Living Architecture Monitor - Green Roofs for Healthy Cities

THEGROWINGMEDIA&PLANTISSUE
“Organic matter breaks
down and goes away; to
replace lost material could
require many trips with
heavy bags up steps or
an elevator.”
Chuck Friedrich
the particles which lighten the load, retain nutrients and water, and
best of all it is permanent. A good quality graded sand as filler is
good for stability, and a measured amount of the proper organic
compost provides microbial activity that is beneficial to the biology
of the media. Currently, there are no standards related to the use of
compost on green roofs.
What about fertilizer? Save it for planting time. There is absolutely
no reason to add fertilizer to a green roof media during the blending
process. For good reason, in most cases the media is installed
months before the first plant is installed. Therefore, why spend
money on fertilizer that will only end up leaching out and down the
drain before the plants show up on the job? The best method is to
blend a good slow release fertilizer into the top layer of the media
during the planting operation.
Once planted, provide lots of maintenance and irrigation; with that
it can be beautiful and last for several decades. I forgot to mention,
also add lots of money, you get what you pay for. If you can keep the
general contractor and schedule in check, the process can actually
go smooth, right? Wrong. Don’t forget about the engineer.
AWEIGHTYSUBJECT
I am amazed how the specification on the weight of the media can
be so tight, while in the same specification, the plan calls for 14
Oak trees to be planted. It tickles me when I am asked at least
twice per month by designers, “how much does a full grown tree
weigh?” What? Dogwood or Sequoia? Does an additional pound or
two per cubic foot of media, one way or another, make that much
of a difference? I guess it will when 350-pound Aunt Bertha and
the twins decide to have lunch up on the roof.
All kidding aside, weight is an issue, especially on retrofitted extensive
roofs. It is important to design using the saturated weight of the
media. The ASTM Sub-committee for Green Roofs has developed
standards for the practice and test methods for the determination
almost as absurd to consider all organics to be the same. There are
some organics that I would never use on a green roof (some of which
were used in the research behind the FLL Standard). From cellular
structure to composting process and beyond, organics are far too complex
to generalize within a Standard.
Secondly, the FLL Standard dictates organic content by mass. I can
understand why, since the dominant testing method for organic content
is the burn method, which can only measure by mass. However, by doing
so, the Standard leaves a lot open to interpretation. For instance, one
can use an extremely heavy inorganic material to achieve a high percentage
of organic content since organics are generally much lighter. I was
able to achieve organic content of over 60 per cent by volume in the
growing medium but only eight per cent by mass. Was this ambiguity
intended by the Standard?
Interestingly, contrary to what I heard in the industry, the FLL Standard
did allow for higher levels of organics. Section 9.2.2 states: “A greater
proportion of organic matter may be required where special forms of
vegetation, such as humus rooting plants, are used.” This shows the
importance of matching the growing medium to the physiological
needs of the plants, another area largely uninitiated by many suppliers
in our industry.
Thirdly, the Standard focused on material specifications instead of
performance specifications and by doing so, essentially closed the
door on innovation. (It seems the lowly sedum is the order of the day,
matched to equally low performance mediums). This caused companies
to scramble to find similar products in North America with the
prize going to those who could quickly identify and corner the market
on certain products. As most of the construction industry in North
America makes the change to performance-based specifications, the
FLL Standard represents a step backward.
Lastly, I concluded the Standard itself was not as much a problem
as people’s interpretation of it. For example; there seems to be much
misunderstanding of what organic content is. One hundred per cent
FORMULATING, TESTING, PLANT
GROWTH TRIALS, PROBLEM SOLVING
“SEND US YOUR
EXTENSIVE/INTENSIVE 5
GALLON PAIL PLEASE!”
SOIL CONTROL LAB
42 HANGAR WAY
WATSONVILLE, CA 95076
(831) 724-5422 PHONE,
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WWW.GREENROOFLAB.COM
FRANK@COMPOSTLAB.COM
CONTACT: FRANK SHIELDS