sumisu: follow the root crop grouping. Garlic (Allium sativum) shouldn't have a lot of water now. Since you already planted (pretty good timing for Mass.), straw or hay is fine, but pack it a bit. I think of mulch in terms of tan bark or pine fines for depths. So don't overdo the straw or hay, as you don't want high water retention. If it's the regular garlic, it's quite hardy.