The dynamics of planted and colonising species on a green roof over six growing seasons 2001–2006: influence of substrate depth

Fifteen herbaceous perennial grass and herb species were planted into experimental green roofs in spring 2001. The species differed widely in their origins, heights, flowering times, life spans and growth forms but all were typical of dry and nutrient-stressed habitats. Three individuals of each species of a standardised size were randomly assigned to a planting grid at 20 cm apart in each experimental replicate plot at substrate depth of either 100 or 200 mm. Each treatment was replicated three times. During each growing season, the mean height and spread of each individual was recorded, together with flowering performance and % vegetation cover. In addition the numbers and % covers of all spontaneous colonised species were recorded. Greatest survival, diversity, size and flowering performance of planted species occurred at 200 mm depth. Bare ground and moss cover was greatest at 100 mm, as was diversity of colonising species. Differences between the early years and the final years of the experiment indicate the need for long-term monitoring of green roofs in addition to short-term experiments.