Try asking the next five people you see why the leaf color is vibrant one year and dull the next, and you will get varied responses. We had a lot of rain. September was dry. We had a frost. We had a freeze. October was hot. October was cold. “Leave me alone and pass the cider,” one guy said.
Well, inquiring minds want to set this straight. One of the reasons your writer joined the FCMG was to understand how things worked; to understand the old adages. “Don’t plant tomatoes till after Easter” said Grammer. I learned this was fine if Easter was in late April, and the soil temperature was 60 degrees. March 15th, not so much. So, Pa went surfing for a consensus on this annual event.
We all know that green leaves absorb sunlight and through photosynthesis food is produced in the form of sugars and carbohydrates. Some is used for growth and some is stored. As fall approaches, the days become shorter, and the amount of sunlight decreases. This signals the plant to slow down the production of chlorophyll. The presence of chlorophyll is what gives the leaf the green color. Leaves have certain colors present that are masked by the green. There are other pigments in certain trees’ leaves that are responsible for the tans, yellows, purples, and reds, violets, and oranges. Oak trees have a lot of another pigment called tannin that contains mostly browns and tans. The colors were already there, we just couldn’t see them. As autumn proceeds, a process also begins to produce cells that close off the stem from the tree, facilitating leaf drop. These processes happen every year.
Now, the vagaries of the weather take over to determine if the leaves are dull or bright and how long they stay on the trees. The most colorful, vibrant, spectacular, awesome, display of color occurs when we have the longest period of dry warm days and cool but not frosty/freezing nights. Like this year! I particularly noticed the wonderful red maples. It is mostly sunlight levels that control the density of the colors that are visible. Overcast days produce more yellows and browns. Bright days enable the production of more sugars, but the crisp nights keep the sugars in the leaves longer. There is a plethora of combinations that come into play.
The cumulative amount of moisture available to the tree in the growing season can also affect color. A late or dry spring and a summer drought can result in muted hues. A stormy autumn helps to whip the leaves from the trees prematurely. An early freeze kills the leaves and to the forest floor they go. The consensus scenario Pa found is a wet spring, moderate summer, and cool nights and warm autumn days is best (and generates the most descriptive adjectives). No matter what play nature presents to us each fall, we can be sure that the fallen leaves will help replenish nutrients and hold water so that our trees will flourish and repeat the cycle next year. And don’t forget that a nice pile of leaves is a great place for your grandkids to play and a wonderful material for your compost pile. You do have one, don’t you gardeners?