are fully developed on trees, they begin making and storing the carbohydrates that will be needed for the new tree growth in the following year. As the season progresses, in late summer or early fall, the trees enter a growth process that produces the colorful fall leaves we have come to love. This process starts as the weather turns cooler and hours of daylight decrease.

During the spring and summer months, carbohydrate-producing chlorophyll is constantly replaced in the leaves. The chlorophyll gives the leaves their green color. As the nights get longer in the early fall, the cells near the juncture of the leaf and stem divide rapidly but do not expand.

This action of the cells forms a layer called the abscission layer. The abscission layer then blocks the movement of materials from the leaf to the branch and from the roots to the leaves.

As the chlorophyll is blocked from the leaves, it disappears completely from them. The lack of chlorophyll allows the yellow (xanthophylls) and orange (carotenoids) pigments to be visible.

The red and purple pigments (anthocyanins) are manufactured from the sugars that are trapped in the leaf. These pigments in leaves are responsible for the vivid color changes in the 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 later.

When temperatures drop below freezing, the abscission layer hardens more rapidly, cutting off the leaf’s connection to the tree. Freezing conditions destroy the leaf’s ability to manufacture the red and purple pigments. Early frost will put an end to the colorful foliage.

A different effect will result from this late October killing freeze. When we get an early hard freeze, 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 branches breaking if we get a wet, heavy snow where the added weight held by the clinging leaves can be a problem.

So what is the bottom line to all of this? Well, the bad news is that trees that had either started or completed their leaf color change may be deficient in phosphorous and potassium next spring because they didn’t have time to move these elements out of their perishable leaves. The worst news is that trees that still had green leaves on them when the killing freeze occurred did not have time to move all of their nutrients out of their leaves and may have lost some primary leaf buds. These trees could have a sparser leaf canopy next year unless they can activate their secondary buds.

The good news is that your affected deciduous trees most likely did not freeze to death.

We won’t know the full impact on these trees until late spring, when their buds come out of dormancy. But for now, providing general tree care is the best option, which means winter watering on warmer days and ensuring that mulch is present around tree bases.

Jim Leser retired to Cedaredge after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.

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