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. Definition. noun, plural: stimuli. (1) (physiology) A detectable change in the internal or external environment. (2) (physiology) That which influences or causes a temporary increase of physiological activity or response in the whole organism or in any of its parts.