You prune in early spring as you can see all the branches but it is mile enough for the plant to recover quickly. They do not like the shock of cutting down lots of branches at one time to near the ground, so you have ot do that over time if it is a well established plant.

Agree with Kildermorie. Just cut one or two older stems before active growth restarts in Spring. Trouble is the old dry leaves can look so messy. At least they help protect the buds which you might see on the stems.

Also, they tend to flower on older wood, so if you prune it then you will not get flowers on that stem again for 3 years or so. If it's very bushy you can take out a third of the stems each year to spread the flowering gap. They do get quite large especially if it is the yellow flowered p. lutea.