Hibiscus syriaca, Rose of Sharon?

Yes. H. syriacus.

Thank you! My neighbor has a Rose of Sharon right over the fence, but I hadn't thought of that! Do the seeds come with the dropped flowers, or fall later? Is there any trick for keeping them from germinating?

Lalala, The petals drop , and the fertilized ovary develops over a period of weeks if I remember correctly. The seeds are ripe when the seed capsule turns brown and hardens. I used to just pull the flowers off when they were spent, to avoid seedlings, but I'm not sure your arms are long enough to pull off your neighbor's flowers from the other side of the fence Lol. Preen might work, but it can cause other problems. If you stay watchful, they are easy to pull out or scuffle when they are little, but by the time they make their 2nd set of true leaves, their roots will be deep enough that a trowel will be necessary.

Hibiscus syriacus ripe seed capsule.

You can just scuffle the soil with a sharp Dutch hoe when you see them germinating. That cuts the stems. Don't wait until they start to get woody and well rooted.