Factors favouring vegetation in quay masonry walls: A pilot field study

Walls overgrown with plants can have a positive impact on urban comfort and contribute to biodiversity in the city. In particular, quay walls, thanks to their close contact with water, have the potential to be ecologically engineered to encourage the growth of herbaceous plants. Different factors can affect growth of vegetation on walls. This research aims at experimentally investigating the effect of several variables, including quay wall design, building materials and environmental conditions, on receptivity of brick masonry quay walls for herbaceous plants. To this scope, ten quay walls (size 2 m × 2 m x 0.43 m), have been built and placed in a canal in the city of Breda (the Netherlands). The survival and growth of vegetation and the moisture content in the wall were monitored during a period of about 2 years. The results show that the presence of a layer of soil substrate with high capillary suction, positioned in between the masonry cladding and the concrete structure of the wall, has the most relevant positive effect on vegetation growth on the masonry. Mortar composition and irregularities of the wall surface influence bio-receptivity too, but to a less extent; orientation had only a limited effect. Moreover, the strategy of using a mechanically strong bedding mortar in combination with a weaker but more bio-receptive pointing mortar has proven successful at favouring growth of herbaceous plants, while providing sufficient strength to the masonry.