Cytisus Scoparius (Broom) Pruning and Stunting

Hi,

I planted cytisus scoparious (broom) last year and it has started flowering from the base right to the top.I believe these plants need pruning. When do I prune, how much do I take off?

All plants have flowered but there are two with one stalk about the height it should be and the rest of the plant seems stunted to about one third of where it should be? The plant is alive and a few flowers showing on the one high stalk. Do I need to do anything to them

regards

barry