After flowering. It'll stand quite a hard cut. I dug one up, snapped it in half and threw it on the fire heap, but it was salvaged by my caring brother, days later, and within a year it had grown back strongly from below the break, and is now a mighty thriving beast. Phil J

Mahonias are just gearing up for flowering, doing so in the winter. so don't prune it back now as above wait until it's finished flowering - though you'll have to get the winter clothes on to go and do it lol

Best time to prune a mahonia is after they have fruited and the birds have finished feasting on the dark juicy berries. They have finished their cycle and someone said on here that the berries can be eaten by us gardeners.