Councillor John Parker wrote in the last Leaside Life about the importance of planting the right trees in the aftermath of the December ice storm. It was a good start, but….
He is indeed correct that not all trees are created equal, but the trees he lists as having suffered the most serious damage are almost all weed trees, ones you won’t find in any nursery and that simply “volunteered.”
Birds, squirrels and the wind dropped seeds in our yards, and voila: Siberian elm (which horticulturalist and author Michael Dirr refers to as “one of, if not the, world’s worst trees…”), Manitoba maple, crack willow, and white mulberry. Three out of the four are non-native and all four are considered invasive.
White elm is usually referred to as American elm in North America and is all but wiped out in the city because of Dutch elm disease.
Black locust is not native to Ontario, is considered a minor invasive species and may withstand urban conditions well, but it is very weak-wooded, dropping branches in even minor wind storms.
Leaside is home to many mature Norway maples, which are non-native and now classified as invasive, as most of you who have spent many hours yanking seedlings from your flowerbeds and lawns will agree. It does make a good urban street tree, as do its many cultivars, but its use should be reduced.
So, where does this leave us? We need to increase the diversity of our urban trees and reduce or eliminate the use of invasive alien species. But it’s not quite that simple.
Trees aren’t like cars – you can’t just pick the make and model you like and expect it to be happy wherever you park it.
They all have particular growing requirements, and sadly, some of our most spectacular native species, like Sugar Maple, are quite intolerant of urban conditions like salt and air pollution. While they might survive in your backyard, they’d be a very poor choice for a boulevard tree on a major street. White pine is also intolerant of air pollution and road salt, but survives in backyards in the neighbourhood.
Ash, with quite a number of native varieties that are very well adapted to urban growing conditions, have become a very poor choice because of the invasion of the Emerald Ash Borer.
Rather than rule out the use of non-native species entirely, our focus should be on creating a healthy mix of native species that tolerate urban conditions, including air pollution, soil compaction, salt contamination and lack of water; and selected alien species which do not threaten our native varieties.
A few species to consider include oaks, hackberry, Freeman maple (a cross between red and silver), gingko, Kentucky coffee tree and cork tree.
Paddy Duncan, Broadway Ave., is a graduate of U of T’s landscape architecture program.