When can i prune back photinias red robin

You can cut at Photinia to keep the red leaves coming; so you can clip it any time you like during the growing season. It tends to take off and get rather large if you don't control the growth. Then you have to get the saw out !

I do mine regularly to stimulate new red growth. I have a hedge I want to keep to about 6 feet tall .If yours needs a severe prune I would do it in easy stages so it doesnt die of shock. Most would survive but you never know. So I would trim it now and then again maybe after the new growth early next summer.