I'm thinking of pruning mine soon, so you can probably prune yours now.

Really? you can prune 2/3rds?

I thought it was 1/3rd.

It is one third, not two thirds. Wasn't thinking this morn.
duane

I wouldn't bother getting out the ruler :-) Prune back hard as soon as you see new growth emerging. You can prune back as hard as you like as long as you do not prune back into old wood that is not producing any foliage. Just keep it above this level.

Pruning more radically than just cutting back by a third or trimming off the top of the foliage will result in a more compact and densely branched plant that produces more flowering stems. Annual hard pruning should begin the first spring after planting - the longer you wait to prune or lighter the pruning 'touch', the greater the chance the plant will become leggy and splay open.