Huge opportunities exist above city street level for urban greening companies, according to experts at the latest conference of the Sustainable Water Industry Group (SWIG).
Ecologist Gary Grant told attendees at the gathering that while building new parks might cost too much, thousands of smaller interventions such as green walls and rooftop rain gardens could still significantly reduce flooding and mitigate against the "heat island" effect.
He added: "A planner looking at a map of a modern city might say that there's not much greening we can do - we don't have the space or the money. But actually if you have a wall with 150mm to spare you probably can do something."
City buildings are ripe for the picking for the urban greening industry. A 2010 audit carried out by the Victoria Business Improvement District found around 30 per cent of commercial buildings could be retrofitted with green roofs.
In addition, SWIG chairman Neal Landsberg said a "tipping point" is being reached where planners are increasingly fearful about future water supply and climate change-related flooding and know they need green infrastructure to take the pressure off systems already straining to cope.
But while solid evidence is being produced to back up the claims of urban greeners, decision-makers still need convincing that greening is cost effective, said Grant.
The key is what he calls "multifunctionality". If one intervention ticks several boxes, such as cooling, cutting pollution and reducing run-off, it is more likely to be accepted by planners, Grant explained.
Individually, building owners could see multiple benefits from a green roof or wall, such as rainwater harvesting to flush toilets, improved worker health and lower air-conditioning costs.
Historically city planners have separated functions into different zones, but that is not how nature works, he pointed out.
"We're mimicking nature, and nature is very good at doing a lot of things simultaneously," said Grant.
Multifunctionality also opens doors to funding from other city departments, he added. One instance is creating rain gardens that double as traffic-calming islands, which are paid for from the highways budget.
"People always say: 'We wouldn't have the money for greening,'" said Grant. "But actually there is a lot of money in cities. We just need to find new ways of getting hold of it."
Awards - Recognising extraordinary work
The annual Sustainable Water Industry Group Awards recognise "extraordinary projects" that promote sustainable water use.
This year a new urban greening category has been added to recognise city-specific projects such as green walls and green roofs.
Judges will look favourably on projects that bust myths about the costs and benefits of urban greening projects and those that can clearly demonstrate value for money and return on investment over time.
Eligible projects should be currently under construction or completed within the past two years. Nominations close on 30 September, with the winner set to be announced on 25 November. For details, see www.sustainablewater.org.uk.