I read that cedar hedges should be pruned/trimmed in September. Is it too late to trim them today, Oct 18th? The temp today is a high 6 going down to -3 Celsius. Day time temps for the next few days ranging from12 to 17 degrees Celsius
To be prudent, an October pruning may be a little late in the season for your cedar hedge. The reason is that pruning encourages the growth of new shoots, and these new shoots will be more vulnerable to cold winter temperatures than well-established growth from earlier in the year. Some gardeners do prune their cedars at this time of year, but a cold winter may result in the die-off of any new tips that emerge over the next few weeks.
Here is a response by the Toronto Master Gardeners to a similar question, plus some good advice on care: https://www.torontomastergardeners.ca/askagardener/pruning-and-fertilizing-cedar-hedge/.
In his popular blog, Mark Cullen discusses pruning the cedar hedge in September: https://mark-cullen.blogspot.ca/2010/09/cedar-hedges-investment-that-grows.html.
Finally, here is a link to one nursery’s take on optimal timing for pruning cedar hedges, with a clearly-written explanation on the way that pruning encourages new growth and why a late fall pruning may be ill-advised: https://www.richmondnursery.com/index.php/en/articles/gardening/item/70-pruning-hedges