THE EFFECT OF SUBSTRATE DEPTH AND SUPPLEMENTARY WATERING ON THE GROWTH OF NINE HERBACEOUS PERENNIALS IN A SEMI-EXTENSIVE GREEN ROOF

Semi-extensive green roof systems combine many of the environmental benefits of extensive green roofs with some of the aesthetic potential of intensive green roofs. Perhaps most importantly they enable a wide range of herbaceous plant material to be grown without significant structural alteration to buildings. An experiment was undertaken in Sheffield, northern England to determine the potential of semi-extensive roofs for diverse herbaceous plantings. A range of drought tolerant herbaceous perennials with different structural characteristics (low growing, medium height and taller ‘emergents’) were grown in two depths of lightweight substrate (100 mm and 200 mm) and with or without limited supplementary watering. Increasing substrate depth without supplementary watering produced relatively few significant improvements in plant performance. However, supplementary watering at both depths promoted growth in the majority of species. Low growing species (those typical of standard extensive green roofs) performed less well under the more productive conditions. The paper discusses the many opportunities for semi-extensive green roofs, particularly where opportunities for additional watering (perhaps from stored rainwater) are available and where public or user access is possible. INTRODUCTION The majority of published accounts of planting on roofs discuss only two possibilities: the ‘intensive’ and the ‘extensive’. Intensive or traditional roof gardens employ engineering solutions to enable standard landscape components of lawns, planting beds, paving and water to be supported at roof level. Such gardens usually require high maintenance, irrigation and feeding, and may differ little in appearance from ground level landscapes. In contrast, the type of roof planting that is of greatest general interest and application is the so-called ‘extensive’ green roof. Such roof greening systems generally comprise lightweight layers that sit on top of the roof and are tolerant of the extreme conditions of temperature, wind and drought that exist on exposed roofs, are relatively low in maintenance requirements and require little or no fertiliser inputs or irrigation. Extensive roofs are composed of thin layers of growing medium (10 cm or less) and therefore the plants that grow on them need to be very tough. A rather restricted range of plants, mostly species of Sedum, is used in most green roof systems. While these are very reliable they do tend to result in monotone effects (apart from the short period when they are in flower). A new concept is beginning to emerge that combines some of the characteristics of the two. ‘Semi-extensive’ roof gardens use the same light-weight materials and layers of the extensive system, and have the same low-input philosophy behind them, but they employ slightly greater depths of growing material (up to 20 cm depth). There is therefore a reduced need (or no need) for additional structural support of the roof and the potential for a far greater and more interesting range of plant material or vegetation types to be grown. As a result, the scope of planting on roofs becomes much wider, as do the benefits in supporting wider biodiversity. They are also much less expensive to construct and maintain than intensive systems. Semi-extensive roof plantings offer the same insulating and protective advantages of extensive roofs but are potentially much more visually attractive. They are therefore best suited to situations where they are accessible and can be seen clearly. Applications include: Proc. IC on Urban Horticulture Eds: R. Junge-Berberovic et al. Acta Hort 643, ISHS 2004 306 • Private or communal roof gardens • Visible flat-roof surfaces • Flat-roof house extensions • Outhouses, sheds, porches and other domestic horticultural applications • Planters, containers and raised beds in exposed urban situations In the UK, the concept of ‘extensive’ roof greening has only recently received attention. Where it has been applied it is generally through the use of systems that are either imported from, or are very similar to, those used in central Europe, and in particular Germany. There is, however, great scope for exploration of different approaches. This is for two main reasons: • The climate of much of the UK is generally less extreme than that of continental Europe, with relatively warm winters and a greater likelihood of regular rainfall throughout the summer • The UK has a long tradition of domestic horticulture, with a relatively high interest amongst the public in the cultivation and care of plants. Roofs and other surfaces that are not directly connected with the underlying soil represent new opportunities for such domestic horticulture. There is virtually no published research in English on plant growth and selection for green roofs. That which has been published relates to tolerance of plants on green roofs to extreme low temperatures in winter (Boivin 2001). Prolonged very low winter temperature is unlikely to be a limiting factor for plant growth on green roofs in the UK. Of greater importance are summer drought and high temperatures. The research reported in this paper is part of a programme of research that is investigating the horticultural potential of green roofs. In particular this paper focuses on the effect of substrate depth and additional watering on the performance and survival of nine drought tolerant herbaceous perennials of contrasting habit in a semi-extensive green roof context. MATERIALS AND METHODS The trial was set up in the spring of 2001 on a flat roof on top of a three-story commercial building in the city centre of Sheffield, UK. The site was relatively sheltered, with an additional fourth storey on two sides of the test plots, and a low parapet (0.8 m) on the other two sides. A single lightweight growing medium was used consisting of 50 % LECA (light expanded clay granules), 35 % green waste compost and 15 % medium loam. A simple timber framework was used to construct six test beds (each measuring 1.5 m x 3.0 m) that contained the growing substrate. A drainage layer of 25 mm LECA was spread over the sealed roof surface, and over this was placed a geotextile membrane to act as a filter layer, preventing smaller particles from the growing medium migrating into the drainage layer below. The growing medium was then spread on top of the geotextile. Three of the test beds were filled with 100 mm of substrate and the remaining three were filled with 200 mm. Each bed was further divided into two subplots, one of which received supplementary watering, and the other which received no additional water throughout. Supplemental watering was given once a week, on the same day each week, by hand. However, when there had been significant rainfall within 24 hours of a regular watering time, no additional watering was given. Irrigation was carried out on six occasions over the growing season of 2001. However, because of regular rainfall throughout the summer of 2002, no additional watering was given. The experiment therefore examined the effect of interactions between shallow and deeper substrate and no watering and additional watering (as required according to the pre-defined parameters of the experiment). The plant species chosen for the experiment all originated from dry habitats, for example dry grassland or steppe, free draining rocky places, or sand dunes, and therefore were expected to possess some degree of drought tolerance. The chosen plants also had ornamental characteristics, either through flower or foliage. None had to have great above-ground biomass because they were only to be supported by thin substrates. One of the main considerations for plant selection however was the structural characteristics of