By now, most deciduous trees have already lost their leaves. And, by now, most homeowners have done what they were going to do with those leaves. One option often used by homeowners is to use fallen leaves as mulch around the base of their trees. Depending on how it is done though, it may actually be causing your trees more harm than good.
First of all, there are many benefits to mulching trees, whether using leaves or woodchips. Mulch keeps grass and other plant material from growing right up to the base of the tree. This reduces competition for moisture and nutrients and keeps lawnmowers and string trimmers further away from the trunk, which reduces the potential for damage caused by collisions. Mulching with wood chips, leaves or other organic material also improves soil structure, increases soil fertility and reduces compaction. Too much mulch, too close to the tree can cause some problems though.
Get ready, this is about to get deep. (get it? Roots… deep…) Roots need oxygen to grow. When excess soil or mulch (generally 4-6” or more) is placed against the base of a tree, or on top of existing roots, soil-oxygen levels can start to drop. The tree responds to that decrease in oxygen by focusing its efforts to re-establish roots that can restore that oxygen exchange. In the short term, this demand for new root growth adds stress to the tree. Another problem is that these new roots create a new “layer” above the other established roots. The long term effect of this new layer is that roots end up crossing each other. And, just like branches and trunks, roots grow in diameter every year. When they cross each other, the force of that annual growth can start to have a girdling effect, reducing the flow of nutrients and water. While this will not necessarily kill the tree, it is an additional stress that can leave the tree susceptible to other problems.
Here are a few tips to reap the benefits of mulching trees while preventing any additional stress. Leave room for the tree to breathe by keeping leaves and mulch away from the trunk. Leaving a gap of open space 1-2” at the base of the tree works great. Think more along the lines of a “ring” or “donut” rather than a “volcano”. Use mulch that contains wood chips or chunks. Mulch to a depth of 3-4”, and a radius around the tree as large as you can tolerate. (It is not possible to mulch too large of an area!)
Follow these few tips, and your trees will thank you!