Those who walk onto Sara and Tom Simpson’s roof in summer will be greeted with the slightly sweet aroma of scented geraniums as well as a cornucopia of  sedums of varying colors and textures, irises here and there and some dwarf  lady’s mantle.  All are part of the Simpsons’  “green” roof, which is enveloped by the long, reaching arms of a 150-year-old bur oak tree.

The residential green roof is unique in Lake County, and it’s Sara Simpson’s place to go to cultivate her passion for gardening and to seek out a quiet respite. (Photo gallery here.)

The Simpsons live in a Sears home that was constructed in 1926. When they decided to put an addition on to this cozy, architecturally significant home in 2002, Sara Simpson thought of having a flat rather than pitched roof to create a garden of delights atop her home.

“Green roofs are relatively new on this continent,” said Simpson, who has a horticulture degree from the College of Lake County in Grayslake.

To learn what she had to do, she took classes and sought advice from green roof gurus and architects. She also researched just the right plants that would do well growing on her roof.

“Once  I got interested there was no stopping me,” said Simpson. She’s visited the green roof at Peggy Notebart Nature Museum in Chicago and marvels at  Millennium Park, which some folks might not know is actually a green roof.  Simpson has stored in her computer many examples of green roofs throughout the nation and the world for admiration and education.

In 2005, the green roof sans plants was installed. Simpson hired workers  to hoist up the layers she need atop the roof to make it habitable for the plants she wanted to grow. A special growth media was placed atop a filter fabric, drainage layer, root barrier and waterproof membrane. These layers keep the area well-drained as well as the media from moving.

The green roof media is not like a garden soil, Simspon said. It’s a lightweight, engineered, mineral-rich material.  Research  is being done on what makes the best media for green roofs, she said.

Next came the  labor-of-love part.   Sara and Tom Simpson lugged  1 foot by 3 feet blue-stone steps up three flights of stairs to the roof, so there would be places to walk around the garden. Then she created a blueprint design to determine which plants to use and where they would be placed.

She learned that  Sedum, also called stonecrop, “is the work horse on green roofs.” This genus can withstand drought and has shallow root system.

“People who think they can grow a prairie on the roof,” she said, but that’s typically not possible because most prairie plants have very deep root systems. One prairie plant she can grow is called prairie smoke; she thinks it thrives in well-drained areas like her green roof.

About 70 percent of the green roof  contains 24 varieties of sedum. Scattered among the sedum are four types  of alchemilla (called lady’s mantle), allium, dianthus, geraniums and some dwarf and medium-height irises, among other plants.

Sedum might be considered the ordinary to seasoned gardeners,  Simpson said.  “But then I started realizing all the varieties, and they are survivors. Sedum has a unique metabolism, which helps it survive drought and other harsh conditions.

Unlike many green roofs, Simpson has some shade from her 150-year-old bur oak tree, so she can intersperse some shade-tolerant plants. She watered her plantings when they first were installed, but now the only maintenance required is weeding and removing the fallen leaves from the oak tree in autumn, she said.

Stepping along Simpson’s green roof is akin to walking on the tundra, where nearly every plant is miniature-sized. As a result, those who come to view the garden must bend down or  get on their knees to thoroughly enjoy all the detail, colors and textures.

A small yellow bloom peers out from a wispy green sedge, while small stands of a sedum known as ‘Bailey’s Gold’ grab the attention. Close up, one sedum looks like a tiny spruce, hence its common name Sedum ‘Blue Spruce.’

Little white ornamental alliums from the onion family are in bloom now — yellow irises have lost their blooms, but the larger Iris pallida, a purplish-blue flower, is at its prime. When autumn comes, some of the sedums take on a reddish-orange hue. And it’s nice here in winter, too, Simspon said,, with the shadows from the bur oak and lingering reddish hues of sedum.

“This garden helps you to slow down,” Simpson said. “It’s so peaceful up here.”

Beyond the peace, green roofs provide cooling effect in the summer, helping reduce the energy for air-conditioning. Green roofs also provide places for bees and other insects to find food, and absorbs air pollution, among other benefits, according to Green Roofs for Healthy Cities. The organization (greenroofs.org) works to increase the awareness of the economic, social and environmental benefits of green roofs.

Simpson‘s green roof  will be open to the public during the Gardeners of Central Lake County’s July 9 walk. A limited number of people will be allowed on the green roof at one time. For more information, go to www.gclcil.com.