- Three pigments – chlorophyll, carotenoids and anthocyanins
- Amount of daylight – causes chlorophyll breakdown, and corky wall formation
- Temperature & moisture – cause the production of sugars creating anthocyanin pigments and brilliance
But what does all of that mean? When most people are enjoying the full swing of summer in August, the triggers for leaves to start turning their fall colors have already been pulled. The longest hours of daylight are during the Summer Solstice in June. From that point, the days begin to grow shorter and shorter which activates the whole process for leaves to prepare for fall.
Leaves primary job is to produce food for the plant in its cells containing chlorophyll, which is also what turns the leaves green. Leaves harness the sun’s energy in a process called photosynthesis, that converts that chlorophyll to carbohydrates; e.g., sugars and starch, which are the food for the plant. What isn’t so obvious is that along with the green pigment of the chlorophyll, there are carotenoids which are the yellow and orange pigments also located in the chloroplasts of the leaf cells. These carotenoids are shy and hidden most of the year by the dominant green of the chlorophyll.
By the time the Fall Equinox rolls around in September, the days and nights are at equal lengths. The shorter hours of daylight has caused the leaves to stop producing food, and the chlorophyll breaks down finally revealing those underlying colors of yellow and orange. Those color changes remain fairly consistent from fall to fall. However, there are factors that create the other colors as well as their brilliance.
As the chlorophyll is dwindling in the leaves, the temperature and moisture also affects the brilliance of colors that they will have. Those warm, sunny days and cool – but not freezing nights, keep the sugar production in the leaves going, which produces the anthocyanins pigments to be produced. These are the stunning red, purple and crimson colors.
When the days grow shorter – it also triggers a chemical change in the plant that cause a corky wall to form between the twig and the leaf petiole. This corky layer eventually seals off the exchange of nutrients and water from the leaf, and will eventually seal completely, allowing the leaf the freedom to drop from the plant.
Finally, another fun fact to keep in mind is that an early frost will weaken the colors, but in these days of Global Warming – we may not have to worry about that! We’re also fortunate here in the Pacific Northwest, because rainy and/or overcast days ten to intensify fall colors – and we’re good at that kind of weather!