One of my favorite cold hardy hibiscus is also my tallest! It's called Hibiscus"Raspberry Rose" and gets surprisingly HUGE. It's is a dieback perennial (unlike the non-native Rose of Sharon which has a tree form), but seems to get more than 6 feet tall with ease!
The flowers have really unusual divided petals that give it a different look than most hibiscus. Apparently it's a cross between the Texas Star Hibiscus (Hibiscus coccineus) and something else, but your guess is as good as mine. It has MUCH larger leaves than a hibiscus coccineus, but the flowers do have a similar shape.
The blooms start in August and keep on going until Frost so it's definitely a later bloomer, but worth it!
Here's a timeline:
Aug 8
Aug 10
August 13
