This chapter introduces sources for the selection of plants for stromwater BMPs, salt tolerance, and green roofs.
The following agencies provide up to date information on plant material selection for vegetated stormwater BMPs .
There are two specific situations in which these above sources should not be used: high salt concentrations (in spray and soil) and green roofs. Recommendations on salt tolerant and green roof plant material selection are given below.
Locations where salt tolerance is a concern include roadsides receiving frequent winter snowmelt spray, vegetated swales or basins where snowmelt runoff infiltrates the soil, and water bodies receiving relatively large volumes of snowmelt. This discussion is limited to selection of vegetation for constructed stormwater BMPs and vegetated areas receiving runoff from high use transportation routes and parking lots, wet and dry infiltration basins associated with regional ponding, county and state roadway swales and filter strips, and winter road snow dumping areas.
Salt tolerance is not a concern for stormwater BMPs such as rain gardens and infiltration swales in low to moderate use local streets and catchments with little or no salt-laden snowmelt runoff.
Salt tolerance is common to many plants of coastal marshy areas. These species are reliableon the east and west coast in their indigenous ranges. Some of these species are very widespread. The populations found in the Midwest are not necessarily salt tolerant. For inland areas the availability of naturally occurring populations of salt tolerant species is limited.
Salt tolerance has been shown in some of the dry grassland species of the west. The range of these species may include Minnesota. The local populations may exhibit salt tolerance and are recommended.
Salt tolerance has also been shown in some of the aggressive and invasive species found in the Midwest. These species, although amenable to the high salt areas are not recommended because of the stress that may be introduced to native plant communities. Depending on the species, their seeds may travel fairly far by wind or water and are not recommended for rural or urban areas, even if native plant communities are not adjacent.
Stormwater BMPs with high salt concentrations will be susceptible to invasion by exotic and invasive species due to multiple stressors from the salt, along with sedimentation and high phosphorus concentrations and petroleum products. Common buckthorn, one of the aggressive Midwest exotic species of saturated soils, has high salt tolerance. Box elder, a native of lowlands, but often a colonizer in disturbed sites also has high tolerance. Reed-canary grass has moderate tolerance, and purple loosestrife has high tolerance.
The table below lists species and plant seed mixes which should be reliable in soils with high salt concentrations. The tolerance to salt spray may vary, and is shown in parentheses if known. The plant materials listed do not include highly aggressive and invasive species.
Salt tolerance ratings can vary across the country and between investigators, depending on the ways the data are collected and the ratings categories selected. Rating systems are not standardized between various investigators for different plant types (trees, shrubs, herbs) and uses (agriculture, horticulture). The sources sited here were used to represent as best as possible recent research, regional evaluations, and results from specialized salinity testing laboratories.
Note that information on salt tolerance for Minnesota plants warrant some interpretation. Much of the salt tolerance information published nationally is oriented toward agriculture rather than stormwater BMPs. The table below attempts to interpret data from the references noted at the end of this Appendix for applicability to Minnesota. The table below provides sources for seed mixes.
Some common Midwest species are known to be intolerant of high salt soil concentrations. Avoid planting these species or seed mixes when salt is expected to be a stressor.
The green roof BMP has the most specialized circumstances for plant materials and thus requires a very different list of materials compared to on-the-ground BMPs. More than any other BMP, it is unwise to proceed on selecting green roof plant materials without full knowledge of the entire green roof structural design system. This has to do with the very constrained growing conditions for this highly engineered BMP. Note that the following discussion relates to plant selection for green roofs and is not a design sheet for green roof BMPs.
The first consideration on plant material selection is the basic green roof design type. Extensive green roofs (EGRs) have been commonly using xeriscape types of plantings in a shallow, draughty growing medium. These are more appropriate for urban rooftops (See Table E.3 for planting recommendations). Intensive green roofs (IGR) include earth-bermed structures and tend to be heavier and reliant on richer, deeper substrates and may also have shrubs and trees (See also Chapter 12 Guidance Sheet on Green Roofs). As such, the plant materials for extensive and intensive green roof systems are not usually the same. For example, the selections for an earth-berm IGR planting may be quite different from an EGR system.
Secondly, the plant material selection, through biological processes and nutrient cycling, may effect whether the BMP exacerbates or mitigates the function of nutrient storage. Research to date comparing EGRs and control (nongreen) roofs shows that green roofs are a poor BMP for nutrient storage and removal from precipitation. In evidence from both southern and northern climates, total phosphorus concentrations are higher in runoff leaving a green roof compared to control roofs, although the mass loading is the same as the control (Moran et al., 2004). Nitrogen losses from a green roof do not differ significantly from control roofs. So for nutrient loading and design of removal systems, other BMP tools should be located ‘further down the runoff path’ from the rooftop BMP to trap runoff from green roofs. It is unclear how the IGRs function for nutrient storage.
Extensive green roofs are definitely a reliable BMP for reducing peak runoff rates. This has been demonstrated in several controlled studies in both southern and northern climates. Intensive green roofs provide the same function. It is not clear if the plant material plays a significant role in this or whether it is related to the design of the planting medium and underlying roof runoff system. To date it has always been assumed that the overall design should support plant materials that are tolerant of drought conditions and not prolonged saturated soil.
There is plenty of opportunity for experimentation on green roof plant material. Most controlled experiments have been limited in the kinds of plant material tested. The Genus Sedum has been widely used in extensive green roof plantings. It is unknown whether plantings dominated by other and widely different plant groups will yield the same results. As such, the function of a green roof as a stormwater BMP may vary: nutrient storage may not be an issue with some plant materials. Until further case studies and experiments are conducted on the nutrient storage function of this BMP, it is wise to assume that all plant material selections will yield added nutrient runoff, particularly if the plants are fertilized. Thus, the green roof system should be designed in series with other BMPs which are expected to function in this respect.
The variety of choices of EGR plant material for warm and cold climates is generally limited in the following ways:
Plant selection is restricted to materials which will be successful in very shallow substrates, perhaps 6 inches, up to 12 inches deep. Long-lived, perennial drought tolerant species commonly display deep taproot growth or deep fibrous root systems. This is the main reason that the Genus Sedum is so commonly relied upon. In contrast, many of the prairie forbs and grasses valued for infiltration BMPs may not be appropriate for green roofs.
The plant material selection for any one specific green roof is also dependent on the substrate content and depth. Intensive and extensive systems were already defined, and will significantly effect the substrate choices. The plant material selections provided here have been limited to those for EGRs. Even within an extensive system, in which the substrate is in general droughty, the specific design of each system will significantly effect plant productivity. Experiments in which the same plant material was grown on several different substrates demonstrates the importance of this. As such, one of the main criteria for selecting EGR plants is the substrate design. And often, this is limited by the structural integrity of the building, particularly for retrofit designs.
For EGRs, irrespective of the specific design, one general consideration applies to establishing plant material. The material will usually be introduced as young plants, and to reduce transplant shock and provide an enriched environment for further growth, an organic substrate such as compost should be used as the immediate transplant medium.
Colorado State University Cooperative Extension Tri River Area. 2003. Salt Tolerance of Various Temperate Zone Ornamental Plants. Curtis E. Swift.
Johnson, Gary R. and Ed Sucoff. 2000. Minimizing De-Icing Salt Injury to Trees. University of Minnesota Extension Service.
Minnesota Department of Transportation. 2003 Seeding Manual. Office of Environmental Services, Erosion Control Unit.
University of California, Riverside. George E Brown Jr Salinity Laboratory
University of Wiscsonsin-Extension.1999. Salt Injury to Landscape Plants. Delahaut, K.A. and E.R. Hasseljus. Cooperative Extension Publishing.
Bengtsson, L. 2004. Hydrological Response of Sedum-Moss Roof. American Geophysical Union, Fall Meeting 2004, abstract #H41A-0285.
DeNardo, J.C., A.R. Jarrett, H.B. Manbeck, D.J. Beattie, and R.D. Berghage. 2003. Stormwater Detention and Retention Abilities of Green Roofs. World Water and Environmental Resources Congress 2003. Paul Bizier, Paul DeBarry - Editors, June 23–26, 2003, Philadelphia, Pennsylvania, USA
Federal Energy Management Program. September 2004. Green Roofs. DOE/EE-0298.
Moran, Amy, Bill Hunt, Greg Jennings. 2004. Greenroof Research of Stormwater Runoff Quantitiy and Quality in North Carolina. NCSU Water Quality Group Newsletter August 2004.
Shooting Star Native Seeds, 2005. Mn/DOT 2005 Seed Mixes. http://www.shootingstarnativeseed.com/mndot_mixes.htm
Uhl, M, Schiedt, L, Mann, G, Henneberg, M. 2003. Long-term study of rainfall runoff from green roofs. Wasser und Boden. Vol. 55, no. 3, pp. 28-36. Mar. 2003