“They paved paradise and put up a parking lot...” Joni Mitchell

Pavement usually means trouble for trees. But at the Chicago area’s first environmentally-friendly parking lot, it means new life for the 41,000 trees that call the Morton Arboretum home. Opened in September 2004, the US$ 43 million dollar expansion called Branching Out! includes a green parking lot — one with a natural filtration system that will actually improve the ecosystem and be a showcase to the world.

Founded by Joy Morgan of the Morgan Salt Company, this living tree museum and park is located on the fringes of Chicago in Lisle, Illinois, and features 3,300 different types of trees, shrubs and plants from over 50 countries. The expansion, including a Visitor Center, Children's Garden, Maze and restaurant is spread across 1,700 acres (6.9 km²), positioning it as one of America's foremost public gardens. But a highlight of the development is the innovative new parking lot.

The Arboretum features 3,300 different types of trees, shrubs and plants.

“Yes, parking lots are not usually a park feature,” laughs Kris Bachell, horticulturist and Director of Grounds and Collections at the Morton Arboretum. “It’s the first time the Environmental Protection Agency has funded a parking lot. We’re a test case.” Bachell explains that while they were restoring the site’s Meadow Lake, they realized that after restoring the shoreline and adding 78,700 perennials and 260 new trees, they didn’t want to sewer the lake with pollutants from the nearby parking lot. So, they decided to investigate healthier alternatives.

Consultations led to Christopher Burke Engineering of Rosemount, Illinois who devised a unique system of cleaning the water. “The key is filtration. Our intent is to slow and cool the water before it reaches the lake,” Bachell explains. To do this, each brick on the parking lot has a hole in its center, filled with gravel. As water drains through the gravel, contaminants are neutralized. Under the bricks is another layer of gravel, until finally, the water leads to a planted wetland that filters it further.

Concave medians with bio-soils, 8,800 natural shrubs and perennials line the parking lot and catch run-off during heavy rains. But although it sounds simple, it wasn’t cheap. The cost for the parking lot alone amounted to US$ 1.2 million dollars. But it was necessary — the Arboretum receives 420,000 visitors annually and expects that number to rise to 750,000 within the next decade.