Black Knight is my favorite and it gets huge. But if you hard prune it down to 12 inches in February it keeps it from getting too big. Same for all Buddleja davidii. I have Asian Moon and Miss Molly and they are said to be sterile. I have had black knight the longest and I cannot find anywhere that says it is sterile, but I never had a single problem and never saw any volunteer seedlings. It was the most attractive shrub I have had as far as butterflies go. I had a Honeycomb and the only thing that it attracted was bees, maybe that is why they call it Honeycomb. LOL.
http://www.oregon.gov/ODA/PLAN...