Is it too late to prune cedar hedges about 1/3 from the top?
The following answer is from Toronto Master Gardeners Guides.
If you do it right away, you should be fine. It’s best to finish pruning about 6 weeks before freeze up and here in Toronto, our first hard frost is around the end of October.
However, there’s always a chance it might be earlier, and there is no harm in leaving the pruning until the spring.
Here is a link to a Toronto Master Gardener Guide on Evergreens Suitable for Hedging, including information about pruning:
and here is a link to Mark Cullen’s Gardenblog entry on pruning cedar hedges: