The Green Roof at the Minnesota Landscape Arboretum

By Brita Thompson, 2010 Summer Intern

The Minnesota Landscape Arboretum’s Green Roof is part of the “Harvest the Rain” exhibit located in the Margot picnic shelter area, just north of the main parking lot. This previously ordinary picnic shelter now sprouts a colorful array of plants in an effort to highlight a growing trend in water runoff management. The green roof was installed at the Arboretum in the spring of 2009. This innovative roofing material demonstrates an increasingly popular method of water management and how it can be good for the environment, the home, and the landscape.

What is a green roof?
Why grow a green roof?
How are green roofs put together?
What did we plant on it?
How do we maintain it?
How does the Arboretum's green roof handle the climate?
Where can I learn more?
Who sponsored the Arboretum's green roof project?

What is a green roof?

Most simply, it is a roof with plants growing on it, and, in some form or other, green roofs have been around for centuries. They were used in hot climates to add cooling and beauty to the harsh landscape and in cold areas to insulate against the bitter winter temperatures. Today, green roofs are usually complex systems of plants, media and waterproofing that can either take the form of elaborate gardens or simple ground-cover plantings.

Why grow a green roof?

In our modern city landscapes a large proportion of the surface area is non-permeable: that is, it does not allow for water to pass through and drain into the water table. Instead of being absorbed slowly into the ground, rainwater is forced to flow along the surface, often reaching a river or lake before it can drain into the earth. This water picks up pollutants along the way and can cause significant issues for our natural freshwater ecology. The large amount of surface runoff also creates a massive job for our sewage and storm-water management systems, many of which are outdated and inefficient. A green roof helps to solve some of these issues by capturing the rainwater that would otherwise add to the runoff problem and using it to grow plants. Besides benefiting the environment, green roofs also help to moderate the temperature of your home, absorb pollutants out of the air, provide oxygen for us to breath, and add a touch of beauty to the otherwise harsh urban landscape.

How are green roofs put together?

The secret to a lush green roof is layers. Our green roof has 7 different layers that all serve distinct purposes.

 

All of these layers were specially chosen to be light-weight, but the original building still needed to be checked for structural strength. We had to add 4 new steel collar ties inside and an extra plywood layer on top so that the roof could hold an additional 30 lb/ sq ft.

What did we plant on it?

Gardening on roofs presents a few challenges; our roof required plants with a shallow root system that were also both cold hardy and drought tolerant. We found 14 different plants from both non-native sedum groups and native ‘goat prairie’ plants that grow wild on the Minnesota bedrock bluff prairies.


 How do we maintain it?

A green roof, just like any other garden or landscape, requires some maintenance to keep it healthy and beautiful. To find out how we care for our green roof click here.

How does the Arboretum's green roof handle the Minnesota climate?

We took special care to plant our roof with cold hardy plants, but the roof top is a very harsh place for a garden. So far, all of our plants have survived the winter, but an especially harsh season with little snow cover could take its toll. We will observe the plants' performance on the roof, and if some species have a hard time in the cold, we will replace them with more tolerant species.

While the cold has not yet caused trouble for us, the gusting wind has. Just a short time after installation, a windstorm blew a good portion of the plant material and erosion control blanket off of the north side of the roof. We put it back as best we could, and the area is growing in nicely now.  The plants appear ready to take on whatever the Minnesota climate throws at them next.

Where can I learn more?

Visit the Minnesota Green Roofs Council website to learn more about green roofs in Minnesota.  You can also read about University of Minnesota graduate student Jonathon Hensley's research from the 2009 Waterosity exhibit.

 Who sponsored the Arboretum's green roof project?

The installation of the green roof was made possible by the generous support of the Minnesota Green Roofs Council working with:

Aloha Landscaping, Inc. Green Acres Sprinkler Company, Inc. 
Berwald Roofing Company Henry Company
Cermak Rhoades Architects Metro Area Sheet Metal JATC (Sheet Metal Local 10)
Coldbond, Inc. Minnehaha Creek Watershed District
CoPu Hybrid Green Roof Media    Roofers Local Union 96 & Apprentice Training Center
Dragonfly Gardens Rosenquist Roofing Company
DripGrip  

 

Gardens & Collections

Display and Specialty Gardens
Annual Garden
Dahlia Trial Garden
Daylily and Chrysanthemum Walk
Dwarf Conifer Collection and 
Waterfall Garden

Entrance Garden 
Fern Walk
Green Roof

Hedge Collection
Home Demonstration Gardens
Hosta Glade
Iris Garden
Japanese Garden
Lilac Collection and 
Blackman Garden
Lily and Dahlia Collection
Maze Garden
Ornamental Grass Collection
Peony Walk
Perennial Garden
Rain Gardens
Rose Gardens
Sensory Garden
Shade Tree Exhibit
Terrace Garden
Woodland-Azalea Garden

Visitor Center
Dahlberg Welcoming Terrace
Keating Terrace
Newton Dining Terrace
Sweatt Entry Terrace
Wall Teaching Garden
Wright Terrace Gardens
Wildflower Garden
Garden for Wildlife

Native Areas
The Prairie
Spring Peeper Meadow
The Bog Area

Shrubs
Azalea Collection
Bailey Shrub Walk
Hydrangea Collection
Lilac Collection
Miscellaneous Shrub Collection
Pea Shrub Collection
Potentilla Collection
Rhododendron Garden
Spirea Collection
Viburnum Collection
Weigela Collection

Trees
Arborvitae Collection
Ash Collection
Birch Collection
Buckeye Collection
Corktree Collection
Crabapple Collection
Elm Collection
Hawthorn Collection
Larch Collection
Linden Collection
Locust Collection
Maackia
Magnolia Collection
Maple Collection
Nut Collection
Oak Collection
Pine Collection
Poplar Collection
Prunus Collection
Small Tree Collection
Serviceberry Collection
Spruce Collection
Weeping Tree Collection
Willow Collection