Large cities commonly face conflicting land demands for housing and urban landscaping. Rooftop gardens have been extensively advocated as a greening solution, but their effectiveness has been limited by the compacted footprint of tall buildings, and roof greenery is competing space with other roof facilities and green energy installations. With the development of high rise blocks which have high wall to roof ratio, and consequently large potential surface area, vertical planting appears to be a better alternative for urban greening. Vertical planting can be categorized grossly into facade greening which involves the spreading of climbers on wall surfaces, and wall planting which usually uses modular planters in pots or trays for plant growth to form a living cover. Soil and water retention are the major design attributes to the success of vertical greening, and carbon emission must be carefully addressed in designing vertical gardens for urban greening. To illustrate the thermal performance of a vertical vegetation system, a prefabricated turfbased modular system which constituted a vegetated cladding on the facades of a public residential building was taken as an example. Cooling effect was closely associated with the green coverage and moisture in the growth medium, which demonstrated the value of maintaining a healthy vegetation cover beyond visual amenity. Drought stress was the major threat to vegetation sustainability and various environmental factors were identified. In contrast to horizontal planting, a distinct pattern of moisture distribution was observed, which may deserve special attention when determining watering regime. The constraints of vertical planting and their solutions would be discussed in biological and ecological perspectives.
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