As cool ideas go, this one tops them all.

Businesses can reduce their air conditioning cost — and in turn, their carbon footprint — by planting a garden on the roof.

The concept is popular in Europe, where 100 million square feet of rooftops have literally gone green, but in Hawai'i, it's only now taking root.

The newest green roof is growing right now on a second-floor deck used by the Honolulu architecture firm of Philip K. White & Associates.

Employees at the firm recently planted 150 square feet of ferns, naupaka papa, rhoeo and plectranthus using a modular system called GreenGrid, which grows plants in removable trays.

The system allows for plantings that are shallow — 4 to 8 inches deep. They can also be grown in relatively small planters — 2 feet by 2 feet, or 2 feet by 4 feet.

"It's a kind of a living laboratory," said Laura Ayers, an architect with the firm. "We had to find plants that would thrive in a 4-inch depth and withstand the elements. We do get a lot of rain but also a lot of sun. We had to find something that didn't require a lot of water."

The soil also had to be light enough so as not to stress the roof of the building, she said. What they used is similar to potting soil, but lighter.

The plants are part of an ongoing green renovation that began when the architects moved into the building two years ago and teamed up with another tenant, the interior design firm of Philpotts & Associates.

Changes included photovoltaic solar panels, sensors that turn off lights when people leave a room and a system that diverts rainwater to dry wells that are designed to put the water into the aquifer and not into storm drains.

Although the dry wells reduce runoff by 25 percent, the GreenGrid plants will also capture water, Ayers said.

The system was donated by Weston Solutions Inc., which helped develop Green-Grid. The architects, who chose the plants, plan to show off its features to anyone interested. They also plan to add another 100 to 200 square feet of plants, Ayers said.

Weston estimates that a green roof system will cost between $20 and $25 a square foot. Savings on utility costs, in return, come primarily through insulation provided by the plants.

GreenGrid estimates that a roof insulated with plants can reduce heating and cooling costs for the floor below by 25 percent to 50 percent.

The architects were looking for low-maintenance plants and actually began testing when they moved into the building, Ayers said. They put a tray with a variety of plants on the roof and left it there.

"We literally did nothing and those plants that survived are the ones we are planting," Ayers said.

The new plants will get better treatment, she said.

"We do intend to water them, and we get rain here," she said. "But let's face it, we are architects, not landscapers — and we will forget."