I have the generic variety and it rarely has a chance to set seed -- it tends to get very tall and robust with tons of blossoms, then it all flops to the ground, still full of bees. I carefully (to avoid the bees) cut it back to 4-6 inches, toss the leaves to my chickens and compost, then wait for it to recover. I think I got 3 flops last year. Sometimes the rain causes the flop, other times I think it just gets top-heavy with blooms. I do often find young seedlings close by and the plant increases in girth every year, but I only allow about a 1 1/2 foot diameter and dig out anything growing outside the 'circle of death.'
