Healthy Plants vs Efficient Living Wall: Laboratory experiment and data analysis

Living walls are increasingly adopted as green infrastructures to reintroduce nature in built environment and provide benefits in addition to aesthetic amelioration such as the improvement of air quality or the increase of biodiversity. As for other nature-based solutions, living walls are complex systems in which vegetation plays a pivotal role in the mitigation of adverse conditions. Indeed, the efficiency and the aesthetic appearance of these plant-based strategies strictly depend by the health status of plants used to build the living wall itself. Substrate play a pivotal role on plant health and growth along with environmental conditions, such as light exposure, temperature and humidity. However, sustainability features of substrate are often overlooked, and non-renewable materials are frequently used to produce growing media for living walls. Therefore, this study presents and discusses results obtained during a 120-days laboratory experiment concerning the effects of some innovative growing media on health and growth status of plants commonly used for vertical greening. These innovative growing media were produced by upcycling some organic waste and by-products collected from local supply chain such as cork obtained by stoppers, raw wool, hazelnut shells, hemp stalks, fronds of invasive freshwater vegetation, and coffee grounds. Each waste and by-product have been mixed with regular soil used for the cultivation of ornamental and herbaceous plants (Chlorophytum comosum, Spathiphyllum wallisii and Mentha spicata). The chlorophyll content was considered as useful index to evaluate plant health status. It was monitored on plants set in modules that contain varied innovative growing media through a non-destructive procedure performed by an optical manual metre. On the other hand, the plant growth was assessed on the same plants by monitoring the “Total Leaf Area” (TLA) and the “Number of Leaves” that were used to calculate the “Mean Leaf Growth Index”. This index was elaborated to provides indications about the mean growth of plants during the experiment running. Moreover, two-way ANOVA was performed to statistically assess the effect of these innovative substrates on chlorophyll content and TLA. Promising results were obtained for some innovative growing media such as those containing hazelnut shells, grinded cork and hemp stalks. Indeed, all three plant species set up in these growing media presented positive results concerning the chlorophyll content and the “Mean Leaf Growth Index”. These outcomes suggest implementing further studies to move towards a techno-ecological strategy that consider plant physiology an essential indicator to design sustainable and efficient living walls. Moreover, this study proposes a ground-breaking methodology supported by the circular mindset that can be adopted to develop further substrates based on the upcycling of undervalued materials at local scale.