Tuesday, December 1, 2020

Site Conditions, Maintenance Costs, and Plant Performance of 10 Extensive Green Roofs in the Research Triangle Area of Central North Carolina

Compared with traditional roofing, green roofs (GRs) have quantifiable environmental and economic benefits, yet limited research exists on GR plant survival, maintenance practices, and costs related to plant performance. The objective of this study was to assess plant cover, site conditions, and maintenance practices on 10 extensive GRs in the Research Triangle Area of North Carolina. Green roof maintenance professionals were surveyed to assess plant performance, maintenance practices, and maintenance costs. Vegetation cover on each site was characterized.

Relationships among plant performance and environmental and physical site characteristics, and maintenance practices were evaluated. Survey respondents ranked weed control as the most problematic maintenance task, followed by irrigation, pruning, and debris removal. No single design or maintenance factor was highly correlated with increased plant cover. Green roof age, substrate organic matter, and modular planting methods were not correlated with greater plant cover.
Results showed a trend that irrigation increased plant cover. Plants persisting on GRs included several species of stonecrop (Sedum sp.), but flame flower (Talium calycinum) and ice plant (Delosperma basuticum) were also present in high populations on at least one roof each. Green roof maintenance costs ranged from $0.13/ft2 to $3.45/ft2 per year, and were greater on sites with more weeds and frequent hand watering.
Proper maintenance protects GR investments and ensures the GR system achieves its original intent (Curtland, 2013). However, limited data are available on maintenance costs and practices for extensive GRs. Green roof maintenance costs have been reported to be between $0.75/ft2 to $1.50/ft2 per year (Prince George’s County Department of the Environment, 2016). Similarly, Acks (2006) reported annual maintenance costs to be $0.60/ft2 formediumperformance scenario GRs and $0.10/ft2 for standard roofs. As previously noted, mostcost-benefit comparisons have not included the cost of maintenance, and no such data are available from environments similar to the climate of North Carolina (Beck et al., 2018). Therefore, further detailed information is needed to determine maintenance costs of site-specific locations and roof types.
The time spent on maintenance practices ranged from 0.01 to 0.22 h/ft2 per year (Table 4). The yearly cost of maintenance ranged from $0.13/ft2 to $3.45/ft2 per year. Although weeding times for the 10 study sites varied from <0.01 to 0.04 h/ft2 per year, among all the maintenance practices, 100% of the survey respondents ranked weeding as the most timeconsuming task (data not shown). Irrigation and pruning were tied for second most time-consuming tasks, followed by debris removal (data not shown). Due to the different conditions on these roofs, the time spent on irrigation varied from zero to 0.05 h/ft2 per year,

Table 4. Estimates of cost, time, and frequency of the four primary maintenance practices of weeding, watering, pruning, and fertilizing green roofs. Results are from a survey of site managers for 10 extensive green roofs in the Research Triangle Area of North Carolina.
z Maintenance costs were calculated based on survey estimates of overall annual maintenance costs divided by area of the site;  $1/ft2 = $10.7639/m2.
y Overall time spent on maintenance tasks was calculated based on survey estimates of hours divided by area of the site; 1 h/ft2 = 10.7639 h/m2.
x Number of times the activity was performed per year.

These four GRs with high desirable plant cover had maintenance costs per year within $0.13 to $0.28 in common and ‘Weihenstephaner Gold’ orange stonecrop as the dominant desirable plant.

by Julieta Trevino Sherk, Wenyan Fu, and Joseph C. Neal all of Department of Horticultural Science, North Carolina State University, 166 Kilgore Hall, Campus Box 7609, Raleigh, NC 27695-7609 jtsherk@ncsu.edu.
Volume 30, Issue 6; December, 2020; Published online 25 September 2020.
This is an open access article distributed under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/).

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