Green roofs are what
they sound like, says Hunt, Extension specialist in the Department
of Biological and Agricultural Engineering. Other names for
green roofs include garden roofs, eco-roofs and roof gardens.
A green roof works like
a sponge atop a building. When it rains on a conventional roof, the
water immediately runs off. The greater the rainfall, the greater the
runoff. With a green roof, the soil on the roof retains most of the
water temporarily and then releases that water at a much slower rate.
In addition to the obvious flood-control benefit, Hunts team is
examining the degree of nutrient removal, and that effort is what makes
this a pioneering endeavor.
Our sites in North
Carolina are among the first in the world to try to discern if this
is indeed the case with respect to nitrogen and phosphorus removal.
Although green-roof research
in North Carolina began in the spring of 2002, and green roofs remain
a relatively new idea to the United States, the concept has been around
for centuries. Native Americans used green roofs in their sod houses.
Green roofs were also popular among the Vikings. Most recently, the
concept has been extensively used in Germany and other northern European
nations. Today green roofs are used to aid in the heat island
effect in highly congested areas, stormwater retention and for aesthetic
purposes, according to Hunt.
When the appropriate soil
mix is used, good drainage is a given, and good drainage promotes
deep roots, which reduces the need for excess watering once the plants
are established, says Friedrich.
Green roofs come in two
varieties extensive and intensive. The main difference between
the two involves the type of plants used for the roofs.
In general, roof plants
should not only be drought-tolerant but should not have airborne seeds
and be able to live a long time [at least 40 years] without adding too
much weight to the roof, says Ed Snodgrass of Emory
Knoll Farms, a private nursery that is donating plants for Hunts
Extensive green roofs are
designed to be virtually maintenance-free and thus require hardier plants
that can survive drought, extreme temperature variation and intense
sunlight. These plants usually grow in soil that is 2 to 4 inches deep
and reach heights of about 3 inches.
that extensive plants be a ground cover and stay low and add as little
weight to the roof as possible, Snodgrass says. In the Mid
Atlantic and Southeast, this means plants like Sedums, Delspermas and
Rosularias. He adds that it is important that these types of green-roof
systems be designed for each specific location and warns that a
cut and paste plant approach would result in low-performing systems.
While extensive green roofs
should need little more than some annual maintenance, intensive green
roofs require a substantial amount of maintenance. Intensive green roofs
are generally used more for show than function and require
regular watering and fertilizing. The soil is always deeper for intensive
green roofs because the plants grow taller. Plants can reach heights
from 3 to 15 feet and include varieties such as Jovibarbas and Sempervivums.
We are trying to identify
which plants work best in eastern and central North Carolina,
We want to examine the feasibility of green roofs as a stormwater management tool and decide if such a practice should be recommended to local governments, says Regans, who adds that state law requires local governments to establish stormwater management programs.