this is a fast growing hedge, if thats what you have "a hedge" needs to be trimmed at least twice a year as not to get out of control, not sure if there is a time thing on trimming it though, i would tend to trim it before new growth starts

Trimming it now until end of March will not harm it.

Sooner rather than later,we've already got blackbirds and sparrows nesting in ours :)

thats another thing, glad you brought that up aster, be care of nesting birds plez :o)