Why leaves change color in the fall

By Kathy Conner Cornell As I look over our farm I am starting to see some color changes in the tree leaves. It reminds me of the first time my husband and I saw the farm. We had told a few of my cousins that we were looking for hunting property. I got a call from one of them that there was a farm available. So the third weekend of October 2002 we headed down here with my parents, Stella and Grover. Dad grew up on an adjoining farm. The weather was nice fall crisp, the clouds were big powder puffs and the leaves were in full brilliant display. We fell in love and couldn’t say no so signed a contact on the spot. As a child I was told that Jack Frost painted the leaves in the fall. Obviously it is more complicated than that. To make it simple, trees get the signal with cooler nights and shorter days that it is time to shed leaves. Chlorophyll production ceases and other pigments in the leaves have the chance to shine though. If that is all you want to know, fine but if you’d like more details read on.
The color of these sweet gum leaves show the significance of the botanical name Liquidamber styraciflua. Fall leaf color happens in temperate regions only. Trees in areas with year round warmth have evolved with different reactions to the weather, usually adapting to less stressful biological actions during the dry season. In temperate areas deciduous trees have survived by evolving to cease photosynthesis in the winter. If trees held onto their leaves and continued to photosynthesize they would wear themselves out. Trees need their winter dormancy. The process starts as day length gets shorter and nights get cooler. This signals the tree to cut off the moisture and nutrients to the leaves making photosynthesis come to a halt. Since photosynthesis is the process of taking carbon dioxide plus water and sunshine with the chlorophyll in the leaves to produce sugars, the food for the plant, oxygen and water. Therefore, with no flow of water the leaves can no longer perform this process. Chlorophyll production ceases and other pigments in the leaves such yellow carotene and light yellow xanthophyllis can show through. Two water soluble pigments anthocyanin and betacyanin may also accumulate in the leaf cells. Athocyanin is red if the cell sap tends to be more acid and blue if it is more alkaline and shades in between the two if the sap is neutral. Betacyanin is usually red but only found in a limited number of plant families such as Cactaceae and Portulacaceae. Bottom line, it is the interaction of these pigments and other lesser pigments in the leaf as the chlorophyll fades away that gives us the brilliant fall display. Some falls offer more vivid color than others. The last piece of this puzzle is soil moisture. According to Will Stafford, Meteorologist on WSET, we should have good fall color this season since we have had ample moisture. Hopefully we won’t have any strong winds causing excessive leaf drop before the color display peaks. While we are all still practicing ‘social distancing’ due to COVID-19, and all Halifax County buildings remain closed to the public, if you have gardening questions, you can best reach an Extension Master Gardener or Extension staff member by sending an email to wmccaleb@vt.edu. If you can’t email, you can call and leave a message at the Extension Master Gardener Help Desk at 434-830-3383, giving us your name, telephone number and nature of the call. The Help Desk phone is checked timely and someone will get back to you, although it may be from a different telephone number. Keep washing your hands, wear your mask and take a walk to enjoy the fall color.