Planting season for basketball arena roof?

The Target Center roof may soon be growing green.

The Minneapolis City Council Friday authorized taking bids for three differing configurations of vegetation atop the city-owned arena.

At 115,000 square feet, the arena would have the second-largest green roof in the state, according to the Minnesota Green Roofs Council.

The city's development agency recommended last December against a green roof, but Council Members Lisa Goodman and Scott Benson pushed hard for further analysis, citing a positive feasibility report by a consultant, Kandiyohi Development Partners.

Kandiyohi said the additional $400,000 cost of a green roof could be recovered in five years. But further study estimated the recommended options could increase the $1.2 million cost of a conventional roof by $300,000 to $1.6 million.

Whether lower costs for cooling and stormwater management would recoup the additional expenditure depends on how long the building stands. Consultants said a green roof has an expected life of 40 years, twice that of a conventional roof. So if the building lasts beyond the point where a new conventional roof would be needed the project could pay for itself.

But the report noted that once the building is paid off and the Minnesota Timberwolves commitment to play there expires in 2025, the building will be 33 years old, about the average age at which pro basketball arenas face demolition. "It's hard to predict with sports facilities," Benson said. "I think the public would be somewhat outraged if that building didn't last 20 years."

Green roofs can cut cooling costs by reducing heat near rooftop air intakes, but the Target Center's side intakes mean less savings, said Kristin Guild, a city business manager.

Plants such as low-growing sedum on the roof can also absorb water and cut runoff.

Bids are due back at the council in October.

DeLaSalle playing field

In other vegetative news, the question of grass versus artificial turf at DeLaSalle High School's controversial planned athletic field was settled when the school withdrew its bid for a synthetic field.

The Park Board said it is difficult to keep grass fields fit during adverse weather and heavy play.

But neighbors who have opposed the expansion onto park land, said polyethelene fibers with a tire crumb filling have no place in a historic setting.

Late Thursday, City Council President Barbara Johnson told the school and a park commissioner that the fake turf plan lacked council support.

One island resident applauded the withdrawal. "It's exceptional, like Leap Day," said Chris Steller, saying it's the first time the council has upheld the city's Heritage Preservation Commission on project issues.

The school said it was giving up on the turf request to avoid more delays.