Why Leaves Fall — And Why They Don’t Fall At The Same Time

Collecting leaves with a Cyclone Rake is a great time to enjoy the beauty of your landscape and its trees. As you pay attention to your trees year after year, you’ve probably noticed that all leaves do not fall equally. Some trees shed their leaves sooner, and that timing can vary from year to year. The reasons for different leaf falls are anchored in tree genetics – especially in the leaves.

Leaves, as you probably know, contain chlorophyll – the main pigment that converts the sun’s light into sugars. Chlorophyll pigments are green. During autumn, leaf cells begin to seal off the flow of water to the leaf. That leaf moisture loss starts to reduce the green chlorophyll pigments. This process helps guard the leaf from frost damage.

As the Chlorophyll decreases, the leaves start to show their true colors. Leaves also contain over 80 differentcarotenoid and xanthophyll pigments, according to a Clemson University Extension publication. Carotenoids, like beta-carotene, appear as orange hues, like the sugar maple. Xanthophylls, like lutein, are more yellow, like the leaf changes of aspen and yellow poplar. Other leaf pigments include tannins, which appear as golden and brown; and anthocyanins, which appear purple or red, like red maple and sweetgum.

During fall’s shorter days and cooler nights, a layer of cells at the base of the leaf’s stem, called the abscission layer, starts to seal off the flow of water to the leaf. That “abscission layer” lets the leaf make a clean break from its branch – keeping tree sap from leaking out and tree diseases from getting into the tree. That’s why leaves fall at the stem.

A tree’s species and genetics largely dictate when the water flow starts to seal off, but weather and environment can also change that timing from year to year. Moisture, temperature and sunlight are most important. A late summer drought can delay the process. Drought in the early fall – right when the tree is starting to decrease chlorophyll production – can cause early leaf drop, according to Colorado State University. Early freezes can also kill leaf tissues, leading to an early drop.

By spending consistent time in your landscape, as you do when collecting leaves with a Cyclone Rake, you’ll start to notice the leaf fall pattern and timing of different trees – and notice how the weather from year to year can affect that timing. As you notice that, you might also take time to marvel at how each tree species has particular genetic designs and varying pigment levels that provide the ideal timing for its leaves to fall.
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