Do Plant Traits Help to Design Green Walls for Urban Air Pollution Control: A Short Review of Scientific Evidences and Knowledge Gaps

Green walls (GW) and living wall systems (LWS) are often claimed to have a positive effect on urban pollution problems if their plants composition is optimal (design of the LWS). An in-depth survey of the knowledge on plants traits maximizing GW effects on air pollution shows that this statement is made too quickly: there are still some large knowledge gaps. Robust conclusions can only be drawn for particulate matter (PM): leaves containing trichomes/hairs are the most effective to capture PM. The rougher the leaf is, the more PM it catches, and small leaves collect more PM than large leaves. After having analyzed the plant composition of six LWS existing in Belgium, we noticed that this knowledge on efficient plant traits is not used to design them with the goal to maximize their PM capture. In addition, studies about other pollutants and other traits are required to optimize the GW plant composition and to maximize their effects on air quality.