Plant selection depends on a few factors. First, a project team should determine what type of green roof is best suited to its project: intensive or extensive. Intensive green roofs have deeper planting media depths and are similar to traditional landscaping. Extensive green roofs, designed to boost building performance and environmental sustainability, use shallower depths and require less maintenance.

Next, teams should consider their roof’s microclimate, which is determined by several factors, including average temperatures, wind levels, sun intensity, and rainfall. The U.S. Department of Agriculture publishes its Plant Hardiness Zone Map (planthardiness.ars.usda.gov) based on the average annual minimum winter temperatures, divided into 10-degree zones; lower-numbered zones are colder.

However, depending on roof height, the conditions on top of buildings may be very different from conditions on grade. Rooftops may require plants suited to a different zone than the landscaping planted several stories below on ground level does. Green Roofs for Healthy Cities offers a database of green-roof professionals (greenroofs.org/index.php/find-greenroofprofessional), who can help sort out the technicalities of selecting plants for rooftops.