I would not use mulch. It has too much moisture in it already. This is what you don't want!
Wood chips , if they are dried out might be o.k. but, not the best choice unless they are more the size of shavings. Here a large bag of shavings are only about $5.00 so, that's what we use.........I wish I could use cedar 'cause a small company right down the road supplies a lot of east Tn. with cedar shavings.