I have a number of rock roses, that are now large clumps and flowering
prolifically. The flowers are starting to die off, now normally I would
dead head them but this is impossible because of the quantity of
flowers. Two years ago I pruned one of the plants back the result was an
awful looking plant, though it did recover to give a good show this
year. How should I treat them? Also are they easy to propagate, as they
must surely after three years becoming to the end of their life cycle?
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Residing on low ground in North Staffordshire

Cistus are mainly the big shrub rockroses, aren't they? I assumed, from the reference to "clumps" that the OP meant Helianthemum, the little ones about 8 inches high, with huge quantities of flowers about 2cm across in shades of white, yellow, red and pink. Same family, so I imagine pruning is much the same.

If it's the little ones we're talking about, there's no point in dead heading. The flowers are short lived, and the plants just produces a succession of them. Dead heading won't encourage the plant to produce any more.

They will certainly carry on for a lot longer than three years, jest spreading and getting larger and larger. I had one for well over 10 years, which had to be removed because we got rid of the bit of wall it was living on.

They are fairly easy from seed.