Sometimes a small single flower in an image can be very effective. The background needs to be complimentary - very plain or nearly plain otherwise you end up distracting the viewer from the main subject.
It is often more effective to place the flower to one side of the picture rather than 'slap bang' in the middle. You may of heard of 'the rule of thirds'?
Imaging drawing a noughts and crosses grid over your image - ie to vertical lines and two horizontal lines equally space - like you would playing noughts and crosses. The idea here is that it is often most pleasing on the eye when the main subject matter lies where these lines cross each other.