I would imagine Kopper King is sterile. Other than that, I don't know.
Barb

There are a number of sterile, or supposedly sterile Hibiscus varieties. Lord Baltimore is NOT completely sterile. It will cross with some other Hibiscus, such as Hibiscus coccineus which is one of Lord Baltimore's parents. I made such a cross, and ended up with an extremely vigorous, free-blooming hybrid with more attractive foliage than Lord Baltimore. A drawback it has is that it needs a LOT of room to spread out. Plant Delights sells a tall, sturdy, very erect hybrid known as Red Flyer. I have NEVER had this plant blow down in high winds! It is said to be sterile, but I have been able to get a few seed pods from it. I am NOT sure about the parent of these seeds, but could be something quite unusual! This season I must label my crosses! Hibiscus mutabilis 'rubrus' is a reddish pink blooming plant that will outperform most, if not all other hibiscus varieties in my experience. This variety has bloomed for me in New York from July to December 24th! Talk about bloom power from a semi-tropical Hibiscus that is easy to grow in New York and New Jersey gardens with only a light mulch and good siting! Wow! This variety is perhaps the most sterile of all! I have not yet been able to unlock its secrets! There are also a couple of Hibiscus syriacus/paramutabilis hybrids which perform almost, but not entirely like the mutabilis clone.