The answer is that, like many plants from the south-eastern states of the USA, although totally frost hardy it is used to a very hot summer with high humidity and plenty of moisture at the roots (see my article on summer rainfall climates). Not always easy to provide here. The hottest spots in many British gardens will also be the driest, and hardy hibiscus are often wetland plants in the wild. The result is that your plants might not even appear until June or even July and may struggle to get big enough to flower before the frost cuts them down again.
What's to be done?
a. Choose a place in the garden that gets the longest period of sun possible. A south or south-west facing aspect is good. Anyway, choose one that is totally open to the sky.
b. You should definitely make sure the site is as well mulched and fed as possible. Clay suits them well but may be slow to warm up so add plenty of organic matter.
c. Hardy herbaceous hibiscus often live in seasonally flooded sites in the wild so contriving some sort of mini bog garden with a small pond liner would not be going too far. Extra irrigation would not go amiss and you could grow them in a hole or trench as you would celery.
d. Talking of vegetables, market gardeners have all sorts of tricks for warming the soil for early crops - cloches, black plastic, fresh manure mulches etc.
All this may seem like a lot of faff to go to, and for most ordinary garden plants that would be true but I believe there are some plants that are just so remarkable they are worth the extra effort. Hibiscus coccineus and it's relatives are just such plants. If all else fails you can start them off in a pot in an unheated greenhouse as you would some Hedychium.
H. coccineus a fabulous tall elegant species with huge vivid scarlet classic hibiscus flowers and narrow red tinted Acer palmatum-like foliage.
5L pot ~ too big to post
H.coccineus Texas White
Like the normal red form, but with (you guessed it) pure white flowers. The foliage has somewhat broader segments too, and the whole plant is plain green.
3L pots ~ £8
An amazing hardy species, but where coccineus is a study in class and elegance, moscheutos is blousy and over-the-top. A much shorter plant (about 3ft high and much bushier) with large, soft oval leaves and enormous (8-10 in!) flowers usually in some shade of red pink or white, with a darker eye. Has to be seen to be believed. Hardy and perennial but easier to grow quickly for summer bedding.
H.palustris - may or may not be a subspecies of moscheutos. Anyway, a less 'in your face' form. Palustris means 'of wetland' but I'm not sure it needs more water than the others - just don't skimp on the irrigation with any of them.
5L pots ~ £10
H.moscheutos x coccineus - should be intermediate between the two species. The seedlings so far are indistinguishable from coccineus.
5L pots ~ £8
H.militaris - a medium sized species with large, dark eyed white flowers.
2L pots ~ £5