How do I keep a betula pendula's height down?

You will have to stand back and look at the shape - has the tree got several leaders? If so, you could cut out the higher ones and grow it on the lower ones. If you just prune it on the main trunk it will look just that - a chopped tree! Very ugly. Our tree surgeon always says - cut a little then stand back and look before proceeding step by step, because you can't stick it back on again! Good advice, eh?

We have a self sown silver birch in our garden which has been there for about 15 years.Since we also don't want it very large we have to cut it back hard every year because it puts on a lot of growth. It looks a bit ugly when cut but soon grows out rapidly again.We cut it back in winter and in spring it all grows back again. We have to do the same with a Robinia which would fill the entire garden if allowed to!