Hi, This tree just started growing in my backyard (I didn’t plant it). Can you please tell me what it is? Should I remove it?
When trees just appear in our backyards, almost springing out of nowhere and growing quickly, we should consider that it may be one of many invasive species that are common in Toronto.
We cannot tell with certainty from your photograph, but this tree could be the Glossy False Buckthorn, Frangula alnus (as opposed to the Common or European Buckthorn, another invasive species whose leaves have a distinctly different venation). Here is a website that may help you with a positive identification: https://mnfi.anr.msu.edu/invasive-species/GlossyBuckthornBCP.pdf
There are several tree species that are prolific self-seeders in Toronto gardens, and while some are not officially designated as invasive, what they have in common is that they are all fast growing interlopers that appear where you don’t want them. In this case, even without a completely foolproof identification, our advice would be to remove this young tree, being careful to dig up all or as much of its root as possible.
A Toronto garden blogger (and Master Gardener) provides some insights on self-seeding trees, in this case the Siberian Elm, that you might find interesting: https://www.torontogardens.com/2009/04/warning-siberian-elm-needs-firm-hand.html/