What really goes on beneath those mulch volcanoes

mulch volcano

This is a mulch volcano created by landscape maintenance companies that regularly pile new layers of mulch against a tree trunk before the older mulch has time to decompose. Mulch should always be placed back away from the trunk and root crown.

During the past three months Danny and I have been asked to check out a number of live oaks with mulch volcanoes and to fix the problems we found. We knew from previous excavation on individual trees that there would often be some nursery ropes and straps left behind to girdle the growing trunk and an occasional stem girdling root, but we weren’t prepared for the extent of the problem. In all we have performed root crown excavations on 84 live oak trees that had been planted approximately eight to 15 years ago and regularly mulched by the common landscape maintenance procedure that piles layer after layer of mulch onto the trunk creating what we  affectionally call “mulch volcanoes.

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 and removed. In most cases the wire baskets and the ropes

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When using an Air Spade or Air Knife near streets and traffic, it is always a good idea to erect a shield to reduce the amount of blowing soil, dust and debris.

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 is 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 20 trees treated was found to have damage from wire baskets, girdling from nylon ropes and root flare strangulation caused by stem girdling roots.

Photographs are included below to illustrate our findings. 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 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. Without the Root Crown Excavation treatment, the lifespan of these improperly planted trees would be measured in decades not centuries as should be the case for live oaks.

We do not believe that another root crown excavation treatment will be needed on these treated trees as long as future mulch applications can be kept 18 to 24 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 roots are cut and removed does result in some level of stress for the trees. The trees are fairly 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.