When is a good time of the year to clip red tips and to grow them?
Certified GKH Gardening Expert
By clipping, I assume you mean take cuttings from them. This can be done during routine pruning, which is anytime between spring and fall (late spring/early summer is ideal). These articles should also help: https://www.gardeningknowhow.com/ornamental/shrubs/photinia/growing-red-tip-photinia.htm, https://www.gardeningknowhow.com/ornamental/trees/tgen/how-to-root-cuttings-from-various-shrubs-bushes-and-trees.htm