Fall foliage is one of those annual events many of look forward to. While the changing leaves certainly represent the end of the growing season, they also bring some wonderful colors against a backdrop of blue sky and crisp air.
Although a particular tree doesn’t change much year to year, the colors the leaves show can be dramatically different from one fall season to the next. Much of the reason the colors are so different each year has to do with the weather during the growing season.
It seems like the brightest colors occur when the summer is not too hot, humid and doesn’t have an excess of rain. This year, our summer just gone by seems to be ideal for bringing out the best colors in the leaves.
Warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. The amount of moisture in the soil also affects autumn colors. A cold spring or a mid-summer drought can delay the color in fall and cause muted colors. Significantly above average autumn temperatures will also lower the intensity of autumn colors. A mild spring with adequate moisture, typical summer weather, and warm sunny fall days with cool nights should produce the most brilliant autumn colors. The good news, that was this year!
The green color the foliage exhibits during the spring in summer is due to the chlorophyll manufactured by the leaves. The green chlorophyll overpowers the other colors in the leaves which allow the yellow and orange colors to shine through. Small amounts of these colors have been in the leaves all along, but are masked by the green chlorophyll.
What you might not realize is the bright reds and purples we see in leaves are made during the autumn. Here’s how it works. In certain trees, like maples, glucose is trapped in the leaves after photosynthesis ceases. Sunlight along with the cooler and longer nights signals the leaves turn this glucose into a red color. The name of the pigment, which tints leaves reds, purples, and crimson is called anthocyanin. These can vary a lot year to year. Carotenoids, responsible for the yellow and gold are always present in leaves and remain fairly constant from year to year.
Oaks show more brown color which is basically the waste left in the leaves after the no longer gather light and carbon dioxide.
The reason the leaves eventually fall off is that as the veins carrying fluids into and out of the leaf stop working efficiently a layer of cells forms at the base of each leaf like a plug in a pipe. These clogged veins trap sugars in the leaf and promote production of anthocyanins. Eventually the separation layer is complete and the connecting tissues are sealed off it is then the leaf will fall.
During this time if the weather doesn’t bring us a big windy storm the leaves can hang on for a while. A cold snap with freezing temperatures will also accelerate leaf drop. You can follow my forecasts on Twitter @growingwisdom
The reason leaves fall in the first place is because they are not built to withstand freezing temperatures and would turn to mush in our cold winter. The plant needs to seal up the spot where the leaf formed to protect the tree or shrub. Needled plants and other broad-leaf evergreens like hollies and rhododendrons are built differently and their leaves and needles can withstand the cold. These plants do lose some moisture all winter which is why it’s critical to keep the soil evenly moist until the ground freezes.
Of course all the biomass falling the ground is then recycled by worms, bacteria and other critters on the forest floor. Leaf mulch is one of the best types of nutrients you can add to your garden. Just be sure it’s broken down.
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