Do you mean a special brand of Wicca they practice, or just Wicca in general?
In general, it's like ancient Celtic nature worship type ritual thingies. Lots of people try to ascribe this to Stevie, and she definitely has thematically Wiccan tendencies and imagery, but...I'm not really sure she is a huge practioner or anything. BTW, FYI, I think that is the third most popular religion where I live...
What with our actual elected green party officials and all.