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DaveMacKenziefoundedHortech,Inc,inSpringLake, MI, a wholesale nursery specializing in

low-maintenance, water-conserving turf alternatives in 1983. In 2006 Hortech started LiveRoof, LLC, as a horticultural science company to design and manu-facture a more innovative, plant-friendly system for green roofs. Dave answers some fundemental ques-tions for Midwest Roofer.

How did you get into green roofs?Hortech started supplying plants for green roofs in the 1990s. It was 2003 when we got into the business in a very big way as a supplier and consultant for Ford on their huge green roof in Dearborn, MI. At about that time, a contractor to whom we’d been supplying plants asked if I would drive over to Chicago to help out with problems on some green roof projects. When I looked at all the problems the contractor was dealing with, I thought to myself, “I could do better.” In 2006 we started LiveRoof to do just that.

What are the problems on those green roofs?It’s more a question of what wasn’t a problem. The choice of plants wasn’t appropriate for the tough environment of the rooftop, the way the plants were planted was far from ideal, the soil in which the plants were planted was wrong, and the design of the modular trays wasn’t plant-friendly. I realized all these issues had one root cause: No one was seeing the rooftop as an entire biological system. Green roofs have to be more than just separate trays of plants set out on a roof or a vegetative mat rolled out on a roof. A green roof has to be green—a unified, horticulturally sound, sustainable ecosystem in which plants can thrive.

So the green roofs were not “horticulturally correct?”Exactly. The one essential point I should communicate to roofers is this: Plants are living things with biological requirements that nature won’t let us ignore. Wherever plants are planted, they have to be right for the environment, and the environment has to be right for the plants. We need to understand that, just as the structure of a roof has engineering requirements, a green roof has horticultural requirements.

is the soil primarily organic or inorganic?The growing medium for some green roof systems is lightweight and consists of mostly organic material with lots of compost and peat. But this sort of soil tends to shrink in volume, decompose, and wash away. Based on soil science, the growing medium for green roofs ought to be more than 90% inorganic material by dry weight, supplemented by a small amount of disease-suppressive organic material.

are native plants good for green roofs?This would seem to make sense intuitively, but it’s not really true. The plants that thrive on a green roof are drought-resistant because they are exceptional water conservers without deep root systems. That rules out most native plants. Succulent, water-holding plants like sedums and alliums are well-suited to any rooftop environment.

Why are prairie plants known for drought resistance?Plants like purple cone flower and prairie dock are very drought-resistant, but it is not because they are water conservers. They are what we horticulturalists call water “sourcers”—a class of plants with wide-ranging root systems that can go as deep as eight feet down in search of water. Extensive green roofs have soil depth of six inches or less. Water sourcers aren’t right for the rooftop environment and won’t survive for long on an extensive green roof without regular irrigation.

Which begs the question: to irrigate or not to irrigate?Assuming proper plant selection, supplemental irrigation is seldom neces-sary; however, a backup irrigation system for prolonged hot, dry, windy weather is a good idea. In addition, a little bit of irrigation—for instance, just one inch per month during the hot season—is usually sufficient to keep plants hydrated, plump, and thick. That minimizes the opportunity for weed encroachment and lowers maintenance costs.

there’s a big debate in the industry over growing plants from plugs versus cuttings. What’s the difference?A plug is like a single starter plant in a disposable plastic container from a garden center. Several plugs are plugged into soil in a modular tray. A plug-planted system is a set of trays each with rows of individual plugs. The area each plug covers is a monoculture of one variety. In season, the plants may look good but in dormancy, not so good. Since plugs are plugged in rows, you end up with rows that don’t look good when deciduous plants drop their leaves during dormancy.

Starting from cuttings, you can establish an intermingled blend of deciduous, semi-evergreen, and evergreen plants. When modules of plants from cuttings are installed, the green roof looks like a meadow and stays beautiful throughout the year. When the deciduous plants go dormant, they are hidden amongst the others. Starting from cuttings, a green roof can go up with full-grown, fully mature plants; with plugs, it can take 2 or 3 years for the plants to grow.

Putting tHE gREEn in gREEn ROOfS: Q&A with LiveRoof’s Dave MacKenzie