Turfgrass Considerations
Vegetation, specifically turf, is commonly used as a surface for applications with light pedestrian traffic, such as parks or ballfields. For it to be a viable cover under vehicle traffic, the pavement design fundamentally needs to prevent soil compaction so that the living root zone for these plants is both porous and permeable to both air and water. Vegetated permeable pavement has void spaces between a load-bearing pavement material, which distributes the imposed load to the underlying base and/or bedding materials. “A reinforced turf surface bears traffic equally directly, …(and) assists the turf in resisting wear and compaction.”⁵ This support condition allows the plants the ability to stand up to increased traffic weight and volume. Root zone areas for vegetated permeable pavements vary by type of pavement, but the more access to root space, the more likely the turfgrass will survive.

Graphic courtesy of Soil Retention Products, Inc.

 

The soil area between cells is also an important factor to turfgrass health. Vehicle tires are flexible, so when void spaces are too large and overfilled, soil compaction will occur, which cuts off the air and water needed for plant growth. For example, choosing sod to top the permeable pavement for a fire lane (hopefully never used) may be an appropriate design choice. However, if the use is daily parking, applying turf by seed, and not over-filling the void space, is likely to give greater protection to the emerging root system. Choosing the appropriate method of turf establishment for the intended use can be especially critical when the pavement is saturated. With heavy and/or constant traffic, significant compaction in the void space can occur along with turf damage.

Another aspect of turf establishment and maintenance is to realize that the width of the load-bearing portion of any vegetated permeable pavement system is important to retaining turfgrass as well as carrying the traffic load. The greater area of contact between the pavement and the vehicle tire, the better the pressure is distributed and the root zone is protected. A relevant ingredient for healthy turfgrass is a bedding course, defined as the underlying sandy material between the pavement and its often heavily compacted base, which allows for a continuous symbiotic root zone and moisture for the plants. The depth of pavement can also have an impact on the ability for roots and moisture to spread along with similar-sized materials for infill and bedding course. That is because root zones are complex systems, with physical, chemical, and biological components. Each of these components together determines the quality of the turfgrass. Pavement systems which maximize the root zone area, while allowing for filtration and aeration, are likely to result in the best long-term vegetative cover.

Specifying the type of grass species or groundcover, and whether to seed or sod turfgrass over the pavement surface depends greatly on the location and intended use. Many geographic locations receive sufficient moisture to support turf without irrigation. Choosing the appropriate vegetation for the site conditions, and anticipating cold climate factors such as freeze/thaw cycle are also important design considerations. A pavement system can be designed to capture rainwater and collect the runoff for reuse as irrigation. If the vegetated permeable pavement also serves to enhance stormwater regulation, this may be considered an appropriate application of water in an arid environment.

Selection of a turf species must take into consideration microclimates like shade, slope, temperature variations, and seasonal conditions. Parking can create a microclimate that casts shade for a portion of the day over the turfgrass. This may affect the density and growth of some turfgrasses; therefore, specifying the correct species can be an important long-term maintenance decision.

Whether to use a warm-season grass or cool-season grass, bunchgrass or spreading grass type, one that is salt tolerant to de-icing or is shade tolerant, are all design considerations that are site and project specific. Ferguson, in his book Porous Pavement (2005), states that a warm-season grass such as Bermuda stands up well to traffic as does Tall Fescue (cool-season). However, some grasses may be considered an invasive species to native ecosystems. Local cooperative extension agents, state agricultural offices, and landscape architects can offer advice on which species are best.