Gardening with Allen: Short days lead to fall color

I have noticed the change in leaf color is starting. Do you know what causes leaves to change color?

I have lived in several different places and have noticed a difference in the way leaves change color. In the eastern U.S. it is a quicker process with most leaves changing color over a shorter time period. I like our change here in Western Washington and Oregon. It is a more gradual process with different trees changing over a longer time period.

It involves the same processes. Leaf color change in the fall is caused by several chemical processes that occur as the growing season ends.

Chlorophyll is the pigment that gives leaves their green color. It is necessary for photosynthesis, which is the process where plants use sunlight energy to manufacture sugars for food. Trees in the temperate zone store these sugars for the winter dormant period.

As the days get shorter and the food factories in the leaves shut down for the winter, the chlorophyll breaks down. As the chlorophyll breaks down, other pigments in the leaf such as carotenoids and anthocyanins become visible.

Carotenoid pigments are always present in the leaf, but are usually overwhelmed by the green chlorophyll in the summer. As the chlorophyll fades, the carotenoids show their yellow and orange colors.

Anthocyanins are responsible for vivid reds. They develop in the fall from sugars in the leaf. The best colors are produced when the weather is dry, sunny and cool. Our recent weather should be conducive to good color development.

At the end of the growing season, as many nutrients as possible are transferred from the leaves to the branches. As fall progresses, trees decrease production of the hormone auxin, which triggers the elongation of cells to form an abscission layer.

This is the protective seal that forms between the leaves and the branches, allowing the leaves to separate and drop to the ground.

Another way to add color to the landscape in the fall is to plant fall blooming flowers like chrysanthemums and asters. Pansies planted now will continue their color through the winter and into the early spring.

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