With green roofs, how tall is too tall to help bees?

Green roofs are good for bees – and butterflies, birds and more. In addition to providing habitat, they save energy by reducing the need for heating and cooling. There’s still room for solar panels. And get this… they last longer – twice as long as conventional roofs.

Creating green roofs in urban cores is especially valuable for pollinators because these areas tend to be far richer in concrete than vegetation.

But there’s a question that keeps popping up. Urban cores have tall buildings. How tall is too tall for bees? 

Could they buzz up to the top of the 110-floor Sears Tower in Chicago if there were flowering plants and vegetation on top? How about The John Hancock, a 60-story building in Boston? And if not, how low must you go? 

Research points to a fading benefit starting at 5 stories and little benefit for bees after 8 floors. 

But can a bee fly higher? Interestingly, in a lab experiment, which removed the cold that comes with high altitudes, some bumble bees flew as high as a lab-simulated 30,000 feet, the rough equivalent of 2,100 stories. But in real life, that’s not happening. 

And yet if some bees can fly higher, why do the benefits begin to fade at higher altitudes?

One answer is wind. It’s windier on tall buildings, and it blows them off the roof. They won’t choose to fly back up. 

Two, the top of a building typically won’t contain all the flowering plants and vegetation that a bee needs, sending them looking for foraging opportunities elsewhere. If they fly over the edge, they’re unlikely to fly back up. 

But even if you lose the benefits for bees, there are still those other good reasons — energy savings and longevity of the roof — for building owners to install a green roof. 

Cities and others should do more — in terms of policy and funding — to encourage green roofs. See more on the topic here.