Are you talking about the shrub or the flowers from seed? The ones you grow in a border from seed are annuals and die at the end of the season. The shrub can be tidied up in winter but very hard pruning can kill it.

It's the shrub type and its about 18inch tall at the moment, okay thanks will do as you say and just tidy it up.

Well it depends whether it is the larger one that grows to at least 5 feet tall and across, or one of the small ones such as Barnsley Baby and if its flowered well at the height you say its probably that.

The large ones can be pruned quite hard but if overpruned they can react by dying. Sorry I don't know about pruning Barnsley Baby .