Geraniums have been popular garden perennials for a long time, and have lots of devotees. Consequently, there are lots of good sources of information on the internet. A good start is provided by the Open Directory Project's geranium category. For a primer on erodiums, this page from the UK gives a good overview.
Stimulus control is the phenomenon of a stimulus increasing the probability of a behavior (operant response) because of a history of that behavior being differentially reinforced in the presence of the stimulus.
What is Stimulus Control? Well established cues are under what is called stimulus control. The stimulus (cue) increases the chance that the behavior will occur because the animal has been reinforced for performing the behavior in the presence of the cue. But what does it really mean for a behavior to be under good stimulus control?