If you had them last year, the seeds can get around pretty well. The seed pods explode with explosive dehiscence, and critters might move them around in various ways.
The previous year in this spot in OH - also Z5b - were 3 individuals. The following year was the solid carpet in the pic below. And I'd though I was deadheading fairly well...
Where I am now, I can't get a seed to sprout to save my life!
