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Why are some maple trees not dropping seeds?

Why are some maple trees not dropping seeds?
Why are some maple trees not dropping seeds? 02:36

MINNEAPOLIS — Trees, yards and gardens are lush with all the rain recently. But some people wonder if the weather has also helped stop a pesky problem this time of year.

Why are some maple trees not dropping seeds? Good Question.

Towering high above Nancy Thue's house is a massive maple tree that creates an annual annoyance for the retired teacher.

"Every school year, the last week of school, I'd be raking up the helicopters," she said. 

Helicopters is the fun name for maple seeds, also known as samaras.

"I would have it all cleared off the walkway to my car and I'd come back, I'd do it again," she said, saying it was a yearly project for several decades.

But this season, her deck and lawn have gone seedless, something she's never experienced. 

"I thought oh, maybe I'm not seeing them yet, maybe they're still green up there, and maybe they're later. And as it turns out, they never showed," she said.

Gary Johnson, an urban community forestry professor, wasn't surprised to hear her quandary.

"It can be genetic, or it can be weather, or it can just be that normal fluctuation of up and down," said Johnson. 

Sometimes it depends on the type of maple tree. WCCO talked with Johnson while standing under a Norway maple tree at Minnehaha Park. It produced plenty of seeds with many still scattered on the grass below.

Pollination is also a factor, which happens via the wind for maple trees.

"If the weather is wicked windy, or really rainy, or the combination, or real cold, the pollen just may never get to the female part of the flower. So, if the pollen can't get there, then it can't be fertilized and if it can't be fertilized, it can't get a samara," he said.

In recent years, some maple trees dropped seeds in overwhelming numbers potentially due to stress from the drought.

"There's a tendency to just flower like crazy and produce seeds like crazy. Plants just want to live, so it's just to keep that next generation going," he said. "However, that would not explain the lack of samaras this year cause we've had three years in a row of drought. So, if that was 100% factual, good grief we would just be in a snowstorm of samaras this year."

Another explanation is simply that some maple trees aren't dropping seeds because it's a down year.

"The same thing happens with oaks, too. You'll have really heavy mast years or acorn years. And then it'll drop down and then it builds back up and it is kind of unpredictable," he said.

Using red maple trees as an example, Johnson said some years they will have 20% female flowers, the part that pollinates and creates seeds. In other years, the same tree might produce 60% female flowers. Exactly why those numbers fluctuate is hard to predict or explain.

"As a forester… we learn to avoid saying never and always. The second thing I could tell you is that there is much more about trees that we don't know that what we do know," he said.

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