Why Leaves Really Fall Off Trees

Why do leaves really fall off trees? It's not the wind. It's not the cold. It's because trees use 'scissors' to cut their leaves off. Around this time of year in the Northern Hemisphere, as the days grow shorter and colder, those changes trigger a hormone in leaf-dropping trees that sends a chemical message to every leaf that says, in essence, 'Time to go! Let's part company!'

According to Peter Raven, president of the Missouri Botanical Garden and a renowned botanist, the wind doesn't gently pull leaves off trees. Trees are more proactive than that. They throw their leaves off. Instead of calling this season "The Fall," if trees could talk they'd call this the "Get Off Me" season.

Once the message is received, little cells appear at the place where the leaf stem meets the branch. They are called 'abscission' cells. They have the same root as the word scissors, meaning they are designed, like scissors, to make a cut.

And within a few days or weeks, every leaf on these deciduous trees develops a thin bumpy line of cells that push the leaf, bit by bit, away from the stem. You can't see this without a microscope, but if you looked through one, you'd see those scissors cells lined right up.

Enlarge University of Wisconsin Plant Image Teaching Collection

The scissor cells are stained red and mark the boundary between the branch (left) and the leaf stalk.

That's where the tree gives each leaf a push, leaving it increasingly dangling. "So with that very slender connection, they're sort of ready to be kicked off," says Raven, and then a breeze comes along and finishes the job.

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