Arborists and horticulturists have done a great job of persuading people that mulching trees is a good idea. Mulch helps retain soil moisture, reduces competition from grass and weeds, and eliminates bark damage from lawn mowers and weed-eaters.
Unfortunately, we haven’t been clear about how to apply that mulch. We should have emphasized not more than 2 inches deep on our poorly drained soils. As is so often the case, people have assumed that more is better. Mulch heaped up in volcano-shaped mounds around a tree can actually be fatal to the plant.
Thick mulch holds too much moisture in the soil. This can cause the roots to rot or encourage the growth of disease organisms that attack the roots. Mulch piled against the tree trunk or covering the root flare (that area where the trunk spreads out into roots) prevents air exchange through the bark and causes death of the inner vessels that carry nutrients through the tree.
Over-mulching cools the soil so much that roots don’t grow well. This is especially a problem in the early spring. The decomposition of hardwood mulch can rob the tree of nitrogen. Both growing and rotting of wood require nitrogen. Over-mulching encourages growth of girdling roots that can encircle the tree and eventually choke it to death.
Mulching can be one of the best or one of the worst cultural practices that can be done for landscape plantings, depending on what is applied and how. That is the message from Chris R. Carlson, Director of Horticulture Technology at Kent State University in the February edition of Arborist News. His recommendations for our heavy clay soils are to mulch wide not deep.
He suggests killing or removing all vegetation from an 8-foot circle around a tree and covering that area with 2 inches of mulch. Deeper layers cause problems; any thinner will allow the weeds to grow back. Use mulch that has large chunks so that air can circulate and water can pass through. Keep the mulch layer 3 to 6 inches away from the trunk of young trees and 8 inches away from older tree trunks.
Make sure your mulch is not too fresh. If it is recently chipped it will pull soil nitrogen away from the roots. Bagged mulch should be OK. Let fresh mulch compost for at least 6 weeks before you apply it to your trees.
After a while wood mulch will decompose and turn into soil. Before you add new mulch, rake the old to loosen it up and check its depth. Otherwise, the mulch can develop a matted waterproof layer that keeps moisture from reaching the soil.
Look at trees in their natural state. Their leaves fall under their branches, keeping down weeds and returning nutrients to the soil. Look at the area covered by leaves and the depth of the coverage. No volcanoes there. Our mulching should copy the natural process.