Why do leaves fall off trees in autumn? Most people say "because it gets cold". But watch a birch during a warm week in October. It turns yellow and drops its leaves anyway, at roughly the same time as last year. The tree is following something that is exactly the same every single year, which the weather never is. And there is one place in town where the tree can be fooled.
What happens when a tree drops its leaves?
Leaf fall is something the tree does on purpose. The leaf does not simply die and drop. The tree lets it go.
At the base of every leaf stalk, the tree builds a thin layer of cells called a separation layer. While it is being built, the tree pulls valuable nutrients out of the leaf and stores them in the branch for spring. Only then does the leaf come loose. The colours that appear along the way are explained in Why do leaves change colour?.
So when does the tree decide to start? Here is the surprise:
The tree does not wait for the cold. It counts the dark.
For many northern trees, shorter days are the most important signal. What the tree really measures is the night. Leaves contain substances that sense the difference between light and dark, and once the nights are long enough, the preparations begin, well before the first frost. In Oslo there are about 19 hours of daylight at midsummer. In early October there are about 11, and the day shrinks by almost five minutes every day. Why it happens so fast is explained in Why do evenings get dark so fast right now?
Cold, drought and storms can hurry things along. Light starts the clock.
Try it at home: the snap test and the streetlight hunt
Suitable for: 4–12 years
- ✅ A broadleaf tree you pass often, ideally a birch
- ✅ A magnifying glass, if you have one
- ✅ A phone for photos
- ✅ A streetlight with a broadleaf tree close by
- Gently hold a leaf stalk between two fingers and tilt it.
- Feel whether the leaf holds on or lets go with a tiny snap.
- Find the small, pale scar on the branch where the leaf was.
- Look for the bud just above the scar. Next year's leaf is already inside.
- After dark, find a broadleaf tree right next to a streetlight.
- Compare the lit side with the side facing away, and take a photo.
⚠️ An adult comes along in the dark, and you stay well away from the road.
Many people notice that leaves hang on longer on the side lit all night. Not every tree does it, so take a new photo from the same spot in a week. What happens if you follow two similar trees, one by a streetlight and one in a dark park, all the way to November?
More autumn experiments are gathered in Why is autumn the best laboratory a child can have?.
Questions to wonder about
- Which tree on the way to school will be bare first, and which will be last?
- Why can a spruce keep its needles all winter when the birch has to drop everything?
- Nights are as long in March as in September. How does a tree know whether it is autumn or spring?
Every child is made of good atoms. Explore free content at goodatoms.com.