I wouldn't say that the winters are "harsh" or "bitterly cold" but I suppose it depends on what you're comparing it to. It can get quite snowy. In more northern or eastern Ontario it can be a lot colder.
The summers are usually quite hot, but not very long. Humidity is high. Smog is likely.
There are lots of homeschoolers and organic junkies I think. Not sure about Mississauga specifically, although I did once visit a store in the Port Credit area of 'Sauga called "Planet Organic".
In the city of Toronto there are lots of organic food stores and you can find a lot of organic products at regular grocery stores. The junkies, well, you never know where you'll find them

"Going green" is everywhere, so it's becoming pretty common to run into Natural Family Practices. Breastfeeding is widely accepted.
There's another Natural Family Living board that you can get more info from and probably meet up with the moms too. I lurk there a lot, post occasionally, but have yet to meet up (I'm not all that social).
Annah- I can help you if you want to PM me with some questions. I've lived in the city for 11 years - I don't know the suburbs that well personally but I can probably shed some light.
