Some leaves change color weeks before the first icy nights. What's really going on is a bit more complex.
Tree leaves contain cells that create food for the whole tree. Those cells use chlorophyll, which gives leaves their green color. In a process called photosynthesis, the chlorophyll absorbs energy from the sun to turn water and carbon dioxide into sugars and starches — food for the tree.
Green fades to reveal hidden colors
Even though green is what you see during the growing season, there also are hidden colors in the leaves. Those are carotenoids. They're the same pigments that make carrots orange and corn and daffodils yellow.
In the fall, shorter days and cooler nights mean there's less energy for food-making. The chlorophyll starts to break down. The green disappears and the yellow carotenoids can finally be seen in such trees as hickory, ash, birch, maple, sycamore, cottonwood, and sassafras.
Contrasting temperatures produce red and purple
The red and purple colors, though, aren't hiding in the leaves. They're newly created in the fall when sugars are made during warm days, then trapped in the leaves during cool nights. The trapped sugars change chemically into anthocyanins, which appear red and purple.
The more sunshine during the day, the more red color is created. That's why shaded leaves will be less red than those that get lots of sun. If the weather is cloudy and the nights stay warm, there won't be as much vivid red in such trees as maple, sweetgum, oak, and dogwood.
Weather extremes can ruin the fall-color season
Other factors besides contrasting temperatures also affect fall color. Trees that don't get enough water during the growing season may just drop their leaves quickly before they color. And if it gets very cold early on, that kills the leaves, too, before they have time for a fall display.
In good seasons, Missouri's fall color may slowly change from mid-September to peak in mid-October. By late October, the colors fade, and the incredible show is gone.