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.
control. 1  to exercise restraint or direction over; dominate; command: The car is difficult to control at high speeds. 2  to hold in check; curb: to control a horse; to control one's emotions. 3  to test or verify (a scientific experiment) by a parallel experiment or other standard of comparison.