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.
unconditioned stimulus. n. A stimulus that elicits an unconditioned response; for example, food is an unconditioned stimulus for a hungry animal, and salivation is the unconditioned response. stimulus that is just strong enough to elicit a response. unconditioned stimulus. any stimulus that is capable of eliciting an unconditioned response.