Jennifer Ryan and JeanMarie Hartman. 2019. Comparing the Performance of Shrubs and Trees in Parking Lot Plantings: Implications for Design. Watershed Science Bulletin, Center for Wateeshed Protection, Ellicott City, MD.

Parking lot, runoff, trees, transpiration, stormwater

Greening parking lots has many benefits such as reducing stormwater runoff; increasing evapotranspiration, infiltration, and interception; lowering urban heat island effects; and making a harsh environment more habitable. Historically, trees represented most parking lot greenery. Unfortunately, much of the intended cover by trees is not realized because of inadequate soil volume, low quality soil, drought conditions, and parking lot management practices. This project compares tree versus shrub performance in parking lots and parks. Our primary question is: Would shrubs sometimes provide faster and more reliable ecosystem services than trees in parking lots?

The transpiration rates of individual leaves of trees and shrubs were measured during the summer of 2016 in Middlesex County, NJ. The study sites included parking lots and nearby parkland. The species investigated include common species of trees and shrubs utilized in this area. Specifically, the study examined three tree species (Acer rubrum [Red Maple], Gleditsia triacanthos var. inermis [Thorness Honeylocust], and Zelkova serrata [Japanese Zelkova]) and four shrub species (Euonymous alatus ‘Compacta’ [Winged Euonymous], Ilex glabra [Inkberry Holly], Spiraea japonica ‘Gold Mound’ [Gold Mound Spiraea], and Rosa rugosa [Saltspray Rose]). Transpiration rates were combined with canopy measurements to calculate rates of total tree transpiration per day. The results show surprising similarities between the amount of transpiration from parking lot trees and shrubs: When comparing whole canopy transpiration rates, similar transpiration rates could be achieved by dense planting of shrubs in existing tree pits or strips or individual trees in poorly designed tree pits or strips.