A Denver initiative on the November ballot seeks to sprout more trees, shrubs and even vegetable gardens atop larger buildings across the city, a move that would accelerate a trend that has been embraced by some developers.

But by proposing a mandate on so-called “green roofs” that in some ways would stand as the nation’s most stringent — leapfrogging San Francisco — the local environmental activists behind the Denver Green Roof Initiative could be taking a gamble on Denver voters, observers say.

The tension between the clear environmental benefits of rooftop gardens and an aversion by some voters to wide-ranging government requirements was clear in a recent comment by Denver City Councilwoman Mary Beth Susman.

“You know, I think it would be great if we all had green roofs,” she said last week during a council review of the initiative with its backers and opponents. “They’re so lovely. But the mandate is what worries me. … If you have so much support for it, then why wouldn’t the market just take care of it?”

Initiative 300’s backers say they’re optimistic that voters will see the value in making rooftop gardens a standard feature of most new structures of at least 25,000 square feet in Denver.

“People love the idea. We have all these flat roofs with all this space, and we’re not doing anything with them,” said Denver resident Brandon Rietheimer, the initiative’s campaign manager. “Why aren’t we putting solar or green vegetation up there? … We hear all the time that Denver is an environmentally friendly city, yet we rank 11th for air quality and third for heat islands.”

Their goal, in part, is to address Denver’s status in a 2014 study by Climate Central, a scientific advocacy group, of having the third-greatest urban “heat island” effect produced by all the radiating rooftops and pavements. Only Las Vegas and Albuquerque ranked higher.

Denver 2017 election guide $937 million bond package and the Green Roof Initiative are on the ballot

Builders could also incorporate solar panels to offset some of the initiative’s rooftop garden coverage requirements, which would start at 20 percent of the roof area and ratchet up to 60 percent, depending on the structure’s total square footage and type. Residential buildings of four stories or less would be exempt.

City officials would have discretion to grant exceptions — although a building’s owner would have to pay an opt-out fee based on what would be spent to build a green roof.

Besides the mandate, a key sticking point that gives some opponents pause is that some existing buildings also would face a requirement to install green roof components.

That rule would be triggered when the roofs of buildings that meet the size thresholds are replaced or when building additions cause their total floor area to reach the threshold.

Denver’s initiative is modeled after one in Toronto, which became the first North American city to require rooftop gardens about seven years ago. Until this year, U.S. cities haven’t set outright mandates, although many — from Portland, Ore., to Chicago to Washington, D.C. — have varying levels of incentives for rooftops gardens or solar-energy systems.

Denver advocates stake out a different path

Denver lacks such incentives, but the Green Roof Initiative would follow on the heels of San Francisco and even jump ahead of it, green-roof advocates say. That city’s similar mandate — a City Council-passed ordinance that combined green roofs with an existing solar requirement — took effect in January. It applies only to new buildings, not existing ones that are undergoing roof replacements.

“In a city that loves opposition, there was no opposition,” said Jeff Joslin, the director of current planning in San Francisco’s planning department.

But while he’s supportive of Denver’s initiative, he noted some differences in how San Francisco’s ordinance came about.

“We’ve had better than three years of process,” Joslin said. “We initiated a stakeholder group that was broadly representative to really look at the issue from all sides, so that was part of that answer. But the bigger piece was simply the economics of green roofs.”

Until Nov. 7, Rietheimer and the other backers won’t know the temperature of likely voters who received their ballots in the mail this week.

It’s hard to predict, said Eric Sondermann, a Denver political analyst. But he said opponents may face a challenge in such a low-interest, off-year election.

“I think the risk to the opposition is that it’s under the radar and it just looks good, looks cutting-edge, feels good and that no one digs into it,” he said. If voters hear few arguments for or against it, he added, it could be “purely a gut-level emotional or aspirational kind of vote.”

Big opposition forms against measure

The initiative’s backers have embraced their David vs. Goliath position as a tool to motivate supporters.

Denver Mayor Michael Hancock last week came out against the initiative, saying its mandate was “not the right approach for Denver” and could hurt attempts to try out different types of green-roof infrastructure. The Denver Metro Chamber of Commerce and the Downtown Denver Partnership are among several business groups to oppose the measure, too.

Last month, members of the Colorado Real Estate Alliance formed an opposition campaign that has quickly built up a 6-to-1 fundraising advantage over the initiative campaign. The group signals its main argument with its name: Citizens for a Responsible Denver.

“This solution is ill-conceived and fraught with unintended consequences,” said Kathie Barstnar, a commercial development advocate who helped start the group. “I think if it were up to building owners to decide yes, they want to do this, or no, they don’t, that would be totally different.

“We actually encourage building owners to consider whether to do this.”

Hancock, in his statement to the Green Roof Initiative’s campaign, said he would have preferred a collaborative approach to address the issue instead of an initiative.

Rietheimer said that’s exactly what he and his compatriots attempted to do. But he said their attempts were rebuffed last year by the planning department, the council and other city officials.

Instead, they have built support from environmental advocates and some developers, including Zeppelin Development’s Kyle Zeppelin. He has incorporated rooftop gardens into recent projects in the River North area and says the mandate has the potential to address pressing environmental challenges while promoting better building design.

Are costs of green roofs worth it?

The opposition campaign is raising questions in mailers and direct appeals about the mandate and potential consequences. Those include higher building costs, which opponents say could be passed on to renters, and increased water usage in the semi-arid city.

Rietheimer and green-roof advocates dispute some of those claims, calling some overblown or misleading.

But they acknowledge higher upfront costs, while pointing to studies of rooftop gardens that show significant energy savings over time, resulting in a return on the investment within five or six years in many cases.

Cost estimates for installing a rooftop garden range from $25 to $35 a square foot. That translates into tens of thousands of dollars for a smaller office building that meets the 25,000-square-foot threshold to the low hundreds of thousands of dollars for, say, a grocery store.

Toronto-based Green Roofs for Healthy Cities and the Green Infrastructure Foundation recently released an economic study aimed at supporting Denver’s initiative, which has drawn interest outside Colorado.

The study estimated that the new ordinance, if approved by voters, would result in the building of 57.5 million square feet of green roofs by 2033. Over the first 40 years, the study says, those new green roofs could generate $1.85 billion in energy savings and other benefits.

“If anything, the implementation of this initiative will help the Denver economy,” predicted Steven W. Peck, the founder and president of Green Roofs for Healthy Cities, while also helping the environment. “It will actually catapult Denver into more of a leadership position nationally on green buildings.”