Hi, do I deadhead yellow loosestrife to encourage
more flowers or cut back after flowering.
Depends on your garden style and how tidy you are.
You are unlikely to get much in the way of new flowers, though they are very willing plants.
I just leave mine as I quite like their rusty brown foliage in winter and it suits my rural garden.
I usually cut mine back down to the ground when they’re past their best in the autumn.