So here is the site that Laura and I are working on, with lots of space for and impressive cafe and green walls.

Starting at $100 per square foot, G-Sky will install plant-filled wall panels that can go on any vertical surface--meaning G-Sky just quintupled its opportunity. After all, "for every roof out there, there are four walls," says Steven Peck, founder of the Toronto- based industry association Green Roofs for Healthy Cities, who says he expects the vertical greenery market to be "huge."

Green walls can provide as much bill-saving insulation as green roofs, but put less load-bearing strain on the building. Whole Foods (Charts), Vancouver International Airport, and the W Hotels (Charts) chain are early G-Sky clients.

And that's just the start. G-Sky is looking long-term, to a world where carbon-trading is king and companies are eager to offset their greenhouse gas emissions. What better way to do that than to cover your building with greenery?

"Even in small plants, there's a very easy calculation for how much carbon they offset," Sichello says.

Green walls can also help offset the newly identified urban heat island effect: All the heat-absorbing surfaces in a city raise its temperature to as much as 8 degrees higher than that of the surrounding countryside. Peck says no North American city will have enough green roofs and walls to combat this effect before 2027--not even Chicago.

It's time for entrepreneurs to find their green thumbs: There's a lot of growing to