Fall in Minnesota is fast approaching – the days are growing shorter and the air is turning cool and crisp. A more visual indicator of the changing seasons are the vibrant greens of summer gradually transforming into warm oranges, yellows, and reds. While beautiful, this annual natural occurrence is a captivating example of science in nature. But what exactly causes the leaves to change color in the fall?
The chemical responsible for the green pigment in leaves is called chlorophyll, which is also crucial to turn light into sugar during the photosynthesis process. During the spring and summer months, these trees are a rich green shade due to the large amount of chlorophyll within the chloroplasts in the cells of the leaves. As the temperatures drop and days get shorter, the trees get less sunlight, causing chlorophyll to break down in the leaves. Once this happens, hues of yellow, orange, and red are visible. These pigments are always underlying in the leaves and are revealed once the chlorophyll breaks down and the green fades away.
As temperatures fall and the leaves start changing color, hues of orange and yellow are most common to see among the trees. However, red and purple leaves can occur due to chemical changes, such as sugars getting trapped in the leaves, and these rarer pigments are called anthocyanins. Oaks and maples are more likely to produce red leaves, while aspens, birches, and ash trees are more likely to have yellow and orange leaves.
Not all trees have leaves that change color. Only deciduous trees go through this cycle, where their large, broad leaves turn red, orange, and yellow as the temperature lowers. Coniferous tree “leaves” are called needles, which stay green to continue the photosynthesis process through winter. While their leaves look different, both deciduous and coniferous trees’ leaves provide nutrients through photosynthesis.
Peak fall foliage is spectacular yet fleeting, with the stunning show of vibrant oranges, reds, and yellows seeming shorter with each year. The Minnesota DNR provides an excellent tool to help determine the “best” time of the season to see optimum fall colors throughout the state. Check out their Fall Color Finder Map, a tool that shows current leaf colors as well as predicts upcoming changes based on data from previous years. Based on their findings, the best times to see peak fall colors in Minnesota is the end of September through early October for the northern region, early-mid October for central Minnesota, and mid-late October for the southern region.
