The largest sources of nectar around here are clover, black locust, tulip poplar, persimmon, and catalpa. Basswood is sometimes, but it's rare around here. There are any number of other minor sources of pollen and nectar, but the big flow is from trees and clover.
Up in the Great White North your main sources are going to be different, as most of what grows here does not grow there. Likely you have clovers of various types, maybe basswood (linden), raspberries (I used to pick gallons of them along the roads in Sault Ste. Marie when I lived there), and some field crops, especially alfalfa grown for seed and rapeseed. Ornamentals can be a nectar source, but it depends on what is grown and how much nectar is made, and they are far fewer in number than things like trees or wild raspberries.
You can tell when the major flow is over with healthy hives -- they go from bees blasting out in a constant stream with clouds of them returning to just a few in each direction. The hives get crankier too!
Peter