I am currently doing the Diploma, and highly recommend it.
I am doing Science of Gardening 1 at the moment, and it includes some quite practical activities - I have done a soil and pH test in my garden, and the next assignment is looking at the leaves of my lemon tree to diagnose nutritional deficiencies. It's really interesting looking deeper into how plants function and I have been learning a lot that will be very beneficial for my future gardening.
It is all online, and very flexible with times - you don't have to be available for any specific class times (some subjects have online tutorials, but they are not compulsory and are recorded so you can watch later). They release content in fortnightly modules, so if you have a busy period coming up you can generally work study time in around it.