Fall Foliage

As a horticulturist I get asked many questions about plants. I’m normally happy to hear them, and from time to time I can even answer one. This is the time of year when inquiring minds begin to ask: why do leaves change color in the fall? Well here is my cocktail party answer for that particular question.

An Acer rubrum looking red

Leaves are cool. They are able to capture the energy from light and turn it into chemical energy. Take some light energy, throw in some water and carbon dioxide, and boom . . . you have carbs (leaves are not very Atkins friendly). Why do I mention photosynthesis? Well chlorophyll is the light receptor in leaves, and here’s the important part, it happens to be a green pigment. So this is why leaves are normally green during the spring and summer.

As the fall progresses there is less daylight and temperatures drop. These are the cues to the plant that it’s time to shut down photosynthesis for the winter. The chlorophyll in the leaves breaks down, resulting in the loss of greenosity (ok, so I made that word up, the important thing is that the leaves are no longer green). At this point other pigments in the leaf get their moment to shine.

A Ginkgo biloba looking yellow

The big group of pigments that cause fall foliage color are called flavonoids, though other pigments may also be involved. Many of these pigments are the same ones that give flowers their colors. In the leaf these pigments may be present all spring and summer but only get revealed when the chlorophyll breaks down, or they may be present as colorless flavonols that develop into a color once the chlorophyll breaks down. Either way the result is fall foliage color.

A Stewartia pseudocamellia looking good

The levels and mix of pigments varies by plant (on both an individual and species level), so that is what causes some plants to look nicer than others. The growing conditions of the spring, summer, and fall may also impact how showy plants are. A warm moist spring is good. Summer drought is bad. A cool and dry fall is good.

By this time the poor people standing near me at the cocktail party wish the question had not been raised. I realize I could have just said “Leaves are colored by pigments that are visible in the fall.” Sometimes, however, parsimony leaves out important details.

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