Depends on the variety of Buddleia you have - the commonest one is Buddleia davidii, with cone shaped flowers in late summer, and that one is hard pruned back to within 2 inches of old wood in late March. You can take some of the height off now to reduce the risk of wind rock, but the hard prune is done after winter.

Buddleia alternifolia and Buddleia globosa must be pruned immediately after flowering, not in March.

I've just trimmed my Buddleia davidii,back,as suggested above..The branches may look strong,but they really aren't ,and will snap off quite easily,in a strong wind..as has happened to mine in the past..

The above advice is good.

However, can I (very) gently remind people that it is spelt with a J, Buddleja.

Lots of refs to it spelt the other way though - never knew it was supposed to be j until I read your posts!

Its Buddleja OR Buddleia (and even Buddlea has been known historically in botanical texts) either is botanically acceptable, both being simply variants of the name and, in fact, Buddleia is more commonly used these days, Buddlejagarden.... Originally to do with a confusion in spelling between Linnaeus and Houston, so the story goes, though you may know this already - for those who don't, the link below explains in detail, but you'll need to scroll down to Buddleia taxonomy, near the end.

http://www.plantdelights.com/Article/Buddleia-Butterfly-Bush

Ultimately, Buddleja, Buddleia or even Buddlea are just fine, its down to personal choice.

WDKS? (What does Kew say?)

Bamboo, ten years ago you would be right, either acceptable, but now it has finally been settled on as Buddleja. Although I concede the i spelling is in such common usage that I would hestitate to call it a spelling mistake.

Kew uses Buddleja. If you submit a paper to a scientific journal you must now use the j spelling; not down to personal choice at all in this instance.

So all together now:
Bee Yu Dee Dee Ell Ee Jay Ay

Got it!

hmm....

http://images.kew.org/buddleia-crispa-fitch-w/print/7192552.html

Old, old, old print from 1854 - everything else is J.

Regardless of the digression, Dorothy, Bamboo's first advice still holds--B. davidii and its hybrids, prune heavily in early spring, and maybe moderately in fall to prevent winter damage; B. alternifolia and B. globosa, prune in late spring, after their early spring bloom, since next year's flower buds form in late summer.