Sorry, Scarlett: I made a mess of posting a reply to your query about gardenias, as I'm new to the forum. The black stuff on the underside of your gardenia leaves could be sooty mould. If you use chemicals a spray with a fungicide should do the trick. Check the plants carefully for aphids: sometimes sooty mould grows on the sugary excretions they produce. I love gardenias but can never get them to flower again...