When good plants turn bad

I absolutely love the delicate beauty of these gaura 'Siskiyou White', and thought I had the perfect spot for them - across our north-facing front fence.

They have grown - and self-seeded - prolifically, but somehow the whole effect is just not working.  Instead of pretty, upright stalks, they lie flat and twisted, spreading too far onto the footpath.

So they're going to have to go.  Eventually.  

The only question is what to replace them with...