I like caring for plants and, well, they seem to like me. And when I'm visiting my family in New York, I can often be found outside doing all the weeding and pruning. As for plants, I suppose my theme is many different plants rather than, say, all ferns. Top Yoga and Meditation It may sound odd, but there's no way you can stay focused on a PhD without serious psychotherapy, a very strong support network of family and friends, some sort of mental control, or some combination of these. For the mental control, I suggest meditation.
Yoga acts as that, as well as providing a means of staying physically fit.
While not following any set meditation, such as Buddhism, I feel that spending 20 minutes or so on as many days a week as possible just simply sitting and trying to clear your mind is a very beneficial thing. A simple exercise is to imagine that air has a color and concentrate on filling yourself with this color until you can see this color. Controlling your breathing goes a long way here. Trust me, it's not as hokey as it sounds. Or maybe it is.