Seeding the Skies

In 2018, Denver residents voted to pass the citywide green roof ordinance, sparking excitement about a greener, cooler, livelier cityscape. Many rooftops transformed into gardens, farms, and pollinator havens. But despite their benefits, green roofs are notoriously difficult to establish—especially in Denver’s semi-arid climate.

On a rooftop, plants endure full-sun exposure, with substrate temperatures soaring above 90 degrees in summer and plunging below freezing in winter. The shallow, fast-draining engineered soil holds little organic matter. As a result, designers often rely on sod seeded with Sedum species.

While sedum roofs offer benefits, they lack the biodiversity, dramatic colors, and rich textures one might see while visiting Denver Botanic Gardens or hiking the mountain trails of Crested Butte in summer. Colorado’s native species are well adapted to such harsh conditions, yet their potential on green roofs remains largely untapped. Recognizing this, Colorado State University graduate student Maria Schonewise partnered with the Gardens’ horticulture research team to develop native seed mixes capable of establishing naturalistic, resilient plant communities on rooftops.

Working with Manager of Horticulture Research Michael Guidi, Schonewise designed a mix of 34 wildflowers, grasses and shrubs, varying seed densities to test establishment. Once developed, her mix was then sown into approximately 60 plots across three Denver green roofs: CSU Spur, Community College of Denver and in the Green Roof garden atop Offshoots Café at the Gardens.

Throughout the summer, Schonewise tended plots and collected data on species presence, abundance and weed coverage. These findings will help answer pressing questions: Which plants are tough enough to thrive? How can we create biodiverse, low-maintenance communities? What pollinators use rooftop habitats, and what resources do rooftop native plant communities provide urban pollinators? 

Her research could help improve upon the industry standard sedum mats used in green roofs by creating a reliable, hardy and biodiverse method for establishing plant communities, making them just as colorful and rich as Colorado’s various landscapes. Next time you visit the Gardens, stop by the Green Roof garden above Offshoots Café to see this living laboratory in action.

This article was contributed by Horticulture Research Associate Reece Bailey.