euphorbia

Early Spring is the best but you will do no harm if you are tidy minded and want to do it in the Autumn.

Which euphorbia are we taking about here? and do you mean the flower stalks or what? I have lots of euphorbias and cut back the flower stalks as soon as they turn brown. But the leaves don't need cutting back unless they are too big for the space. They are mostly evergreen and look nice in winter especially when frosty.