Green roofs can provide many benefits to urban communities, including reduction of urban heat island effect, reduction in building cooling costs, improvement in air quality and habitat for winged wildlife, and enhancement of urban biodiversity and urban aesthetics. Green roofs are gaining popularity in Colorado, thanks in part to the Green Buildings Ordinance, which took effect in Denver in November 2, 2018. The ordinance encourages the installation of green roofs on new and existing buildings larger than 25,000 square feet.
Similar to landscapes at ground level, seeds of undesirable weedy plant taxa find their way onto green roofs and take root. If left unmanaged, these weedy plants can become established in a green roof and displace desirable plant taxa. Weedy plants in a green roof are removed by hand; however, such removal is labor intensive and increases maintenance costs. Although there are numerous pre-emergent herbicides on the market, few if any studies have been conducted to determine the safety of these products on plant taxa commonly grown on green roofs.
In 2019, a study was conducted outdoors at Colorado State University’s Horticulture Center, to evaluate a common pre-emergent herbicide for safety on four taxa of succulents commonly grown in green roofs. The plants were grown in 2-foot by 4-foot extensive green roof modules, filled with a modified green roof growing medium.
Pre-Emergent Herbicide Product Evaluated
| Trade Name | Common Name | Number of Applications | Treatment | Amount of Product / Acre | Ounce(s) Product / Module |
| Pendulum 2G | Pendimethalin (2.0%) | 2 (6 weeks apart) | 1X 2X 4X | 150 lbs. 300 lbs. 600 lbs. | 0.40 0.80 1.60 |
Plant Taxa Evaluated
| Scientific Name | Common Name |
| Sedum sexamgulare | Tasteless Stonecrop / Watch Chain Stonecrop |
| Sedum hybridum | Mongolian Stonecrop / Siberian Stonecrop |
| Sedum ‘Razzleberry’ Plant Patent #22,457 | SunSparkler® Dazzleberry Sedum |
| Hylotelephium sieboldii | October Stonecrop / Siebold’s Stonecrop / Siebold’s Sedum / October Daphne |
Study Findings
All rates of Pendulum 2G injured the three species of Sedum and the one species of Hylotelephium evaluated in the study.
Effects of Pendulum 2G on Four Taxa of Green Roof Succulents
| Plant Taxa | Treatment | Effects on Plant Growth |
| Sedum sexamgulare | 1X 2X 4X | Stunted growth; poor root growth; several plants died at 2X and 4X rates |
| Sedum hybridum | 1X 2X 4X | Stunted growth; poor root growth; nodules on stems |
| Sedum ‘Razzleberry’ Plant Patent #22,457 | 1X 2X 4X | Stunted growth; poor root growth |
| Hylotelephium sieboldii | 1X 2X 4X | Stunted growth; poor root growth |
Discussion
While the goal of using a pre-emergent herbicide product in a planted landscape is to suppress the growth of undesirable weedy plant species, it is essential that the product does not cause injury to the desirable plants in that area. According to the Pendulum 2G label, at the rate of 100 pounds per acre, the product is safe for use around Sedum spurium, planted in landscapes. Unfortunately Sedum spurium was not included in the 2019 study at CSU, so the safety of Pendulum 2G when applied at rates greater than 100 pounds per acre is not known. Also not known is the safety of the product when applied to Sedum spurium grown in a green roof growing medium, as well as the safety of this product to roofing membrane materials. Based on the findings from the 2019 study, Pendulum 2G should not be used for pre-emergent control of weedy plants in green roofs planted with these taxa.
