When I was taking the Master Gardeners course, the instructor said not to worry about wether you were pronouncing the botanical name of a plant correctly, because chances are the person you are talking to doesn't know either.
Fine Gardening has a pronunciation guide on their site. The best way to use it is to scroll down the list of plant names, clicking randomly on those you recognize and listening carefully to the way they're pronounced. Eventually it will stick in your brain.
However there is no substitute for conviction, even if your pronunciation is a little off, most plant people will understand what you are talking about, and if they don't you can always revert back to the plants common name, assuming that you know what it is.