Against the wall—Root growth and competition in four perennial winter hardy plant species grown in living walls

Abstract

Plants in living walls face challenges from intraspecific and interspecific competition from plants around them, as well as water and nutrient availability in the growing media. This paper explores these challenges using four different species of hardy perennials.Campanula poscharskyana ‘Stella’, Geranium sanguineum ‘Max Frei’, Sesleria heufleriana and Veronica officinalis ‘Allgrün’ were grown in two types of vertical growing media, made of either coir or stone wool, in transparent boxes under greenhouse conditions. In the media, plants were placed above each other, two plants of same species, two plants of different species, or a plant grown alone. Root frequency was registered over 56 days and the activity of individual root systems was studied through uptake of 15N. In addition, plant dry weight and N content as well as water content in the growing media were measured at cessation of the experiment.Shoot and root growth as well as nutrient content in plants were higher in coir than in stone wool and plants placed at the top position had significantly higher biomass than at the bottom position. The stone wool media had significantly higher water content in the lower part of the media while the coir media had water more evenly distributed. Species differences in root frequency were found. Campanula and Geranium showed strong root growth and had root frequencies of up to 0.9, whereas Sesleria and Veronica had less root growth, in some cases only root frequencies around 0.3. The species reacted differently to root competition and planting position and there were differences in the competitive ability between the species. Campanula and Geranium were not affected by competition, whereas both Veronica and Sesleria showed altered root growth due to competition depending on the growing medium. When Geranium was grown above Veronica in stone wool, plant biomass and 15N uptake increased in Veronica indicating environmental modification, with one species improving the growing conditions for another.The results revealed that growing plants vertically in a living wall is complex, and that choice of growing medium and species composition is important for a successful living wall. Planting combinations should therefore be tested before being used in commercial applications.