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.
Sometimes the property of the stimulus which controls an operant performance may not be found in any single instance of the stimulus. When such is the case the control may be through an abstract property of the stimulus, such as its shape, position or size. In each of these cases, the controlling property of the stimulus is found in a class of stimuli, and reinforcement is determined by a general rule rather than a specific form of the stimulus.