Christopher Fuhrman wrote:

I'm in USDA zone 4 (Montreal) and last winter killed some of my lavenders
that flourished last summer. I removed the dead plants two weeks ago. The
variant was "Goodwin Creek" lavendula heterophylla, grown and sold in
Canada. I was surprised and disappointed that this wasn't hardy.

According to my gardener, this past winter killed off a lot of lavender.
I'm in zone 5 and my lavenders didn't have any problems in previous
years. This year, a lot of foliage died but the plant survived. The
problem was a combination of bitter cold and lack of snow (which
provides insulation).