Sometime around Labor Day is a good time to trim back lavender. In colder climates I believe this trimming is recommended to be done by Labor Day. There is a little more leeway for my lavender at the beach because of the very mild climate.
The deep purple lavender heads were still so beautiful it was difficult to cut them all off, but I knew that this trimming would keep my lavender plants shapely and healthy. My labor was pleasant with the air pungent with lavender perfume.
Now I can look forward to another year of elegant and luxuriant blooms.