Does anyone know how to gather seeds from this plant? I know they have them in profusion because it re-seeds itself like crazy, so much we have to remove the plants in the fall before they die from frost. I haven't been able to see any seeds from the small pink flowers that show in the fall. I'd like to gather the seeds myself so I can limit the amount of seedlings that show up in the garden. We spent a lot of time weeding them out this past spring.