Nearly Perfect Plants, Hiding Up on the Roof

Ed Snodgrass is on first-name terms with bizarre and beautiful plants that are strangers to most people, even avid gardeners. With names such as delospermas, orostachys and jovibarba, they are tough succulents with fleshy nodules for leaves, and flowers of intense colors. The plants survive 120-degree summer environments and frigid winters with little soil, no weeding, fertilizing or watering. Indeed, they insist upon it.

Why don't we know about them? They are raised primarily to fashion green roofs, which are sprouting on buildings in cities across the United States, typically flat-roofed institutional offices. Just as a hat will shade the face and cool the brain, a chapeau of plants will lower building temperatures by several degrees. These gardens in the sky have another major plus: They absorb storm water and reduce runoff pollution.

Snodgrass, the 53-year-old owner of Emory Knoll Farms in Street, Md., is mindful that he is growing large quantities of amazing plants that only a pigeon or two might admire. "There's a certain amount of renunciation that comes with being a nursery selling to the green-roof industry. As a plant lover, that's hard. But you have that secret smile knowing that there are all these gardens out there."

Or up there. Time, perhaps, to bring them down to earth.

A few of these plants are not entirely strange: Hens-and-chicks, or sempervivum, has been around for a long time, handy for sticking in hot, dry areas where little else will grow. It is named for the fact that rosettes of scaly leaves produce baby offsets. And rock garden fanciers have been growing some of the other succulents.

But the green-roof movement has brought many more varieties into commerce and produced them on a large scale. It has also introduced the idea that instead of placing one or two in a lonely spot, you can paint with them in the broad brush stroke of mass plantings.

In a plant artist's hands, these succulents could produce landscapes that are different, easy to care for, and of interest year-round.

If they have a problem, it's that they don't have common names for the most part: The succulent seeker must delve into a world of botanical Latin.

Snodgrass loves to see the six-inch-high Sedum kamtschaticum massed against the ground-hugging Sedum sexangulare . Both produce carpets of chrome yellow blossoms, but their subtle difference in height creates an effect of crashing water. "You get bloom at one level and it sweeps up almost like a wave," he said.

Sedums, or stonecrops, reflect the single largest offering. Most are fine-textured and prostrate in leaves of green, silver, blue, pink or purple, and flower colors that range from bright yellow to pink. They are related to the common garden sedum Autumn Joy but are far more diminutive and graceful. Snodgrass sells 21 varieties of one species alone, Sedum album .

Another eye-catcher is Talinum calycinum , whose foliage looks like bundles of tiny French beans. In May it sends up wiry flower stalks that are topped by flags of violet, poppylike blooms that open by day and furl at night. It blooms continuously into the fall. "It's a North American portulaca, fabulous for green roofs because in the afternoon it seems to have all these flowers dancing in the air," said Snodgrass. Assuming you can see it aloft, of course.

Some of the sedums have red foliage, which changes in intensity through the seasons. Sedum spurium Fuldaglut has pink-tinged leaf rosettes that turn a deep maroon in winter; a variety of Sedum album named Coral Carpet has extraordinary red-pink foliage in winter.

Another class of succulents are the delospermas, a few of which have been popular with rock garden enthusiasts, but are now widely used for green roofs in ever-expanding choices. They have daisylike flowers in intense shades of pink and violet. Snodgrass eagerly points out a new variety named Kelaidis that flowers in a more subtle shade of salmon.

These tough plants outdo the weeds by growing in hot, dry conditions and in a very little soil. They have delicate, feeble roots that grow happily in a couple of inches of gravel for the most part. Snodgrass raises them in 72-cell flats in a mix of 90 percent expanded shale and 10 percent compost.

To cultivate them in the garden, thus, one has to give them the growing medium they prefer. If you coddle them in rich, organic soil, they can grow lanky and then splay under the weight of the flower heads. To prove the point, Snodgrass points out a Sedum selskianum in rich soil (used to push growth to take cuttings) and the plant has flopped to expose its reddish stems. Another growing in his own garden, in poorer, drier soil, remains agreeably compact.

Most of these plants are for areas of full sunlight. A few will take some partial shade, but these are not perennials for the woodland or gloomy border.

A hot, exposed site where other plants struggle is ideal, especially a free-draining hill, though plugging them into hardpan clay won't do because of the risk of poor winter drainage. "But you can take an English fork and make the clay drainable, and then throw gravel or sharp sand on top and they would be happy," he said. The flatter the grade, the deeper the gravel layer needs to be: five inches, instead of two or three on the hill.

In his own garden, Snodgrass has been able to blend some of his succulents with regular summer perennials by placing them at the edge of a stone retaining wall. Here, they quietly perform as filler plants in a border of coneflowers, black-eyed susans and daylilies.

Behind his farmhouse, a far bigger wall is evident: A sweeping, 250-foot-long dry-laid stone wall he built one winter and spring. He estimates it holds 200 tons of stone he had delivered plus another 100 tons of inner rubble that he collected from the ground of his 145-acre farm in Harford County. It becomes a perfect perch for more succulents, both amid the capstones and wedged into gaps in the side.

Snodgrass is a fifth-generation farmer on this land, and stumbled into the world of succulents in the way that small-time farmers who survive these days find their niches. For years, he raised llamas until he could no longer stand the agony of destroying animals that had come down with a neurological disease caused by a parasitic worm carried by the local deer population.

Faced with the loss of the countryside around him to suburban development, he decided to tap into this new market. Builders, he noted, will take a "nice, wooded piece of property and bulldoze all the trees down." The new homeowners would flock to the garden center to buy containers of plants that would then fry in the absence of shade. "They would go to the ocean for a week and these containers would be toast. So my idea was that you set up container gardens by your pool or deck and you never touch them again," he said. "That sort of plant ended up being the palette for green roofs."

The market potential for custom-made container gardens pales in comparison with the demand for plug plants for green roofs. In the five years since he began specializing in furnishing green roofs, he calculates he has grown enough plants to cover more than 650,000 square feet of roof.

Of course, if these plants were to catch on at ground level, Snodgrass and other growers of these plants might find the sky's not so much the limit but just the beginning.

Emory Knoll Farms' plant catalogue is on the Web at http://www.greenroofplants.com/ .