I just found this article:
https://www.thespruce.com/get-...
I'm going to try this in the fall: "When the flowers of your shrub are done blooming, simply deadhead them. This will nip seed production in the bud and eliminate all those annoying seedlings. Make sure you remove not just the flower, but also the developing seed pod at its base. With rose of Sharon, the seed pods develop in October and take 6 to 14 weeks to mature, so you have a little time once the blooms have withered. But if you do this carefully, you will be rewarded next spring with a garden free of unwanted seedlings."
