These two are both leaf colour forms of the same species, Acer platanoides (Norway Maple) so they can be pruned almost any time though personally I would prefer they were done in their dormant period with late winter or early spring being the optimum time.

Thank you Fractal. I do occasionally trim branches back at any time of the year in order to keep them from infringing on my neighbour's garden, and this doesn't seem to do any harm. I was just wondering whether a proper trim would have to be done at a particular time - thanks for the advice, and I think I agree that perhaps when they're dormant would be best, to be on the safe side.

i would do it early spring as it doesnt have to sit there with wounds all winter with the spring surge of life just round the corner.i doubt you would have to do it a lot more often than once a year apart from the odd branch.

Thanks Noseypotter, that sounds like good advice!

your welcome jan xx