If you’re adding mulch specifically for weed control, you’ll want to be sure that all the weeds are removed in that area first. When you spread a layer, make it a thick one! In a sunny area, you’ll want to use 4″ – 6″ of mulch.
You may only need 2″ – 3″ in the shade. Usually, what the bed looked like before you mulched will tell you how thick you want the layer. If there were a lot of weeds growing in that area, you’ll want to be fairly heavy-handed. If there were only a few straggler weeds, you can get away with less.
Don’t place the mulch up against the base of the plants, nor against the trunks of the trees. Mulch really belongs from the drip line of the plant and outward. I have to admit, I do go a little bit in from the drip line because it make me feel like I’m making plant roots all nice and cozy, but be careful not to get too close.
When mulch is piled high on tree trunks fungal diseases find it easy to move in and take hold. It encourages rotting, plus rodents think it makes a cool fort and could set up home there. Think “doughnut” as opposed to “volcano”.
Another thing to consider when spreading mulch is how much “settling” there will be. Ideally, you want to mulch in a way so that when it settles (and I mean it’ll condense), that the thickness is around 4″ high. It’s always a good thing if oxygen can reach the soil and if the mulch is 12″ deep it’s very hard for that to happen.
It comes down to eyeing the mulch and taking a best (or sometimes educated) guess at which mulches will actually settle. When you picture say leaves or straw, you can surmise that these are going to settle quite a bit.
So, go ahead an pile that type of mulch on at about 6″ – 8″ high because they’ll end up around half of that in just a couple of days. If you’re using wood chips spread 3″ – 4″ because those guys aren’t big on settling. Don’t freak out if the mulch stays a bit taller – we’re artists here, not scientists.