It depends on what hedging plants you've used, and whether they're deciduous or evergreen.

I can't remember all the names of hedge plants but there's a selection of beech, hornbeam, gelder rose etc. all are deciduous except for the hollies. Have put some photo's of my hedge on my photo's so you can see. I think only tops need to be pruned?

Amsterdam, I'm afraid that my ignorance is showing again. What is a wildlife hedge, and what is its purpose?

I meant to say wild life friendly hedge. The plants have been chosen to attract wildlife into rhe garden and at the same time work well to grow a more natural looking hedge with a variety of different hedgerow plants. Horn beam, beech, gelder rose etc... If you look at my photo's you can see what it looks like now and back in Dec 2014 when it was planted from bare root stock. It's getting a bit straggely and I wanted to know when would be best to cut the tops off to make the hedge a bit bushier. Hope that explains it a bit better.

Well you normally trim beech in late summer/early autumn. I don't think guelder rose will flower if you trim it. It flowers on the new wood but if you cut it after flowering you won't get berries.

I suggest you email the Woodland Trust who will give you the best advice.

Hedges of any kind should be cut back at planting time to encourage bushing out at the base - its a bt late to achieve that afterwards.

Ah! I see!
Well, consider how much room you have, then. Full-up hedgerows take a fair amount of space, and frequent, informal pruning takes more time and brain sweat than formal hedging. So, the available space will dictate some compromises. Your fellows in the UK will definitely be able to discuss timing better than I can--here in the desert, we prune anytime from September through April.

Thanks Tugbrethil. Think pruning in the autumn/winter would be best as well.

Best of luck with the wildlife, then. You only see such things on large estates, here, since wildlife here includes rattlesnakes and skunks! :D

Crumbs! We have a lot to be thankful for...