Earlier studies have reported that deeper substrate promoted coverage and survival of plants in extensive
green roofs. However, little is known about the influence of substrate depths on the distribution of roots
and the physiological response of green roof plant to severe drought stress. Two controlled experiments
using simulated green roof were conducted with the objective of evaluating the effects of substrate depth
on growth, morphology and drought tolerance of Sedum lineare. It was found that plants grown in
shallower substrate (4 cm) had better biomass accumulation and bigger root size (total length, surface
area, volume and number of tips). This was likely because shallower substrate allowed more roots to
distribute into the drainage layer with a reservoir where extra water can be stored under sufficient water
supply. Under persistent drought stress, however, shallower substrate led to poor performance of the
plants. With the addition of a 5-week continuous drought treatment, plants under the 4 cm depth
substrate treatment displayed lower relative water content, root dehydrogenase activity and higher
malondialdehyde (MDA) content than plants under the 10 cm depth substrate treatment by the end of the
drought period. Therefore, if green roof is established with a drainage layer that could store water and is
provided with moderate supplemental water, shallower substrate can be used to improve biomass
accumulation and promote initial plant growth. If the water supply is restricted, however, deeper
substrate is recommended to ensure better drought tolerance performance of plants in extensive
green roof.