Urban Roofscapes senior engineer Joris Voeten gave several examples of how innovative designs, some from the UK, are helping trees in Dutch cities solve environmental challenges.
With its X-shaped underground concrete supports, the Treebox HP system from Leicestershire-based supplier Polypipe has been used in Amsterdam streets for some years, he said. "It creates an uncompacted root space – it’s foolproof."
Meanwhile Green-tech’s ArborRaft system of geo-cellular units "allows cars to park between trees without soil compaction, giving you healthy trees within the urban streetscape", while also storing rainwater, he said.
On this latter point, he added: "If we want trees to perform functions, water is next most important after soil and root space. Without water, they won’t bloom, transpire and so on."
With this in mind, Voeten now advocates "blue-green" rather than green roofs. "Landscape architects want a wider species selection which means more soil. The structural engineers aren’t happy about that because it’s really heavy, and so has cost implications."
This can be got round by including a drainage layer separate to the soil, but connected by capillary cylinders, he explained. This has been deployed in the town of Heerlen in southeast Netherlands, where rooftop trees "are doing very well" – though had needed extra watering during the recent hot dry summer, he said.
Meanwhile Amsterdam is creating entirely new islands to accommodate its growing population, but in the case of IJburg it is installing a central green square of mature trees first. "They will then build the rest around it," he said. This will act as an invisible rainwater reservoir "as the city doesn’t want rainwater discharged into sewers".