All spring and summer, leaves work like little food factories for the tree.
Leaves have a green color in them called chlorophyll. It catches sunlight. It helps the leaf turn water and air into food for the tree. The food is sugars and starch.
Leaves also have yellow and orange colors in them all year. They are called carotenoids.
They are the same kind of color that makes a carrot orange.
The green color is stronger, so it hides them.
In the fall, the days get shorter and the air gets cooler.
The leaves stop making food. The green chlorophyll breaks down and fades away.
Now the yellow and orange colors can show. That is part of the fall color show.
Some trees make a new red color in the fall. It is called anthocyanin.
Warm, sunny days help leaves make lots of sugar. Cold nights trap the sugar inside the leaf.
The red color is made from that trapped sugar.
Frost does NOT make leaves change color.
Some leaves start to turn before there is any frost at all.
When it is time, a special layer of cells grows where the leaf joins the twig. It slowly cuts the leaf loose.
The wind blows it off, or it falls by itself.
The tree heals the spot. The mark it leaves is called a leaf scar.
Fallen leaves are not wasted.
As they rot, they put good things back into the soil, like calcium and potassium.
Pine trees keep their green needles all year.
One needle can stay on a pine tree for 2 to 4 years or more.
All spring and summer, leaves make food for the tree.
Food-making happens in leaf cells that contain the pigment chlorophyll. Chlorophyll absorbs energy from sunlight and uses it to transform carbon dioxide and water into carbohydrates — sugars and starch.
A pigment is a substance that gives something its color.
Yellow and orange carotenoids are in the leaf all season. They are masked by the larger amount of green.
In fall, changes in daylight length and temperature stop the food-making. Chlorophyll breaks down, the green disappears, and the yellowish colors become visible.
Other chemical changes form more pigments, from yellow to red to blue.
The colors come from mixing different amounts of chlorophyll and other pigments.
Brilliant red forms best with warm sunny days followed by cool nights with temperatures below 45°F.
Lots of sugar is made in the daytime, and the cool nights stop the sugar from moving out of the leaf.
Anthocyanin is formed from the trapped sugars.
Trees with red or scarlet fall leaves include red maple, silver maple, flowering dogwood, sweetgum, black tupelo (blackgum), northern red oak, scarlet oak, and sassafras.
Leaves on the sunny side of the same tree may turn red while leaves on the shady side stay yellow.
A warm, cloudy, rainy fall makes less red.
Less sugar is made in the weak sunlight, and warm nights let the sugar move out, so no extra sugar stays to form the pigments.
Frost is not the cause — some leaves begin to turn before any frosts.
At the base of the leafstalk, a special layer of cells develops and gradually severs (cuts) the tissues holding the leaf.
The tree heals the break. After the leaf falls, the spot is marked by a leaf scar.
Most broad-leaved trees in the North drop their leaves in fall, but the dead brown leaves of oaks and a few other trees may stay on until spring growth starts.
Conifers — pines, spruces, firs, hemlock, cedars — are evergreen. Their needle-like or scale-like leaves stay green all year, and a single needle may stay on the tree for 2 to 4 years or more.
Fallen leaves enrich the top layers of soil.
They return elements such as calcium and potassium and add humus, which helps the soil hold water.
Trees that drop their leaves in fall are called deciduous trees.
The eastern United States and southeastern Canada have large deciduous forests and some of the world's showiest fall color.
USDA Forest Service, "Why Leaves Change Color" (fs.usda.gov).
Checked November 2026.