Buddeleia

They all grow pretty fast. I find the Buddleia Davidii with darker blue/purple colours such as Black Night have grown faster that the ones with lighter blue or white flowers. Having said that I put in a white one last August, and at the end of winter it was only small about 2ft at the most, and now it is 6-7ft at the tallest point. I think they all grow fast!!!!

If you want a permanent bush then go for B. globosa, this is BIG and has orange ball shaped flowers. Or slightly smaller is B.x weyeriana Sungold. This is a little smaller and had flowers which start purplish and turn yellow. Neither of these need pruning unless you want to.

Weyriana is to me a nicer plant than globosa, but I am predudiced and hate globosa. Having said that there are lots of wonderful Buddleias and they are all fairly fast growing try Pink Bouquet it has huge flowers see my photos.