What really goes on beneath mulch volcanoes.
Trees should be mulched with organic mulch, but many trees in our landscape are mulched excessively. When the mulch is too deep (over 3 inches deep) and piled up against the trunk of the tree, problems begin to occur that can negatively impact the health, structure and longevity of the tree (Figure 1). A laborious treatment used to investigate what is going on beneath the mulch volcano and correct what can be corrected is called a root crown excavation.
To do the root crown excavation, we used an Air Spade excavation tool which uses a 200 CFM air compressor and directs a stream of high velocity air at the mulch and soil safely removing the soil and mulch from around the roots without damaging the roots. Once the soil and mulch have been cleared away, the exposed wires from the wire basket were cut and removed and nylon ropes used by the nursery to lower the tree and root ball into the planting hole were also cut. In both cases the wire baskets and the ropes have either become completely enveloped by the roots or were beginning to cause girdling and strangulation of the roots.
We also found that the excessive layer of mulch placed around each tree has encouraged root growth upward from the soil into the mulch. These vertically growing roots eventually began growing horizontally once they reached the top of the mulch pile and then started growing downward again. The result of this unnatural root growth pattern was many of these newly formed roots above the natural soil grade had become stem girdling roots that were strangling the root flares. Every one of the 63 trees treated was found to have damage from wire baskets, girdling from nylon ropes and root flare strangulation caused by stem girdling roots.
In our opinion the root crown excavation treatment was highly successful and valuable in helping maintain and promote continued growth of the trees along the parkway. Some of the crowns of trees were beginning to show partial areas of sparse foliage and delayed leaf flush. Without these corrective root treatments the condition of the trees would have continued to slowly worsen each year as more roots were impacted by the numerous conditions we found.
We do not believe that another root crown excavation treatment will be needed as long as future mulch applications can be kept 12 to 18 inches back from the trunk of each tree and kept to a depth of no more than 3 inches. We have found over the years that a 3 inch layer of mulch will decompose over a period of approximately 3 years. Consequently annual mulch applications are not needed and will only result in more mulch being applied at a rate greater than the decomposition rate resulting in an accumulation of mulch and the stimulation of more stem girdling roots.
A root crown excavation treatment in which a number of large groups are cut and removed does result in some level of stress for the trees. The trees are young and quite tolerant of this type of stress. However, making sure the trees are adequately irrigated at least once a week through the spring and summer will help reduce any negative effects of this stress. Some root sprouts can also occur.