Most lavateras are pruned very hard in Spring (around now) but as this is the Tree Mallow, I'm not sure how it will react to this treatment, and I've been unable to find any info regarding pruning of this variety. You might have been better off buying one of the other Lavateras which get to 6-8 feet in a season, but are pruned back to the base in spring.

rhs book of mine says.assurgentiflora should be pruned in spring like you said.prune dead stems back to healthy growth or if necessary can be cut right back to the base.'prune out stems of the previous year's growth to within 2-4buds of the older wood to stimulate new flowering shoots'.

There you go, good news then - I must admit I didn't look in my RHS book but relied on google.