Home buyers love ravines. A typical Toronto home listing promises “Stunning Classical Gardens Overlook Breathtaking Ravine.” But as picturesque as they are, the ravines have problems. To cite one example, for years cities and landscape architects across Canada have planted Norway maple, a tough species good at surviving on the mean streets. In fact, Norway maples succeed too well, and move into parks and ravines en masse, where they shade out native plants. Caterpillars shun Norway maples’ waxy leaves, and then birds get no dinner. Thus the alien tree weakens the food web. 

Toronto’s Biodiversity Strategy acknowledges that “non-native, invasive organisms pose a significant threat to Toronto’s natural systems and urban forests.” Jane Weninger, a senior planner for strategic initiatives at the City of Toronto, says, “We are in a global biodiversity crisis, and Toronto needs to do its part.” 

Simply requiring developers to plant “native species” is not enough to keep out foreign interlopers, however. Tree nurseries and garden centres sell red oak trees, for example, that they grow from “liners” (15-centimetre bare-root saplings), often imported from, say, Tennessee. The acorn that grew the oak came from far south, so the tree may flower earlier than a tree grown from a local acorn. This puts the red oak out of step with pollinators. An oak from a local seed is also adapted to local threats, such as diseases. Hence Toronto’s Green Standard, which aims to promote healthy ravines by surrounding them with trees and plants “from ecodistricts that are adjacent to Toronto.”

Some tree planters already follow such rules. Forests Ontario plants millions of trees across Ontario each year; its nursery partners grow all its saplings from tree seeds collected locally. Mark McDermid, Forests Ontario’s stock and seed co-ordinator, notes that his task is easier, growing just 15 or so tree species. “For the landscape industry to source these seeds is more difficult, because they may need over 100 different species,” he says. “It’s going to be a totally different way of doing business for the growers. Now they’ve got to go into the seed business, or hire tree seed collectors.”

The current seed collector base has limited capacity, McDermid adds. “They are kind of a dying breed.” The collectors, however, will soon have more work: in November, Toronto City Council approved a deal whereby Forests Ontario will supply and deliver to the city about 200,000 source-identified native trees and shrubs between 2022 and 2031.