Okay, so I pruned them back when they reached 20cm. I felt sorry for them because the plants at the back of the border started making buds, and the front ones had their heads chopped off.. so I decided to let them grow and see what happens.
The front plants started flowering about three weeks after the plants that weren't pruned. They are more bushy, and doesn't flop much. Funny thing is, the plants only started flowering once the stems grew as tall as the plants' that weren't pruned.
I will definitely prune my plants in future. I looks so much tidier, and you end up with more flowers.
