I learned this trick for my Mom when I was forced to help mulch in my earlier years of life.  Funny how I now mulch out of my own free will.  Oh how owning a home changes a person.  So the trick, layer thick newspaper before you lay your mulch to prevent weeds.  It helps to water the news paper to keep it in place while you put the mulch on top.  Rocks and other objects are known to help keep the newspaper down if its windy.  I get that some super organic gardeners would disagree because the ink in the newspaper will leach into the soil where the plants are growing, but since we are doing it where flowers grow and not vegetables, I'm cool with it.  Several sheets of newspaper plus thick 3-5" of mulch and then a nice dose of Preen and weeds are prevented quite well.  Yes weeds might still grow, but I have found those that do grow pull easily because they tend to be growing on top of the newspaper.  I prefer the newspaper method to the black fabric people use any day.