Plant on a Hot Tin Roof

Gardening in Austin is a perpetual and mysterious experiment. Our confusing climate, varied and inhospitable soils, and brutal terrain force gardeners to come up with creative solutions to many horticultural problems not faced in cooler, less geologically challenged areas of the world. Those same areas with milder climates, such as Chicago and New York, also happen to be epicenters of popular gardening magazines, books, blogs and websites, which find their way into our hopeful-gardener souls—creating visions of verdant vistas and explosive wildflower meadows. What results is typically a heartbreaking chasm between what we might want in our gardens (hydrangeas, camellias, hostas), and what we can actually have. 

Perhaps nothing, though, has fostered such a gut-wrenching reality check quite like the relatively recent advent of the “green roof.” Green roofing began in Germany in the 1970s, and is now a growing horticultural trend worldwide. And if industry claims are true, it’s popular for good reason. Urban cooling, storm water filtration and reduction, interior cooling, and lower utility bills are but a few of the benefits noted by green-roof manufacturers. So…can we install successful green roofs in our region? And if we can, should we? Can our sweltering, fry-pan rooftops possibly support the same oceans of green that we see in those tempting photos of high rises in Germany and Denmark? 

According to three prominent green-roof pioneers in Central Texas, the answer is a resounding yes. Local garden designer and green-roof expert, Casey Boyter, insists that anything that can be grown on the ground can be grown on the roof, as long as there is ample structural integrity and adequate horticultural resources, such as soil quality and depth, as well as enough water for your chosen plant palette. On her rooftop projects, Boyter has grown a wide variety of plants—from vegetables and hardy perennials, to Hill Country native xeric species such as beargrass (Nolina texana) and bunchgrasses. She’s also quick to note the importance of a good soil mix that will hold up over time without shrinking or rotting. 

Designer Pat Kirwin—who has installed several notable green roofs and rooftop gardens in Austin—says the key is to keep it simple. He has a rooftop soil blend he developed with the folks at Geo Growers—it’s a no-frills blend of mostly coarse, easy-draining minerals and just enough manure compost to keep the microbes feeding and the nutrients flowing. Although his plant palette is often a mix of the hardiest of desert bromeliads, agaves, bulbines and other Texas-tough succulents, he’s had great success with fruit trees, tender vegetables and perennials where deeper soil profiles are possible. Boyter and Kirwin both agree that in the hotter months, most plants will require extra watering to stay alive and verdant. A dash of afternoon shade doesn’t hurt, either.

For larger commercial and residential installations, Georgetown green-roof manufacturer, David Scott of Joss Growers and LiveRoof Texas, recommends using modular trays of pre-grown plants. Scott employs hardy native grasses, sedges and succulents in his trays—grown at his nursery prior to installation. Scott’s method, which has been tested on large installations in San Antonio, Houston and Dallas, includes achieving full coverage of the soil surface by the plants before installing the trays. This allows him to keep the growing medium (soil) relatively moist and cool throughout the day, which places less stress on plant roots and those all-important soil microbes.

Of course, we can’t deny the harsh reality of life on a Texas rooftop: hotter in the summer, colder in the winter and windier year-round than anywhere else in the garden. Add to that a roof’s necessarily thinner soil profile, its slope and irrigation challenges, and we get back to the question of should we build green roofs in our area? The fact is, not much has been published analyzing the overall costs and benefits of green roofing in the South. Until that data exists, we have but a few metrics to judge by, such as the aforementioned storm water filtration and evaporative cooling. And of course, there are those ethereal, immeasurable qualities—beauty, novelty and emotional well-being and so on—that come with hands-on participation in nature. Regardless of which side of this complex rooftop fence we fall, though, rest assured that local green-roof pioneers are working hard to fine-tune the plants, materials, technology and methods to help bridge that ever-present gap between what we see in popular media and what actually thrives on—or above—our sometimes tricky local landscape. 


by Matt Welch