braca, the person at home depot is about half right. you will need to add to your mulch every year because the much is organic and will break down (which is good for your beds). But a total replacement is unnecessary unless it starts to show signs of disease or fungus.

that said, there are times when replacing it all is in order, even if all is healthy. those times are usually when you are wanting to clear out a bed for reorganization and/or redesign. then, you put the mulch in the compost bin to further get your moneys worth and to continue recycling.

now, about the lava rock. great idea except.

you generally cannot work in a bed that has rocks deep enough to deter weed growth, the rocks are just too heave, sharp, cumbersome, and will get in the soil and they do not break down. so the bed has to be "finished" before you put the rock down. however, using rocks in a bed in areas that have large foundation plants is great, because they don't shift as much and animals don't usually dig in them.

i hope this helps!

if you are starting from scratch, you definitely want to go with the bark. it is so much easier to work with, and if you decide you dont like the look, much, much easier to remove/relocate. just make sure you get the natural color stuff. that painted stuff isn't good for your garden.