Here is where our hardy installation crew got a workout hauling heavy buckets of rock to all corners of the roof.  Swarthmore College gardener Bill Costello made many trips up and down the roof’s access ladder helping to load the pallets in the parking lot down below and then unload them on the roof.

This larger stone is used in places where the plants have a harder time growing, for example under the edges of the pitched roof where the water drains.  It is also used along the edges of the roof to prevent material from being blown off in this windier zone.

The majority of the roof is covered with our final layer of material, the growing media. The essential attributes of green roof growing media are: it is lightweight and drains well.  It also must contain some compost to provide nutrients to the plants.  On the Wister Center roof, as on our previous green roofs, we used a growing media called rooflite that is blended in nearby Chester County and delivered to us in large sacks weighing hundreds of pounds. Two of these sacks were lifted by crane and suspended over the roof so that they could be slit open and pushed around to pour the material roughly where we wanted it. The media was then raked around the roof to an even 2-3” depth.

Two weeks later, when work schedules allowed and rain was in the forecast, the stars of the show, the plants, were put to work. We were able to dig several dozen clumps of various species of sedum from our David Kemp green roof as well as harvest a variety of sedum cuttings. This plant material was immediately carried up the ladder to the Wister Center roof and planted or, in the case of the cuttings, evenly strewn over the growing media and watered in.

Now it is up to the plants to do their job of establishing themselves and filling in the bare spaces to create the colorful mosaic of a vegetated roof. We continue to be awed and inspired, as well as educated, by these green roof installations on campus. In addition to the significant ecological services they provide, green roofs are ever-changing kaleidoscopes of life.

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The Wister Center green roof has earned a special place in the hearts of arboretum staff because it is the first one we have installed (almost) all by ourselves. Previously the college has employed outside contractors to design and install our green roofs, and we gardeners have served as enthusiastic acolytes and willing apprentices, as well as loyal maintainers. This time, however, we came close to flying solo.

Our co-pilot was Peter Philippi the seasoned green roof guru who designed our Alice Paul and David Kemp roofs. Peter has direct experience with many vegetated roofs including a thirty-year-old green roof in Germany that is still going strong.

Most of our materials needed to come via pallets hooked to the end of the very long arm of a crane.  Local arborist Knight Brothers very generously donated the services of this crane and its operator.

On top of the roof’s waterproof membrane goes a thick blanket of recycled polyester fabric that both protects the waterproof membrane and holds on to moisture.

Next came a drainage layer: thin, stiff plastic sheets with drainage channels molded into them. This was a change from our earlier green roofs, which, for the drainage layer, had used an expanded shale product called Solite that was developed and generously donated by an alumnus of the college.

The plastic drainage sheets were used because, unlike our other roofs, the Wister Center has sections of pitched roof that drain onto the green roof.  We anticipate these pre-molded drainage channels will be more efficient in handling the increased volume and velocity of run-off from these pitched roofs.

On top of the drainage layer is a second, thinner blanket of recycled polyester, acting as a filter to keep particles of growing media from clogging the drainage layer and to retain moisture.

Stay tuned for Part II of the Green Roof installation: a glimpse at the installation of growing media and planting of sedums.

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In another sure sign of spring, many of these birds are molting into their bright breeding plumage. Just yesterday evening, Arboretum gardener Nicole Lewis and her young daughter, Maysie, observed six very yellow American Goldfinches cavorting high among the flowers of the massive Bur Oak, Quercus macrocarpa, behind the Scott Arboretum office. Maysie is already quite the young plant enthusiast, and she is developing into a good birder for a four year old.

Also yesterday evening, I spotted my first warblers in breeding plumage. These Yellow-rumped Warblers were also high up among oak flowers very busily chasing insects. Keep your eyes and ears open because spring has definitely sprung.

If you are interested in learning how to attract birds to your garden, Nicole and I will be teaching a workshop titled “The Bird-Lover’s Garden” on Saturday, May 3 from 10 am to Noon. Nicole and I will also be leading the Early Morning Bird Walk on Sunday, May 4 from 7 to 8:30 am. Come join us and maybe we will see some of Maysie’s American Goldfinches.

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