
Pushing through the concrete is a large tree-of-heaven (Ailanthus altissima). Native to China, this tree can grow as much as 6 feet per year, with mature trees reaching 70 feet tall and living to be over 90 years of age.
Rarely planted, trees-of-heaven are often referred to as weed trees because their ability to readily reproduce and tolerate harsh conditions allows them to grow in large numbers wherever they take root. Their prevalence poses a nuisance to homeowners and maintenance workers who are constantly pulling saplings out of gardens and cutting them back from buildings and fences.
The seed that this particular tree grew from was carried here by the wind, coming to rest in a crack in the pavement. After a cool spring rain, water running into the crack wet the seed, which germinated and took root in the soil below the concrete before rapidly growing into the large tree it is today. In the fall, this tree will be covered with thousands of seeds of its own, which in turn will fall to the ground and find their way into other cracks around the neighbourhood.