I have had a beautiful Plumbago for several years and cut it back at the end of the flowering time to see it grow enormous again each May. This year, it seems to be slowly dying. It looks as if someone has put weedkiller on it but this isn’t possible. what can I do?
Plumbago is such a tough plant, it's hard to imagine what could cause an established one to die back, unless it's cold or poison. I don't think your winter was cold, was it? Are you sure it couldn't have been hit by herbicide? The stuff can drift quite a distance from where it was sprayed, and it can also wash through the soil for several hundred feet.
If you use a lot of water on it, you might be able to wash bad stuff out of the soil, if that's the problem.
Can you take soil and leaf samples to be analyzed by a university or agricultural lab? The most you can do, unless you can identify a disease, is to cut back dead material, and hope for the best.
I love plumbago. Good luck to you.