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Good Question: How Do Trees Know When To Bloom?

ST. PAUL, Minn. (WCCO) -- In a matter of days, we've gone from a cold spring to one that's bursting with warmth and color.

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

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

From the edge of the Mississippi River to parks and yards, another sign that spring has sprung hangs from above like a colorful canopy.

"It's so much more green and everything's blooming," said Anna Doolittle, a student at St. Thomas University as she walked with a friend along a trail near the river. "It's crazy the difference."

"When they get what they need, they'll leaf out and they'll bloom," said Val Cervenka, forest health program coordinator at the Minnesota Department of Natural Resources.

How do trees know when to bloom?

"It depends on the tree, but most trees need a period of chilling. We know that as dormancy," said Cervenka.

Blooming Trees, Sunshine, Spring Generic
(credit: CBS)

That's right -- the spring bloom starts with the fall chill into winter. Dormancy is when a tree conserves its energy into its roots to survive the frigid season, making it look lifeless above ground.

Then three pieces to the puzzle emerge in March and April, allowing plants to know that breaking their buds is almost safe to do.

The first step is more sunshine. Trees can sense spring is arriving by the days getting long, meaning more sunlight each day.

Moisture is another factor that fortunately hasn't been much of a problem in Minnesota this year. Melting snow is the first step, then rainfall, both of which were plentiful in March and April.

Lastly, there needs to be warmth.

"They're called 'degree days,' but it's not temperature, it's a heat unit. When that number has been reached, those leaves will push out," said Cervenka.

Did the bloom happen late this year?

"It feels like especially this year it's been like a longer wait to wait for the bloom to come," said Doolittle.

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

"The daylight isn't gonna change from year to year. We're still gonna get the same amount of daylight in the spring as we got last year and the year before that," said Cervenka.

But the amount of moisture and warmth can vary from year to year, which could speed up or slow down when the buds decide to pop.

"It depends on kind of the ratio of those things for the tree," said Cervenka.

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

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