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.
Definition of stimulus for Students. stimuli. 1 : something that stirs or urges to action The reward was a stimulus for greater effort. 2 : an influence that acts usually from outside the body to partly change bodily activity (as by exciting a receptor or sense organ) Light, heat, and sound are common physical stimuli.