I am wondering what variety of tree has heart-shaped leaves, with serrated edges, that has been giving
off lots of grey-ish fluff the past few weeks in Toronto?
I believe what you have seen are the airborne “cotton” puffs that carry the seeds of Poplar trees . Poplars are most easily distinguished by their leaves, which are oval, tapered to a sharp point, with fine teeth that line the edge. Mature trees have long, straight trunks and narrow crowns. Poplar fruit are egg-shaped pods that contain cottony seeds that mature in May or June, and each seed has a tuft of silky hair.