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.
Proper stimulus control, is said to have four characteristics: 1  The behavior occurs immediate when the conditioned stimulus is given. 2  The behavior never occurs in the absence of the stimulus.  The behavior never occurs in response to some other stimulus.