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.
This spontaneous transfer of stimulus control from tact variables to the conditioned establishing operation is consistent with results showing the emergence of mands without direct mand training (Hall & Sundberg, 1987; Sigafoos, Doss, & Reichle,1989).