Choosing Trees and Growing Media for Intensive Green Roofs

Incorporating trees into green roof designs and building facades is not only appealing, but there is more demand than ever to plant roof-top trees. The Bosco Verticale (a.k.a. “vertical forest”) in Milan, Italy is home to 700 trees, 4,500 shrubs as well as 15,000 herbaceous plants. Designing for tree survival and longevity, however, is challenging. Intensive green roofs, which can include trees and shrubs, require deeper planting mediums and more maintenance than extensive green roofs. The new vertical forest design movement integrates trees, shrubs and other plant material into multiple façades of a building and requires treating each floor differently because of variable micro-climate conditions.

Harsh microclimatic conditions can include varying sun and wind exposure, which can result in higher rates of evapotranspiration. Low nutrient availability due to the absence of natural soil-derived inputs and limited soil volumes make the planting medium critically important. As more buildings are designed with trees taking centre-stage, arborists, urban foresters and landscape architects will be tasked with creating conditions that can support tree growth, as well as selecting trees that can withstand other conditions not easily ameliorated. Here are some key considerations for selecting growing media and trees for rooftop projects.

Growing Media Quality and Volume

Growing media quality is a primary determinant of the success of tree plantings. Substrates should have the right air and water balance, which is easily achieved by ensuring mixes have adequate amounts of organic matter. Organic matter is also a critical component of plant-microbial interactions which help trees access nutrients and tolerate challenging conditions (e.g. drought stress). The substrate needs to be dense enough to anchor tree roots in fixed volumes, as well as light enough to fit the design requirements. The width of the growing space is critical for the structural roots of trees to become properly anchored in the soil. Therefore, space for lateral root spread is more important than having a very deep container. Tree selections should be made with this volume in mind; compact, non-tap-rooting species are preferable. The substrate volume has to match the trees requirements in terms of rooting space required and moisture regime. According to James Urban, FASLA, mature shade trees require at least 30 m3 of soil to meet their physiological requirements in the built environment.

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