(KMID/KPEJ) – With the official start of Autumn coming on September 23rd, people across the country will start seeing the colors of leaves and shrubs change.

While scientists have been working to figure out how and why the colors turn from green to warmer colors like yellow, orange, and red, there is still a lot that is not known about this autumnal occurrence.

According to the U.S. Department of Agriculture Forest Service, there are three primary factors that determine the color of leaves:

  • Leaf pigments
  • Length of night
  • Weather

The Forest Service explains that as the night gets longer, the production of chlorophyll slows down. This eventually results in the destruction of all the chlorophyll. With the absence of chlorophyll, the carotenoids and anthocyanin are able to show their colors.

Chlorophyll is the pigment that gives leaves their green color and is important for photosynthesis. Meanwhile, anthocyanin give the red colors, like in cranberries, red apples, concord grapes, blueberries, cherries, strawberries and plums. Carotenoids give the yellow, orange, and brown colors that are often seen in things like corn, carrots, daffodils, rutabagas, buttercups, and bananas.

The shortening days and declining intensity of sunlight also cause the veins that carry fluids in and out of the leaves to close off and form the base of each leaf. This promotes the production of anthocyanin and seals off connecting tissues before the leaf is ready to fall off the tree.

During days of warm, sunny weather, and cool but not freezing weather, lots of sugars are produced in the leaf and veins going in to the leaf gradually close, preventing the sugars from moving out.

You can learn more about how leaves change colors and what causes them to turn one color versus another on the Forest Service’s website here.