When and how much to cut back perennials?

My small front garden is heavily overhung by two large, listed lime-trees. The shrubs and perennials there are all looking rather straggly. Should I cut them back now or in the early spring. I certainly don't want to kill them! There are a couple of fairly robust, established shrubs. But also penstemons, Siberian wallflowers, pinks; all of which would look much better if they were of more compact form next summer.

 

I am glad you mentioned the type of herbaceous perennials you have as most varieties can be cut back either in the Autumn or in the Spring with little difference to the outcome . However, Penstemons not being the hardiest of plants are best left to the Spring to prune unless perhaps you live in a very mild area. If they are very tall I don't think it would do much damage to prune them lightly to stop wind rock during the winter and then hard prune them in Spring. I don't think pruning is of benefit to Pinks. I don't grow them now but used to and from memory the only "pruning" I did was to remove the dead flower stalks. Maybe someone more of an expert on Pinks will advise you differently.
Again I don't grow Siberian Wallflowers myself. I think you are meant to trim immediately after flowering depending on variety and many are treated as annuals. Hopefully someone else on GC can be more helpful.
Visa vie the Lime problem. I would have thought you would have the right to prune it back to you boundary whether it is listed or not? Could be wrong but maybe check with your local council.
I have a huge Lime in my garden (about a 1/10th of it shows in the background of the photo!) It's been aged at approx 200 years old and they can live for about 400 years so I think you are stuck with it as a neighbour for a while yet!