May 22 2008
Annual conference review: plant performance
While my interest is in the model development side of green roof research, there were several interesting talks on plant performance beyond the characteristics of depth and slope . Kristin Getter from that other large university in Michigan presented results from sun and shade studies of sedum. She noted that while species dominance changed depending on whether the plants were shaded or received full sun exposure, the absolute cover did not change. However, there were differences in biomass production with a shaded roof clearly producing higher amounts of biomass. For climates similar to Michigan, she recommends s. acre, a. cernuum, s. kamschaticum, s. spurium in shade, and a. cernuum, T. calycinum in sun. While this information is useful to many, I am eagerly anticipating Getter’s results on a green roof’s CO2 sequestration abilities. Too bad we have to wait until next year.
Manfred Kohler spoke on studies evaluating installation using vegetated turf mats versus cuttings. His study also showed effects from sun or shade exposure although this was not the focus. Cuttings of sedum outperformed cuttings of grasses due to the slower growth rate of grasses. Grasses became established with the second year. While mats outperform cuttings in the first year, cuttings provide a greater diversity of plant species. It was observed that the grasses did better on the north facing roof while sedum did better on the south facing roof.
In a separate session, strategies were discussed for creating and maintaining successful green roofs in practice. Unfortunately, the talk was not included in the conference program, and I did not catch the speaker’s name although she’s worked on a number of green roof projects in the DC-Baltimore area. She showed a variety of roof “failures” and her investigations to determine the source of the failure. Wind affected several of her projects through scouring and increasing rates of evaporation. Excessive sun and shade also proved disastrous when plant selection and irrigation schedules were not compatible with the soil media depth and roof’s orientation. While this certainly is a fair challenge for the landscape architect or horticulturist, I also feel that there is a role for the engineer. Of course speaking from the hammer’s perspective, things can only improve upon hammering. Regardless of my bias, engineers can determine from location and building orientation the effects on the rate of evapotranspiration from wind and sun/shade exposure, which would assist in plant selection.