Why do we mulch? Mulching helps maintain moisture for the roots, evaporation is reduced and the need for watering can be minimized. A mulch layer will suppress weeds from germinating at the soil line but lawn mower clippings blown onto mulch may bring in weed seeds that can germinate on top of the mulch.

Many organic types of mulch can improve soil aeration, structure and drainage over time as they decompose. Mulch can reduce the likelihood of damage from weed trimmers which can cause the tree to fail. Mulch can give planting beds a uniform, well-cared for look.

Mulch should be between two to four inches deep. Often, when applied, it appears deeper, but after settling you should end up with a two-inch matted layer. At this time take a rake and loosen the mulch. It can become hard and crusty to where water will just run off and not penetrate.

Mulch around the base of trees is good, except when it is placed up the trunk of a tree and creates that “volcano” shape. Being able to see the root flair of the tree is necessary for good health. Tree roots grow in the top 18-24” of soil. A tree will grow tiny fibrous roots out into the mulch which can dry out quickly and eventually encircle the base of the trunk.

Mulch will also hold too much moisture at the base of the trunk and cause it to rot which invites pest and disease to enter. I asked Dr. John Ball, professor of forestry at South Dakota State University what to do if mulch is covering the root flair. He recommended removing all the mulch from the base of the trunk going down to the soil and with a sharp pruner, cutting off all the fibrous roots that are growing into the mulch.

Remember to add a small amount of mulch back over the root zone area but not touching the trunk. This will force the roots to grow below ground. For a tree that has had mulch up the trunk for many years, lowering the mulch and exposing the root flair may take a few years, but it’s worth it to keep a large tree healthy.