City Hall Roof Garden Cools it Down

Ornamental catnip, loved by bees, blooming on Toronto City Hall green roof.

Ornamental catnip, loved by bees, blooming on Toronto City Hall green roof.

While the goings-on at City Hall get heated in the summer of 2013, the plants growing on the vast roof garden are keeping it cool overhead.  Toronto City Hall’s roof garden has been in place since 2010 and it really is worth a trip up that curved, modernist ramp when you’re in the area. It’s the only way to see it, and what a pleasant surprise it is.

green roof, Toronto city hall, chives blooming

Chives: swaths of pink orbs bloom in June on City Hall’s green roof.

Roof gardens are a practical solution for the hot, concrete city, and they offer a lesson for gardeners who have hot, dry locations on the ground. You can’t go wrong by copying what’s growing here at City Hall. Swaths of tough, drought-resistant perennial plants cool down the concrete, (alleviating the Urban Heat Island effect UHI) while creating oxygen and making pollinators happy. Here’s what makes Summer in the City so hot:

the progressive replacement of natural surfaces by built surfaces, through urbanization, constitutes the main cause of UHI formation. Natural surfaces are often composed of vegetation and moisture-trapping soils. Therefore, they utilize a relatively large proportion of the absorbed radiation in the evapotranspiration process and release water vapour that contributes to cool the air in their vicinity. In contrast, built surfaces are composed of a high percentage of non-reflective and water-resistant construction materials. As consequence, they tend to absorb a significant proportion of the incident radiation, which is released as heat.

At City Hall, short species like chives, ornamental catnip, sedum and prairie smoke bloom in early summer, while tall, graceful grasses take the stage in late summer and fall.

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