July 15, 2013
By Mary Jane Clark and Dr. Youbin Zheng

The University of Guelph and Vineland Research and Innovation Centre, with funding from Landscape Ontario, conducted a study to determine if fertilization is beneficial for overwintering survival and growth of Sedum species in a fall-installed green roof.

Green Roofs for Healthy Cities recently reported that in North America there is 48.5 billion square feet of roof space fit for installing green roofs. The area of installed green roofs has quickly increased, encouraged by incentives and bylaws in many major cities.

Strategies are needed to ensure green roof plant survival when installed in the fall. Fertilization with phosphorus, potassium, or both has been shown to enhance cold tolerance of a wide range of plant species, and the type and amount of fertilizer can play a large role in overwintering success. When fertility remains within an appropriate range for individual species no winter injury occurs, although there is potential for winter injury if plants are over-fertilized or nutrient deficient. In general, an adequate supply of potassium reduces the risk of frost damage in plants, and is required for most plant metabolic processes.  

The trial was conducted on the fifth story rooftop of the Science Complex at the University of Guelph. Sedum mats, grown by Sedum Master in Princeton, Ont., were installed on top of a 10 cm depth of Sedum Master’s Pre-Engineered Growing Medium substrate in September of 2010. The mat had a mix of six Sedum species (i.e., S. album L., S. acre L., S. ellacombianum Praeger, S. reflexum L., S. sexangulare L., and S. spurium Bieb. cv. ‘Dragons Blood’). On Oct. 8, 2010, three replications for each of eight fertilizer rate treatments were applied to green roof plots in a completely randomized design.

Plant survival

Green roof plots experienced the greatest cold stress in late Jan. 2011, when the daily maximum (-13.4 °C) and minimum (-21.4 °C) air temperatures reached the lowest point of the winter.

Despite the temperature fluctuations, Sedum plants survived the winter and rooted into the substrate with no plant mortality. In addition, no leaf browning from winter injury was observed for any Sedum shoots in the control. Leaf browning did not differ among fertilized treatments for either S. album or S. acre, and did not occur for S. sexangulare shoots. However, considering all fertilized treatments, S. album had higher (i.e., 13 per cent) leaf browning from winter injury than S. acre (i.e., 5 per cent).

At installation, the average vegetative coverage of all Sedum plants within green roof plots was 80 per cent, increasing to 100 per cent for fertilized plots and 90 per cent for control plots by spring. Over time, vegetative coverage did not differ among fertilized treatments, however, vegetative coverage of the control was smaller than all fertilized treatments in May, Aug., and Oct. 2011. In addition, vegetative coverage for all treatments was larger in May 2011 than Oct. 2010, indicating that Sedum plants grew in all plots between fertilization and spring.  

Fertilizer addition also influenced the coverage of three individual Sedum spp. (i.e., S. album, S. acre, and S. sexangulare) within green roof plots. S. album covered a larger area than S. acre in all treatments and S. album coverage was larger than S. sexangulare in the NPK+Low K treatment. S. sexangulare coverage was only larger than S. album in the control treatment. Between Oct. 2010 and Oct. 2011, S. album coverage increased in all treatments except the control, NPK, or NPK+Medium K treatments, with the greatest increase for NPK+High P and NPK+High K treatments. For all treatments after one year of growth, as S. album coverage increased, S. acre and S. sexangulare coverage decreased, and S. sexangulare coverage increased with increasing S. acre coverage.

At installation, plant height of individual species did not differ among treatments. After seven weeks of growth, S. album shoots were taller in all fertilized treatments, except NPK and NPK+Medium K, than the control. In the following year (May to Sept. 2011), S. album, S. acre, and S. sexangulare shoots were taller in all fertilized treatments compared to the control, however, no difference among fertilized treatments was observed.

Plant biomass

After one year of growth, shoot fresh and dry weights were lower for the control plots compared to all fertilized treatments. Among fertilized treatments, shoots in the NPK+High P treatment had a higher fresh weight than the NPK and NPK+Medium K treatments and dry weight was higher in the NPK+Low P than the NPK treatment.

In 2010, we observed S. album leaf colour differences between the fertilized and control treatments during the fall, with plants in fertilized treatments remaining green longer in the growing season, compared to control plants, which changed from green to red and/or yellow. From May to Oct. 2011, S. album leaf colour in the control treatment was different (i.e., within the red to yellow spectrum) than all fertilized treatments (i.e., within the yellow to green spectrum). S. album leaf colour was not different among fertilized treatments throughout the study.

Visual appearance of green roof plots was not different among fertilized treatments, but was higher for all fertilized treatments than the control in July and Oct. 2011. The difference in leaf colour was a main factor influencing visual appeal of green roof plots, as treatments with greener leaves appeared healthier than those with red and yellow leaves.

In conclusion, Sedum vegetation mats were able to survive the winter following installation, regardless of fertilizer type or rate, after a fall green roof installation in a northern climate. Although fertilizer with or without added phosphorus or potassium was not required for Sedum plant survival over winter, fertilization increased vegetative coverage, plant height and biomass, leaf greenness, and visual appearance compared to the non-fertilized controls in the following year. Therefore, adding a controlled-release fertilizer such as 16N–2.6P–10.8K, without additional P or K, after a fall installation can benefit green roof plants. Future work is needed to determine the influence of fertilization on overwintering for different green roof plant types (cuttings, seed, or plugs).

This work was financially supported by the Ontario Ministry of Agriculture, Food and Rural Affairs/University of Guelph Research Program, Landscape Ontario, LiveRoof Ontario and Sedum Master.
Dr. Youbin Zheng (yzheng@uoguelph.ca). is an associate professor of the University of Guelph, and the environmental horticulture chair for both University of Guelph and the Vineland Research and Innovation Centre. Mary Jane Clark is a senior research technician, environmental horticulture at Vineland Research and Innovation Centre.