By Bruce Schundler
and Jennifer Appel, ASLA & LI
Green Roof Technology
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| The "red" plants started out as 4" coleus. Normally an annual, these have grown to over 7 feet in width and continued to bloom throughout the winter. This can be attributed to the soil-less media. Jennifer fertilized the same plants in regular soil at the same intervals, and the plants in regular soil reached only about 4 feet in width. (With no fertilizer, coleus normally reaches 2 to 3 feet in height and width, and dies in the winter.) |
So-called "Green Roofs" are being built more and more thoughout the world, especially in Germany and increasingly in the United States.
And contrary to popular opinion or conventional wisdom, they don't have to be complicated or expensive.
Jennifer Appel is a landscape architect in Houston, Texas who had to solve a problem: how to retrofit "an intensive green roof on a 1921 historical structure." Needless to say, the job required "knowledge of structural loading, wet weight loading, drainage, soil media selection, plant selection, irrigation, fertigation, and long term maintenance, while creating the design of the client’s dream requests."
After researching different possibilities on the internet and talking with representatives of The Schundler Company, Jennifer decided on a rather new and unique soil mix: perlite and packing peanuts.
As Jennifer wrote in an email to us:
"I am the Landscape Architect and Licensed Irrigator of record for this project
as well as the person in charge of maintenance.
When I was researching different media sources I talked to someone at Schundler
who said it would not be possible to grow trees in the mix. (FYI - it works
better than soil.)
The mix is: 70% perlite horticultural grade and 30% peanuts.
The peanuts are at the bottom with the perlite on top.
The depth of the planters is 3'6 -(as a force protection barrier).
Next time I have an opportunity to repeat the process, I will go more to a
50/50 and blend it during installation.
When we were installing this - the biggest problem was wind. So, we pulled (2)
all nighters, because the winds are almost null after dark.
Another factor to consider: the perlite and peanuts have a tendency to float -
but with about 2" of shredded hardwood mulch - the media stays put. Do NOT use
pine bark mulch - this will exacerbate the problem.
I use 1-2 minutes of water per day in the fall, winter, and spring.
I use 2 minutes (2 times a day) in the summer (we are in Houston and it gets
really, really hot here at mid day).
I have gone to the all organic program, feeding once a week.
What we learned:
All the one gallon plants exceeded 5 gallon size within 4 months.
This is huge... it normally takes one whole year to get plants to this size.
Personally, I would like to believe that it is the fertilizer in combination
with the well aerated soil less media.
11 of the crepe myrtle trees flowered for 11 months and 1 tree has flowered
continuously and is setting new flowers right now.
I sent the same package to a hot house grower in Dallas. She had the same type
of results indoors with Broccoli and all of her vegetables using the same mix
and the same fertilizer."
Green Roofs are a method for making a potentially greener cities.
The two main constraints are usually wind and weight.
The logical position is to make the green roofs planting area light and
thin. (Geo mats)
If we remove weight as a constraint - based on super lightweight soil-less media, then plant selection based on wind would become the most critical factor.
So far, what I have noticed are a series of geo textile mats of various
depths that provide for very little root mass.
In my landscaping experience, when the root zone is stunted (bonsai), the plants do not perform as well as they do when they have a large, well formed root system.
When you asked about wind loading - my answer is this: Most plants have a root zone between 6"-30" below the soil line. If we can provide a planting area that is 30" deep, then we can expect the regularly fertilized root systems of the plants to intermingle and create a "mat" within the soil-less media.
Once a root mass is established, then single plants will not have a > tendency to uproot with high winds. (At least that is one of the theories that I am successfully testing at the Humble Building.)
One catch to this is that the plants must be spaced appropriately. When at full 'mature' size, plants do not overlap each other or the root systems will not form correctly due to a lack of space. (i.e.: if a plant grows 3' around, do not put it on an 18" center.) I could
diatribe this for days.
Another issue is plant selection.
I believe that the geo mats do work. They work very well in their
current applications.
They would not work for my application.
I would really enjoy the opportunity to consult on another project of this scope to test additional theories about making green roofs
mainstream in urban areas.
And if you would like to contact Jennifer Appel, or use her as a consultant, she can be contacted at appelgreenroof@yahoo.com
Read more about this fascinating and ongoing project --- an article by Jennifer on Green Roof designs at
Other useful links about green roofs and green roof technology:
If you need any more information or think we can help in any way, please call us or contact us at:
The Schundler Company
I have done some plant pull tests to see how well the root systems are forming
on some of the plants. These plants have some of the largest and well formed
root systems that I have ever seen. (I have been in the landscaping industry
for over 10 years.) Hyphae, usually not visible, are extensive and the root
masses are very strong. Even replacing the plants - they had no noticeable
transplant shock.
In another email, Jenny addressed some of the problems of weight and wind uplift:
wet weight loading of only about 23 pounds per cubic foot
and it is supporting the growth of trees, the chef’s herbs,
and more than 5,000 year-round flowering plants.
How to Build an Intensive Residential Green Roof
10 Central Street
Nahant, MA 01908
732-287-2244 or www.schundler.com