In the experiments reported here, species that naturally grow in shallow, well-drained soils were grown in "dry-down" trials together with sedums and other succulents. The plants were raised singly in containers with a 100mm depth of substrate and were grown for 10 weeks in a greenhouse with irrigation.
The interval between successive irrigations was then gradually increased until eventually no more water was given. Each species was then left without water until no viable green tissue could be seen above ground. At this point, irrigation was restarted and the proportion of plants of that species that revived was recorded.
In general, succulent plants were better able to resist being dried down and were also more likely to recover again. The succulent plants Sempervivum 'Royal Ruby', Sedum lanceolatum and Allium cernuum proved to be ideal for green roofs.
Among the herbaceous plants tested, Antennaria parvifolia, Buchloe dactyloides and Thymus pseudolanuginosus survived for a reasonable time without water and the latter species was thought particularly suited for green roof use.
Dr Ken Cockshull, Associate Fellow, Warwick Crop Centre, University of Warwick
Moisture Content of Extensive Green Roof Substrate and Growth Response of 15 Temperate Plant Species During Dry Down by Bousselot, Klett and Koski (2011). HortScience 46 (3): 518-522.
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