Hello

I have a mature hibiscus (syriacus I think) which is giving me trouble, and
I'd appreciate some advice.

Some of the branches are coming into leaf later and later every year, and
are getting covered in what might be lichen. The problem is spreading and
now affects most of the plant, and I want to take some action to save the
rest of it.

I live in Cambridge, the plant's in full sun, but the soil is low-lying and
rather heavy, and sometimes gets waterlogged in winter. I've read that
hibiscuses don't like that, and wonder if it might be causing the problem.
Alternatively, could it just be poor watering? poor feeding? or simply old
age?

There are two other hibiscus plants nearby, which have a similar problem but
to a much lesser extent.

Do you know what the problem is? And what should I do about it? I'm a
newcomer to gardening, and would be very grateful for any advice you can
give.

Many thanks

Chris