Living Architecture Monitor - Green Roofs for Healthy Cities

THEGROWINGMEDIA&PLANTISSUE
“DON’TCALLITDIRT!”
By Chuck Friedrich, RLA, ASLA
My college soil science professor always reprimanded the class
for referring to soil as “dirt.” He would always say: “dirt is
something that is tracked in onto the carpet.” Actually the ideal
green roof growing media should NOT contain any natural soil;
therefore, we should not even call it soil. Most green roof professionals
prefer the term growing media or medium, substrate, or
planting media. My definition for green roof media is: “the particulate
matter or substrate that anchors the plant roots to sustain plant
growth.” Sounds simple, but it can get very complicated.
For proper terminology, we should refer to the growing media as
intensive or extensive green roof media. The media should be designed
for the physiology of the plant material growing in the media.
Sedums require an extensive media that has a coarser gradation than
those used for grasses or other non-succulents. In addition to plant
material, other factors affecting media selection are climate, weight,
fertility, depth, location, microclimate, and irrigation. With all these
factors to contend with we can then add a bunch of engineers to the
equation. Over the past 15 years I have supplied green roof media on
over 140 green roofs. It has been a learning experience offering much
success. (Having a German last name didn’t hurt.)
LOSINGORGANICMEDIA
Trees, shrubs, lawns and flowers need water, air, space for the roots,
and nutrients. Within the proper microclimate, many plants can
grow better on the roof than on the ground. Why? With a green roof
we can create a very big pot filled with the perfect growing media
that is fertilized, irrigated, and most important, well drained. What
we don’t want is a natural soil or a 100 per cent organic mixture
GOINGFORORGANICS
By Rick Buist
When considering the writing of this best-practices article on
using organics in growing medium, I reflected on the experiences
I have had in the green roof marketplace to date. Since first becoming
aware of the green roof industry, I had numerous people tell
me how you shouldn’t use more than a certain percentage of organics
in the growing medium. One says five per cent, another 20 per cent,
and yet others argue it would be best if there were no organics at all!
Now at the time, I found this all rather odd. Our company has many
successful installations (over 80 to date) with few observed problems,
and our sister company has been producing millions of cubic meters of
organic-based growing mediums for decades to the nursery industry.
Then I was told that our organic growing medium projects we installed
did not have a long enough track record to be sufficiently evaluated.
“Just you wait,” they ominously warned, ‘eventually the organic growing
medium will disappear — it’ll be a disaster.” One person even
claimed they had pictures of a green roof we installed showing exposed
roots — impending doom was just around the corner!
So I took all of this great advice to heart and also personally visited the
site in question, only to discover the growing medium depth had not
changed in four years. The roots were not exposed (although clumping
fescues could give that appearance) and the owners were very happy.
Naturally I had to ask myself: Why the strident accusations against organics
in growing mediums?” Is it because some have invested in proprietary
products associated with growing mediums? Is it because of the European
experience? Or do they really have a sound scientific argument?
LIVING ARCHITECTURE MONITOR WINTER