Keep a green roof thriving with the right soil and plants

It's been a few years now since green roofs have become more viable option than oddity. They absorb storm water and insulate the interior climate of your house while adding more green space to the world, rather than one more impervious surface. And they're beautiful — or can be.

Unfortunately, many green roofs quickly turn brown, and I've heard tales of planting and replanting and still not getting it right. Just think: scalding sun, drought, shallow soil and whipping winds add up to an inhospitable environment for living things. But without thriving plants, green the roof is not.

Which is why I've been paying close attention to the green roof on a picnic shelter at Clinton Beach, a little park at the end of the Whidbey Island ferry dock. Planted two years ago, the roof's a rich tapestry of color and texture in all seasons. Whoever planted this, I thought, clearly knows what they're doing.

Jason Henry of the Berger Partnership was landscape architect on the project. He worked in collaboration with Taproot Design, a Whidbey Island husband-wife team of Matthew Swett and Sarah Birger, architect and garden designer, respectively. Their own studio has been sprouting a green roof since 2000, so they've had a chance to vet plants over time.

What makes the picnic shelter roof so beautiful? "Sedums, sedums, sedums," says Henry. "I've almost given up using anything else." Not only do sedums offer the best chance of success, but they come in so many colors and textures that you'd never guess the roof consists of mostly one kind of plant.

One of Henry's colleagues recently returned from an international green-roof conference to report that Europeans, too, are extolling the virtues of sedums. They're using shallower profiles (meaning less soil) so fewer weed seeds can settle in, and planting thickly with sedums, which need little water or nutrients. The idea is that, after a year or so of watering and weeding them, the sedums can be left alone up there and get along just fine.

The picnic-shelter roof holds just 4 inches of soil, the minimum to cover the root ball of a plant that comes in a little plastic pot. In Europe, they're grinding up sedums and spraying them on ever-shallower soil. With deeper soil you have more plant options, but your roof will weigh much more and attract more weeds.

What kind of soil works best up there? Henry is still experimenting with various mixes. He's found that gritty soils similar to those formulated for bonsai work best. High-mineral, low-organic content is vital because you don't want the soil to shrink very much, which is a characteristic of rich soils. "There's not an absolute recipe," says Henry, but he recommends mixes devised by Specialty Soils in Covington (253-638-2272).

Designer Birger says most people's biggest mistake is scanty planting. "Don't skimp, put as many plants up there as you can afford," she says. "You want plants to fill in as soon as possible because weed seeds blow into bare soil." Even with generous planting, you'll have to get up there and weed a couple of times the first year or two. "Just enjoy the breeze and the view," Birger advises, with the experience of caring for her own green roof for the past eight years. The price of green roofs is coming down, she says, concluding, "I don't plan to replace our studio's green roof in my lifetime."

Valerie Easton is a Seattle freelance writer and author of "A Pattern Garden." Her e-mail address is valeaston@comcast.net. Benjamin Benschneider is a Pacific Northwest magazine staff photographer.