I have a number of buddlias and I cut some back servere and others a gentle chop,the only thing is if you chop to severe they will die,I have lost a few that way.they are of course great at seeding themselves and from a seedling they grow quickly or even a stick cutting,for the best effect with out taking over the border is to keep it as a bush.If you have very old lanky ones ,get rid of and start with seedlings or cuttings.They are so lovely giving butterflies,bees and all manner of insects the nectar they need and hurs of watching for you,they come in many diffeent colours ,mine,deep purple,lilac,yellow(double) and I long for a white one.