There are bad mulch jobs and then there are bad mulch jobs.
Just because I’m not a fan of red mulch (a N.Y. favorite) or big pine bark mulch (like my Dad used), I still have to allow for people‘s preferences. Recently, however, staying at a nice hotel - obviously professionally landscaped - I was shocked and awed to see the trees mounded so high with mulch that I feared for their lives.
Mulch is one place where more is not better.
In fact, mounding mulch at the base of your shrubs, plants and trees could very well smother them. And, in fact, the younger the tree and thinner the bark, the more the plant is susceptible to “strangling” by this practice.
In fact it’s recommended to keep mulch a few inches from the base of a tree (not touching the bark) and to replenish annually, but not to keep adding and adding - thereby creating a barrier for water to get to your plants. No more than 3” are recommended.
If, for some reason, your trees have more than 4" of mulch around them or layers of old mulch from years of adding it, I would recommend gently scraping back to a few inches or replacing altogether.
You may be wondering by this point what the Art of the Mulch is for trees.
When possible, look for organic mulch. Some of the best materials to use include shredded bark, wood chips, mushroom compost, composted sawdust, cocoa bean hulls and leaf mold. Avoid peat moss, whole leaves and uncomposted grass clippings because they tend to mat together, and once they dry out, they are difficult to rewet.
Two to four inches of mulch provide the benefits without endangering the health of woody plants. Mulch can be placed within a couple inches of the trunk or stems as long as it does not make contact. It is ideal to mulch trees out to the drip line (the ends of the branches) when possible.
Placed properly, mulch will help conserve moisture for your trees, moderate soil temperatures and (hopefully) control weeds. While I agree that it would be nice to not have to buy it in plastic bags, still, I would not recommend getting your mulch from the county “free” sites, because it often contains weed seeds or fungi spores that may give more trouble than they’re worth. (One year we ended up with some weird tiny barnacles on our car that jumped from the mulch!)
But whatever you do, please skip the volcano approach to mulching as seen in the picture above. If the base of the tree is covered up, there will be poor air exchange and the mulch can harm the tree's growth. It is better to create a (very subtle) trough that will help direct water to the base of the tree.
A good rule of thumb: If you don’t see it in nature, there’s probably a good reason it doesn’t work. Hope this helps you mulch with confidence!