Why do buds open when spring gets warm?

Run your finger along a twig in early spring. You can feel small, hard bumps on it. Those bumps are buds, and all winter they stay shut tight.

Then the days grow warm. One morning, the buds are open, and green leaves peek out. Why do buds open when spring gets warm?

Each bud is like a tiny package. Folded inside are baby leaves, squeezed up small. A hard brown coat wraps around them to keep out the winter cold.

All winter long, the tree rests. Soft new leaves would freeze in the cold. So the tree keeps its buds closed and waits.

In spring, the sunshine warms the ground and the branches. That warmth wakes the tree from its long winter rest. Now it is ready to grow.

The roots wake up first. They start drinking water from the soft, wet ground. The buds will need that water to grow.

The water climbs up the trunk as sap, which means the watery juice inside a tree. The sap travels out along every branch, all the way to the buds. What do you think the sap does when it gets there?

The sap flows right into the baby leaves inside each bud. The leaves soak it up and begin to swell, which means growing fatter and rounder. Each leaf gets bigger and bigger.

The swelling leaves push hard against the brown coat. More sap keeps coming, and the coat is too small to hold them now. Crack! The coat splits open and slips away.

Out come the baby leaves, soft and crinkly. Slowly they unfold and spread out in the sunshine. The bud is open at last.

Sap and swell. Spring warmth wakes the tree, and sap makes the buds swell and pop open.

Sleep warm and cozy tonight, just like a bud waiting for the spring sun.