Six flights up a concrete staircase that serves the parking garage at Burlington International Airport, a heavy, unmarked door opens onto a $500,000 taxpayer-financed rooftop garden.

Back in 2011, when the so-called "green roof" was installed, thousands of succulents stretched out along slate paths in a patchwork of greens, reds and yellows — a project meant to help reduce rainwater runoff, moderate temperature and provide a "perfect place to relax," according to the airport's website.

But a visit last week revealed that large bare spots had replaced the sedums — a type of succulent — that once sprouted throughout the approximately 13,000-square-foot public garden. Dandelions and crabgrass pushed their way between the heartier varietals that remained. Weeds grew from beige flowerpots that lined the garden's fences. Leaning against one of them, a volunteer aspen sapling had taken root.

"It's in need of some TLC. Lots of it. Boatloads," said Michael Lawrence, a landscape architect who helped design the project and has kept tabs on it since. Lawrence said he is one of several experts who have notified the airport of the roof's deterioration — to no avail.

"The city never did anything, nor did they weed and water," claimed Rebecca Lindenmeyr, a landscaper for Linden L.A.N.D. Group, which planted the garden six years ago. From the looks of it, she said, "they basically haven't touched it since we put it in," she said.

Yet the BTV website boasts a photo of it featuring lush, bushy plants and bright flowers in front of a surreal sunset.

The garden is "nothing I've focused on," said Gene Richards, who has been the director of aviation since 2012. He's no green thumb, he admitted, but said when he stopped by the plot in early June, it looked like it was "performing."

The green roof project began in 2010 when Burlington voters approved a $21.5 million bond to add two stories and 600 new parking spaces to the north end of the existing garage. The airport, which is owned and managed by Burlington but is located in South Burlington, had to comply with the host city's zoning laws, which mandate that a certain percentage of the project cost be directed to landscaping, according to Paul Conner, the South Burlington director of planning and zoning. Besides, Conner said, the verdant roof "coincided nicely with ... the airport's mission and vision" for a green footprint.

The city's Development Review Board granted the airport's request for an exemption from the traditional "trees and shrubs" requirement so they could build the green roof, Conner said. Officials funneled $456,000 of the $21.5 million bond into building the garden, which was "intended to be a no-maintenance solution" to the need for green space, according to Richards.

The installations use sedums planted in about four inches of a gravelly soil mixture, said Ray DeFeo, a sales representative at Prides Corner Farms, a Connecticut-based nursery that supplied the plants for the BTV project. The soil mixture and the plants absorb the first inch of rain, reducing stormwater runoff. They're also easy to maintain: The plants transpire, or release oxygen, at night, so they require little water and low nutrient content in the soil. They're perennials, meaning the plants die back in winter and reemerge each spring.

Stonewall Hardscapes installed slate paths, which snake through the garden plots with lampposts lighting the way. Prides Corner Farms provided 3,955 modules — each two square feet in size — planted with combinations of different-colored sedums that make 42 different patterns.

"It was to demonstrate that you can have a public space on a wasteland area and ... it could be very attractive," said Lawrence. "If you can get down on your hands and knees and look at them, there's a whole world of beauty."

The completed green roof generated feel-good stories from WCAX-TV and the UVM Extension's television program, "Across the Fence." The Burlington Free Press dubbed it a "rooftop oasis."

The airport planned a party to commemorate the opening, according to airport commission minutes, and at least one private wedding was held at the site soon after, according to airport commissioner Bill Keogh. The space can still be rented for events, Richards said.

In 2013, the Vermont chapter of the American Society of Landscape Architects awarded the green roof a "public space" award.

But those who helped install the garden say airport officials soon lost interest in promoting or maintaining it.

Lindenmeyr said she supplied signage to direct people to the roof that was never installed; now, there's no indication at the airport that the oasis even exists.

Under her contract, Lindenmeyr continued upkeep for a year, through the summer of 2012. When airport staff didn't pitch in, Lindenmeyr said, she continued to weed and water for a second year. "It's hard for me to invest so much into it — financially, emotionally, time-wise — and see it die," she said.

Lawrence said he drove over occasionally to care for the space as well. After a few volunteer trips, he stopped. "It was more than I could do," he said.

It's gone downhill from there. "It's not that a green roof takes a lot of maintenance, but it takes a little bit," Lawrence said. "If you don't pull the weed right off the bat, one turns into 10,000."

By the winter of 2014, the red sedum had died out, Lindenmeyr said. By the next summer, a 2,000-panel, 500-kilowatt solar array was sharing the rooftop with the plants.

At the behest of Lawrence, Prides Corner Farms representative Ben Lucas visited in August 2015 to assess the struggling garden. The following January, Lawrence sent Richards a letter asking for intervention. He noted "sparse areas" and asked that the green roof be "restored to its original usefulness and beauty." The letter recommended a course of action, including soil sampling, weeding, weekly maintenance and reestablishing the devastated greenery. Richards never responded, Lawrence said.

According to Richards, the maintenance team reviewed the letter and visited the garden — but found no need to implement any of the changes. In an email to Seven Days, Richards said the maintenance workers "were very happy with its performance."

Alex Halpern of Freeman French Freeman, the architectural firm that designed the parking garage, praised the airport's initiative in building a green roof that supplies spectacular sunset vistas across Lake Champlain from the "highest spot in South Burlington."

But he, too, has nudged BTV officials to take better care of it.

"It's a gem," he said. "It'd be nice if it was green again."

Airport staff does weed "a few times a year," Richards said in an email, though "the focus for landscaping is on the high-congested areas of the airport first, with the roof garden secondary."

When they're not plowing runways, up to five staffers tend to 44 acres of lawn mowing, said Nic Longo, the airport's director of planning and development.

"As you may notice, we do take incredible pride in providing great landscaping around the entire airport," Richards added.

This year, he confirmed, no one has weeded the rooftop garden — "not yet" — and it "probably could use a bit of a boost."

Airport commissioner Keogh hasn't heard any complaints. Then again, he added, Richards is more inclined to share good news than bad.

Nor has anyone notified Conner in South Burlington. There's no regular inspection process, he said, though property owners are expected to maintain required landscaping.

"We communicate with property owners when something becomes known to us," he added. Following a conversation with Seven Days, Conner vowed to investigate.

In the event of a violation, he said, the case could go to the Development Review Board and, eventually, to environmental court.

"Our primary goal is to get compliance," Conner said.

For guidance, the airport could look to its neighbor across the runway. "There's not much to it," said James Soulia, facilities manager at the privately owned Heritage Aviation, which installed a similarly sized green roof in 2009. That garden is thriving, with healthy-looking plants that cover the plots in a vibrant green. Soulia said it requires weeding every few weeks and an annual soil test and fertilizer application.

The University of Vermont and Middlebury and Champlain colleges all have green roofs.

The pros aren't yet calling BTV's a lost cause. Lindenmeyr dubbed sedums "tiny little aliens" for their ability to adapt and rejuvenate. During a visit last week, the green sedum variety appeared small, likely as a result of malnourishment and stress, but the roots were intact, Lindenmeyr said; they'd grow as summer progressed. The red varietals could be replanted.

Lawrence, too, said it would take only a small time investment to revive the project.

"It's not beyond hope," he said. "All the ingredients are there."