Photo of Bluebell Woods in Sussex Photo of Bluebell Woods in Sussex

How Do Trees Know When It’s Spring

How Do Trees Know When It’s Spring?

Broadleaf trees have to work out the best time to come into leaf each year, they need to know when to call it spring. How do they do that?

They use two clocks together, a solar one and a temperature one.

The trees gauge the length of night and when that grows short enough then they know they are getting near spring. But, as the weather above proves, that on its own isn’t enough. (I filmed this on the 8th March, less than a fortnight before the spring equinox in the northern hemisphere.)

The trees also use a temperature clock, they actually count the number of warm hours.

If the nights are short enough AND there have been enough warm hours since winter, the trees call it SPRING! And that is when they burst into leaf.

Each species has its own way of keeping score, which is why they don’t all come into leaf at the same time. It’s also why trees of the same species come into leaf at different times depending on where they are and their microclimate – trees in frost pockets come into leaf later than trees in warm spots.

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How to Read a Tree – The Book