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by High Country Gardens

The University of Pennsylvania's Morris Arboretum Rooftop Garden in spring. Photo by horticulturist Louise Clarke.

A Q & A With Horticulturist Louise Clark

Tell us about the rooftop gardens?

The smaller roof (the extensive garden) has only about 4” of soil and is filled with sedum. Because of the soil depth, not much else will grow there.

The larger roof is the intensive garden with about 7” of soil depth. We implemented them because we were aiming for Platinum LEED certification (the highest level). It is also part of our educational mission to show visitors what you can do.

We have a couple cisterns that capture water. The buildings were engineered for the additional weight of the soil, which is a mixture of gravelly, porous, and very fine materials. It includes crushed brick, expanded shale and clay; there needs to be macro-pores and micro-pores to hold water for the plant roots. We originally had 10% spent mushroom compost as the organic material. It’s free draining and fairly lean, to keep weeds down.

What are the advantages of a green roof?

If you’re in an enclosed building, it can lower your energy costs, so you’ll save money heating and cooling that space. The roof lasts longer than a traditional roof, about 20-40 years, because it isn’t exposed to the elements. It assists with storm water management, acting as a sponge during rain events, and helping to avoid peak flows that could overwhelm sewer systems. It’s also aesthetically pleasing and has wildlife, including nesting birds, insects, and spiders. Because of it’s height, it offers pest free gardening, with no squirrels, deer or groundhogs.

What is the biggest challenge in managing a rooftop garden?

Initially it was the irrigation, because the architects didn’t take into account the need for watering a green roof, especially needed to get plugs established. We don’t water regularly, unless we haven’t had rain for a few weeks.

Weeding was also a challenge at first. There was lots of crab grass. But that’s under control now, and we use no fungicide, no pesticides and no fertilizers.

Schizachryrium adds fall interest to the rooftop garden. Photo by horticulturist Louise Clarke.

What types of plants do you look for?

The landscape architects originally wanted to replicate a Pennsylvania shale barrens landscape, but we thought that was a little boring. This was an intern project for the implementation. We looked for plants that would do well in full sun with sharply drained soil, and hot conditions in the summer/cold in the winter.

The intensive roof garden includes Campanula rotundifolia, Geum, Salvia reptans , Penstemon pinifolius (a hummingbird magnet), Rhus aromatic ‘Grow Low’ and some junipers, as well as an eastern native Sandcherry (Prunus pumila var. depressa).


We also have a multitude of spring bulbs, including species Crocus, species Tulips. Camassia bulbs worked really well, as well as Japanese Roof Iris (Iris tectorum).

What we didn’t plan for, but turned out really well, is the fabulous fall color. The grasses look nice and tawny and I leave them for the winter. The ‘Grow Low’ Rhus is fiery. The fall effect is really nice.

The Morris Arboretum in fall. Photo by horticulturist Louise Clark.

What are some dos and don’ts that you’ve learned over the years?

The long tap root of Yucca gloriosa turned to adapt to the 7" soil depth. Photo by horticulturist Louise Clark.

Last spring we thought we’d lost the yuccas. I dug out the taproot, and found it went down 7 inches and took a 90-degree turn. But it turns out we had not lost it, it was just slow to wake up. I’ve tried aloes with fleshier foliage, but with the crowns rotted out. Fleshy, tender things don’t work in our winter wet weather.

Also, we don’t advise planting prickly pears. I have a few up there on the lower roof. They’re not fun to maintain.

The pH is around 8, which is extremely high. I sampled the water runoff from the roof and it is around 7-7.8. The plants are pretty adaptable to these conditions and they have impressed me greatly.

If someone wanted to do this on a small scale, how would they go about it?

I’ve seen green-roof bird houses, green-roof dog houses. My biggest caveat would be to make sure the structure can hold the additional weight, which in our case is around 22lbs. a square foot.

Nigel Dunnett (, has written a couple excellent books on green roofs, “Planting Green Roofs and Living Walls” and Small Green roofs: Low-Tech Options for Greener Living.” Edmund Snodgrass is another authority. His nursery is exclusively geared toward green roof plants, and he’s written the book “Green Roof Plants: A Resource and Planting Guide”

Viewing the Morris Arboretum Rooftop Gardens

While the rooftop gardens are not generally open to the public, they are available for viewing during the annual plant sale in spring and during specific open house days. People can contact the educational department for information about tours.

Louise Clarke is horticulturist at the Morris Arboretum at the University of Pennsylvania in Philedelphia, and responsible for caring for the Bloomfield Farm Section. This 65-acre parcel houses the Arboretum’s Platinum LEED Horticultural Center, materials handling and two equipment garages, which are home to the rooftop gardens.

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