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.
In physiology, a stimulus (plural stimuli) is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity. When a stimulus is applied to a sensory receptor, it normally elicits or influences a reflex via stimulus transduction.