What causes leaves to drop off trees in the fall?
MINNEAPOLIS — The latest map shows fall colors are just starting to emerge across parts of northern and southern Minnesota. That means it's only a matter of time before the leaves drop.
You may think they shrivel up and fall in the wind, but that's not the whole story.
"I love that we get a rainbow in the fall. It's my favorite season," one person said.
The rainbow of colors show up once trees stop producing chlorophyll.
"They start to fall typically when the photoperiod, meaning the length of the day, begins shortening," said Kira Pollack, a University of Minnesota Extension Educator.
How they fall might surprise you. Once each leaf receives the chemical message it's time to prepare for winter, little cells form a bumpy line where the stem meets the branch. They're called abscission cells.
"It's creating cells that push the leaf off. So it's not quite as delicate, but it is on purpose," Pollack said.
So when the leaves fall off, they're getting kicked out by the tree on purpose.
"Trees lose their leaves to conserve energy and to reduce the amount of damage they could sustain during the winter," Pollack said.
If trees kept leaves during the winter, the water inside them would freeze and expand, rupturing cells and causing damage.
"Trees in the fall, our deciduous trees, will essentially turn off the lights. They go dormant and they don't need as much energy so they are able to release those leaves," Pollack said.
Having leaves on the ground can help insulate the tree's roots and bring nutrients back into the system, continuing the cycle of winter survival.
"This is generations and generations of adaption," Pollack said.
Leaves can drop early for many reasons including an early cold snap, drought or disease.
Colors typically peak in the Twin Cities between late September and mid-October.
