Has your car been getting pelted with acorns? Have you found yourself running through a hailstorm of walnuts with your hands protecting your head? Chances are the trees in your region are experiencing a mast year.

Nut-bearing trees, like black walnuts, beeches and those acorn-producing oaks, have “on” and “off” years. “On” years, called mast years, see vigorous production of nuts across an entire species throughout a region.

During mast years, a single oak can drop thousands of acorns, forcing you to rake your lawn even before any leaves drop, or sweep your driveway to avoid twisting your ankle while navigating a blanket of marbles.

After expending all that energy on an abundant crop, the trees take a break, producing few if any nuts during “off” years.

The cycles for mature red oaks and black walnuts typically run two to five years; pecans tend to alternate between boom and bust, often taking just one year off. Most fruit and nut trees undergo these cycles, although each species has its own timeline. And even then, their schedules are not set in stone.

“There’s no general consensus (among scientists) on why we see these mast years,” according to Jonathan M. Lehrer, associate professor and chair of the Department of Urban Horticulture and Design at Farmingdale State College in New York. “There’s a lot of conjecture that it’s caused by variations in temperature and natural rainfall, but we’ve never been able to hammer out exactly why some years have greater production than others.”

Mast years help to ensure the propagation of a tree species because at least some of those acorns and nuts, which contain seeds, take root and grow into saplings.

The phenomenon also affects the wildlife population, as bumper crops during mast years provide abundant food for woodpeckers, deer, mice, wild turkeys, squirrels and other animals, resulting in more breeding. Less food during off years tends to keep those populations in check.

“The production of trees has far-reaching consequences on wildlife and other organisms, and fewer rodents (during off years) affects larger animals higher on the food chain, like foxes, owls and bobcats,” Lehrer said. “Although an abundance of acorns may be a nuisance, the ecological consequences of a mast year can extend for a year or two or three afterward.”

The reasons and stimuli for the cycles may be unknown, but one thing is clear: “As plants go, we go,“ Lehrer said.

So, if you’re finding yourself ducking for cover, take precautions: Avoid parking under masting trees to protect your car. Take care when walking so as not to slip or trip. And, most importantly, remember to look up!

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Jessica Damiano writes the award-winning Weekly Dirt Newsletter and regular gardening columns for The AP. Sign up here to get weekly gardening tips and advice delivered to your inbox.

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