Good question: How do trees know when to bloom? – WCCO

ST. PAUL, Minn. (WCCO) — In a matter of days, we have gone from a cold spring to one full of warmth and color.

That got us wondering: How do trees know when to bloom? And did it take longer than usual this year? Good question.

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Jeff Wagner explains why nature follows its own schedule and not ours.

From the banks of the Mississippi River to parks and patios, another sign that spring has arrived hangs from above like a colorful canopy.

“It’s much greener and everything is blooming,” said St. Thomas University student Anna Doolittle as she walked with a friend along a path near the river. “It’s crazy the difference.”

“When they get what they need, they will leaf and bloom,” said Val Cervenka, coordinator of the forest health program for the Minnesota Department of Natural Resources.

How do trees know when to bloom?

“It depends on the tree, but most trees need a cooling period. We know it as latency,” Cervenka said.

(credit: CBS)

That’s right, spring flowering begins with the autumn cold until winter. Dormancy is when a tree conserves its energy in its roots to survive the freezing season, making it appear lifeless above ground.

Then three pieces of the puzzle emerge in March and April, letting the plants know that breaking their buds is almost certain.

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The first step is more sun. Trees can feel spring coming when the days get longer, which means more sunlight each day.

Humidity is another factor that thankfully hasn’t been much of an issue in Minnesota this year. The melting of the snow is the first step, then the rains, which were abundant in March and April.

Finally, there has to be warmth.

“They are called ‘degree days’, but it is not temperature, it is a unit of heat. When that number has been reached, those leaves will come out,” Cervenka said.

Did the flowering occur late this year?

“It seems like this year especially has been like a longer wait to wait for the bloom to come,” Doolittle said.

April in the Twin Cities this year was six degrees cooler than average, including three nights below zero from April 25-27. That stretch that might have delayed the trees from showing their true colors.

“Daylight is not going to change from year to year. We will still have the same amount of daylight in the spring that we had last year and the year before,” Cervenka said.

But the amount of humidity and heat can vary from year to year, which could speed up or slow down when the buds decide to open.

“It depends on the kind of ratio of those things to the tree,” Cervenka said.

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Sudden warm spells in late winter and early spring can confuse some plants to bloom early. But with the freezing nights that follow, the leaves rarely survive.

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