Sooner or later most bedding plants will need a drastic cutting back. Chelsea chop is a phrase used to describe this operation which should take place after the Chelsea flower show.
This well established horticultural practice is used to prolong the overall flowering period, albeit at the expense of some flowers at the time of this operation.
Let me emphasise that this can only be done safely with the bedders, and should on no account be used for the Florist cultivars, as this treatment would probably kill them.
In six weeks or less the plant will have produced new growth and flowered again. Not quite to the same standard but looks much nicer than an overgrown specimen. Attachment 1.