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US (NC): More plants for stormwater projects

North Carolina’s stormwater design manual gives technical guidance to professionals on stormwater control measures (SCM). SCM are structures that treat and manage the water that runs off parking lots, roof tops, and other impermeable surfaces during and after a rain event.

Green stormwater infrastructure measures like bioretention cells and stormwater wetlands use plants, soils, and natural systems to slow and collect runoff, filter out pollutants, and increase water infiltration.

In North Carolina, the individuals who design these green SCM tend to stick to plants listed in the state’s stormwater design manual. In many cases, the designers are engineers or landscape architects who have little or no familiarity with the plants. As one stormwater engineer told us, “I just pick plants off the list in the manual.” Otherwise, they believe they run the risk of their design not getting approval from the state.

Until recently, the plant choices in the manual were limited. For bioretention cells, there were eight tree species, 22 shrub species, and 26 herbaceous perennial species. There were similarly short lists for stormwater wetlands, another SCM that relies heavily on plants. The limited palette made it difficult to create diverse and attractive plantings. It also made it unlikely designers would use other plants that were appropriate for the projects if the plants weren’t listed in the manual.

In 2016, the NC Stormwater Program began revising the design manual at the behest of the state legislature. Debbie Hamrick, a green industry veteran and advocate for green infrastructure, saw an opportunity. She approached Annette Lucas, the NC Stormwater Program Supervisor, and helped her understand the limitations of the plant lists. Lucas welcomed the input and was very open to bringing in outside experts to review the plant information.

In a series of meetings in 2017, a group of nursery and green industry professionals worked with Annette Lucas and members of NC State’s Stormwater Engineering Group. They reviewed sections of the manual that pertained to plant material and made suggestions for changes. The guidelines for bioretention cells and stormwater wetlands received the most attention, as they are the most plant-heavy SCM in the manual.

Hoffman Nursery participated in the design manual review group and helped expand the recommended planting lists. They were already growing many grasses and sedges that would be appropriate—they just had not been included in the manual up to then. The revisions also included clarifications. For example, earlier plant lists included, “Carex spp.” With hundreds of Carex species native to North Carolina, that guidance wasn’t particularly helpful. In the case of bioretention cells, Hoffman specified 15 Carex species that will perform well.

Instead of just listing genus, the latest version lists individual Carex species.

The group’s input more than doubled the number of tree, shrub, and herbaceous plant species on the lists. The revisions added information on which plants provide support for birds and pollinators and updated guidelines on installation and plant specifications. The revisions have given engineers, landscape architects, and SCM designers better information and more choices. State officials expect it to improve the functionality and appearance of SCM in the state. Hoffman also hopes it will increase demand for green infrastructure plants regionally.

The updated plant lists included more tree, shrub, and perennial species for bioretention cells.

To see which grasses, sedges, and rushes are in the NC Stormwater Design Manual and which ones we grow, download the PDFs:

To see the full lists, go to the NC Stormwater Design Manual webpage.

For more information:
Hoffman Nursery
5520 Bahama Road
Rougemont, NC 27572
Phone (919) 479-6620
Toll-Free 1 (800) 203-8590
Fax (919) 471–3100
[email protected]

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