I have a row of a dozen silver birch trees at the front of my
property, they are about 30 years old and 15 metres high.

I don't want them to grow any taller and would like to take maybe 4
metres off the top of each of them.

I understand silver birch sometimes don't take kindly to this sort of
thing so want to minimise the risk of killing them !

When is the best time ? Now when the leaves are off and they have
plenty of time to recover - but the cut is open all through winter -
or should I wait until say March when I assume there is less chance of
frost / rot but also less recovery time.

Thanks,

David C