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 conÂ·trol (kÉn-trÅl') v. conÂ·trolled, conÂ·trolÂ·ling, conÂ·trols. To verify or regulate a scientific experiment by conducting a parallel experiment or by comparing with another standard. To hold in restraint; check. n. A standard of comparison for checking or verifying the results of an experiment. In an experiment to test the effectiveness of a new drug, for example, one group of subjects (the control group) receives an inactive substance or placebo, while a comparison group receives the drug being tested.