Green facades are often described as lush vertical oases, but in reality many fail due to poor plant selection
Unfortunately, the strategy of plant selection is not well understood in design and architecture where aesthetics dominate, according to John Rayner, senior lecturer in urban horticulture, Burnley Campus, University of Melbourne.
Green facades can sustainably support the thermal performance of a building. In addition, they help improve the air quality for building inhabitants while helping to reduce pollution in cities. A visually striking facade is always welcome in a big city, but it can’t always be achieved if the plants selected don’t work within the climate.
“Many design professionals will choose flowers for facades and won’t let them grow first,” Rayner said. “They need to choose plants that will meet the functional outcomes for the site – plant selection is crucial to this process.”
If cost is the concern, Rayner said the most affordable and fastest growing plants are climbing plants.
These must be grown in the ground, with Rayner recommending the Boston Ivy, Virginia Creeper and Climbing Fig plant species.
“They’re not that fashionable with building owners, particularly if the plants need to be grown in containers,” he said, pointing to the fact that creepers have massive roots and can quickly outgrow containers.
Other plants that can be grown in containers or in the ground include the Chinese Star Jasmine and Wong Wonga Vine, the two most widely used climbing plants for facades.
“A lot of climbers originate in the deep forest where their whole reason for being is to get to light and elevation,” said Rayner. “When you’re growing plants on buildings, you want even coverage and vegetation across the whole facade; instead climbers will go right to the top and not spread out.”
Rayner believes air plants such as the Tillandsia, which does not require soil and can absorb moisture through its leaves, provide a wonderful aesthetic for cities, but to reap maximum cooling benefits, the key is to go for size through trees and large green leaf vegetation.
Green facades are a testament to this as reported in recent study titled New Green Facades as Passive Systems for Energy Savings on Buildings, which looked at the behavior of green facades as passive systems for energy savings in buildings in the Mediterranean Continental climate.
The research team confirmed a “great capacity of the green facade to intercept solar radiation” during the summer of 2011 with a covering of approximately 50 per cent of the south facade of an experimental cubicle.
“Green facades and green walls act basically as passive system through four mechanisms; the shadow produced by the vegetation, the insulation provided by vegetation and substrate, the evaporative cooling through evapotranspiration, and finally the barrier effect to the wind,” the study states.
The next consideration is sunlight which, according to Rayner, is crucial to the survival of a green facade but is woefully underestimated.
He noted that most facades are being developed in dark inner city canyons, which can further compromise plant growth. As a result, shading and location should also be a decision factor when selecting plants.
The table below further details scientific plant names and their lighting requirements:
So when making the decision to build a façade, who should be involved? Perhaps counterintuitively, Rayner believes landscape architects are not the best people for this job but commends architects on “getting it.”
He cited maintenance concerns, noting that some plants can be technically difficult to install and maintain, with access, irrigation and pruning all potential obstacles.
“I’ve seen a couple of projects that are impossible to maintain,” he stated.
He added that the industry is just now learning what does and does not work and the technologies that help green facades survive and thrive while remaining distinct.
“Singapore has a huge number of facades but the city is moving away from them because they’re dominated by the same plant,” he said.
Rayner cited One Central Park in Sydney as a good example, with facades growing in a relatively small volume of soil, but he pointed out that the long-term success rates of these types of sites will not be known for a while.
The key consideration, he said, is soil substrate.
“Most of the research demonstrates that the container sizes are inadequate for longevity,” he said. “The challenge is not now, it’s in 10 to 20 years time because if we get mass amounts of failure or really expansive plant replacement…replacing plants on a side of a building means containers, soils and plants.
“The best thing to do is get these plants in the ground with a good quality soil based planting location.”