I have a red robin which has got out of control, probably 20 feet high. When should I prune and by how much? Thanks
Take a look at https://www.rhs.org.uk/advice/profile?pid=145.
This should give you a good idea.
Normally prune a red robin shrub in March or early spring as this stimulates new growth. As it 20 ft, I would attempt to reduce its height now so that high winds do not break its branches. They are tought shrubs and normally take a good prune quite well without any major problems
In the autumn of 2016 I had to get rid of mine as it kept encroaching onto the road.
I cut it back to almost ground level as it could not be dug out.
For the whole of 2017 it kept sending out new shoots and I took all these off regularly and it's still not dead yet!!!