I bought some Lavaterra a few years ago and took a flying guess from things I heard on the Internet that I should prune it down to 12-18" in late fall ... then prune it hard, like 6" or less, in early or mid-winter.
I never knew why someone advised TWO prunings, but I have to do SOMETHING to keep it from growing to 6+" feet and flopping flat.
I suspect that I should prune it again in late spring, maybe just before flowering or even before the buds are big, if I want to keep it from flopping over - which I am starting to think means keeping it down to 4-5 feet at most.
I also read that it would be more restrained if the soil were even worse and it got almost no water. Well, I interplanted it with things that DO want water, and I didn;t realize that soil could GET much worse than mine!
This is allegedly Lavatera thuringiaca 'Barnsley', a perennial woody bush in my Zone 8 garden. I've been cutting the wood back back year.
smith, I could sure use a few tips on tree pruning. 
