科学美国人60秒:Bees Have a Goldilocks Lawn Mow Schedule

Add up every golf course, athletic field, industrial park, and yard in the U.S. and you have an area nearly the size of Florida. Upon first glance, all that lawn might seem a biological wasteland—a monoculture of grass. 

But while natural areas in the U.S. continue to decrease thanks to urbanization, urban green spaces—including lawns—could become more important reservoirs of biodiversity. 

"What happens if we mow our lawns less? Do we get more lawn flowers? And if we get more lawn flowers, can we get more bees?"

U.S. Forest Service ecologist Susannah B. Lerman.

She and her colleagues devised an experiment to see if front lawns could in theory provide decent habitat for bees – and if so, how to do it. So they recruited 16 homeowners from a Massachusetts suburb and monitored for flowers and bees throughout the summer for two years. Each of the homeowners agreed not to use any kind of pesticide or herbicide. And none had cultivated any sort of pollinator or vegetable garden that could skew the results. Some of the lawns were mowed weekly, some every other week, and others were mowed every three weeks.

"When we mowed the lawns less, we got more flowers, roughly two and a half times more. But it was those that we mowed every two weeks that had the most bees."

No surprise, flowers were most abundant on the lawns mowed least often. But why do bees like a bit more frequent mowing? Lerman thinks that's because most of the bees she found were tiny native sweat bees, roughly the size of a grain of rice. Critters that small could find it difficult to navigate through the taller grasses.

In all the researchers found 111 types of bees over the course of the study. That represents a quarter of all bee species known to occur in Massachusetts. 

"When you get down on your hands and knees, there's a lot going on in these lawns."

So not only do lawns hold more biodiversity than it might seem, but it turns out that one way to manage them for wildlife is to be lazy—but not too lazy.

"A lot of people have been telling me that they feel vindicated now. That they have a realize to tell their neighbors why they're not mowing, it's for the bees…By mowing your lawns every two weeks or so, you're letting these flowers grow and bloom and they seem to be having a positive impact for bees."

—Jason G. Goldman