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.
stimulus. pl. stimuli [L.] any agent, act, or influence that produces functional or trophic reaction in a receptor or an irritable tissue. conditioned stimulus. a neutral object or event that is psychologically related to a naturally stimulating object or event and which causes a conditioned response. See also conditioning.