Hello Diana,
It is certainly rather late to be cutting back. However, if the 'leggy'
growth is likely to cause damaging wind rock during winter storms, then you
should cut the stems back. Choose a dry and sunny day {:-) if possible (to
limit the entry of diseases) and make your cut in ripened green tissue.
Avoid cutting back into old 'wood', as this may not regenerate.

My own plant is only partly cut back because it still had a flower or two
last week, which I left for winter-flying insects. Where I have cut stems
back, there is already sign of new growth.

Good luck with your plant come Spring.
Spider
wrote in message
...
I have posted in the past but not for a while.
I grew a plant from a unlabelled packet (HPS)
and it flowered for a long time last year, Spring and into Summer.
It has bright acid yellow flowers. Does anyone know when would be the
best time to prune this perennial?. I should probably have done it
after flowering although I would not describe it as a shrub. Do you
think it will grow well again if I prune next Spring? I believe it is
described as the perennial wallflower (according to a sample in a
local garden centre). At the moment it is just leggy stems with a few
small leaves at the end of each. Diana