Washington ? Here are some tips from Edmund Snodgrass, co-author of “Green Roof Plants: A Resource and Planting Guide,” on planning a green roof:

¢ Succulents are the most popular plant.

“The best quality of the (succulent) plant is that it lives on existing rainfall. If you get into a lot of irrigation or maintenance, it just drives the cost of the roof up,” Snodgrass said. “Look for plants that can withstand drought, wind and all the things that make surviving on roofs more harsh.”

¢ Shop around for plants that are both attractive and practical. Many of the sedums are shallow-rooted and well-suited to roof plantings. Certain cactus varieties are rugged enough to make it in a thin, inorganic medium. And they don’t go dormant, either, meaning they provide color all year.

¢ If your plant material is 4 inches deep or more, then consider adding some herbaceous perennials (phlox, dianthus, campanula, salvia and potentillas, among others). Kitchen herbs can be a smart addition if you have a half-foot or more of soil depth. Grasses are good, too, although some may need to be mowed and irrigated, and they can become a fire hazard.

¢ Use plugs, two per square foot. They’ll grow into a vegetative roof in about a year and a half.