Herbacious plants? You can keep planting them through this month.

you can grow pot grown herbaceous/perennials plants just about anytime but preferably when temperatures are 50f and the plants are still actively growing

I would add that if we're going to keep getting colder winters then planting after summer or autumn doesn't really leave a good amount of time for their roots to establish well.
I've found that to be the case here in the south west, and it's a very mild climate.

I'd agree Louise plant up to say August but no later.