There are two cardinal rules for using mulch to combat weeds. First, lay the mulch down on soil that is already weeded, and second, lay down a thick enough layer to discourage new weeds from coming up through it.
A four-inch layer of mulch will discourage weeds, although a two-inch layer is usually enough in shady spots. If you know that a garden bed is filled with weed seeds or perennial roots, try a double-mulching technique to prevent a weed explosion. To do so, set plants in place, water them well, spread landscape fabric, and top it with mulch.
Mulches that also retains moisture (like wood chips) can slow soil warming. In spring, pull mulch away from perennials and bulbs for faster growth. A wet mulch piled against the stems of flowers and vegetables can cause them to rot; keep mulch about one inch away from crowns and stems.
Mulch piled up against woody stems of shrubs and trees can also cause rot and encourages rodents (such as voles and mice) to nest there. Keep deep mulch pulled back about six to 12 inches from trunks.