As temperatures climb and days grow longer, budding trees prepare to unveil their summer regalia. It seemed we were almost there a couple weeks ago, but a cold snap put things on pause.

The phenomena we call green-up, which is when the leaves come out in the spring, is rather complicated. It starts months earlier, back in the fall, and uses signals from the environment in order to execute the perfect leaf-out.

Fall Prep and Winter Survival

In the fall, as temperatures drop and nights get longer (the secret is in the night, not day), trees go dormant. Trees in northern locations with harsh winters do it to survive the frigid conditions and lack of water.

Trees have to be dormant for a certain amount of time each winter before they start preparing for spring. This way, they avoid budding during a mid-winter heat wave and then dying as temperatures drop again.

Once trees reach the minimum number of dormant days they need, they get ready to wake up their dormant buds.

At this point, many factors cause trees to sprout, but one proves most important in ensuring their survival – temperature.

Spring Wake up

All of the factors that play into trees going dormant in the fall also help them come to life again in the spring, but it’s mostly about temperature. Warmth that sticks around is what signals to trees that it is time to wake up and leaf-out.

Instead of the length of the night, it’s the hours of daylight that make the difference. Increased daylight and increased water amounts signal to trees that spring is around the corner. But before it fully arrives, it waits for warmth, which is the most important signal of all.

Chemical processes developed through evolution help trees to know when warmth is around to stay and early buds won’t be lost to bitter cold. When the trees sense that warmth, green-up begins.

Timing

Temperature may be the most important factor in getting those green leaves to sprout come springtime, but a series of requirements must be met in order for the whole process to begin.

A mild fall might delay the spring’s start simply because there hasn’t been enough elapsed time since bitter cold set in for the trees to think it’s safe to bud.

Likewise, a cold spring might signal to trees that buds may be in jeopardy if they start sprouting too early.

All of these different factors play into when we see green-up, or leaves in the spring. And just one little change in one of these factors can make a difference of a couple weeks when it comes to the return of leaves.

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