Green roofs have special appeal on buildings in northern climates. They can insulate against extreme temperatures, conserving warmth in the winter and reflecting the sun's hot rays in the summer, in addition to limiting water runoff.
But most of what's known about designing modern green roofs comes from Germany, Toronto, and Chicago--places not as, let's say, rich in climatological variety as the Twin Cities.
With the wrong plants for this climate, well-intentioned and otherwise well-designed green roofs fail. So University of Minnesota horticulture professor John Erwin and graduate student Jonathan Hensley set out two and a half years ago to study which plants were best to plant on top of buildings.
They tested 88 plants on the roof of
Williamson Hall, a university building that is mostly underground. But it wasn't ease of access that led them to choose a test site with a roof at ground level, says Erwin. The plants actually have a tougher time of it there, where the air is warmer and moves less.
Their focus was on "extensive" green roofs--those where plants grow in shallow tray systems that are light enough to retrofit. It's a matter of supply and demand: "Most roofs are already built," Erwin says. ("Intensive" green roofs use deeper soil that can need the support of structures such as underground parking ramps.)
The findings: forty-four plants, half of those tested, will work in Minnesota. Hensley's thesis containing the list will be made public this fall. That will help building owners inspired by examples at
Target Center,
Minneapolis City Hall, and Mystic Lake Casino have more success with green roofs of their own.
Source: John Erwin, University of Minnesota
Writer: Chris Steller