In message , Nick Maclaren
writes

Secondly, the problem is primarily the weather. Despite the fact that
we have been having very warm winters recently, we have also been
having very cold springs. That shrub comes from relatively hotter and
drier conditions (as its name suggests) and won't put out leaves until
the weather warms up. If you think that it has problems, think of my
poor Erythrina crista-galli, which has only just got a 2" shoot :-(


Hibiscus syriacus is a misnomer; the wild range of the species is Korea
(where it's the national flower) and northern China. But it's
naturalised in quite a few warmer places. It comes from a more
continental climate, and according to the literature, requires the aid
of a wall to encourage flowering in the cooler-summered parts of the
British Isles. As it's very late into leaf I assume that it doesn't mind
cold springs.
--
Stewart Robert Hinsley
http://www.malvaceae.info/Genera/Hibiscus.html