Subject: Tree Mulching

Carsten Gleaser,  a Queens resident and independent consulting arborist. He has over 25 years of education, training and experience providing Arboricultural Consultations on a broad range of matters.  From experience in Arboriculture & Urban Forestry, Horticulture, Instructor/Lecturer, to Depositions & Expert Witness.  In 2004, Carsten received his Doctorate from CUNY Grad school in Biology speciality in the Plant Sciences and Urban Woodland Ecology.  He worked in NYC Parks for 12 years, first as a Horticulturist/Arborist then as Director of Organic Recycling Program for Brooklyn Forestry.  Carsten's knowledge of horticulture and arboriculture, and his time in Parks, makes him a wonderful Friend of  Our Park and community of street trees.

There are numerous American National Standards that have been developed for the care, treatment and management of trees, vegetation and soils within the landscape.  These standards have been developed over decades and are contributed to by industry experts and practitioners in the Tree Care Industry, revised periodically by new tree research findings.   Those experts have worked with the American National Standards Institute (ANSI) to develop acceptable standards and Best Management Practices (BMP) for use by professionals, hobby gardeners and others in order to best optimize the Tree’s Needs.  Wood mulching and mulch objectives for trees and shrubs are located within the ANSI A300 Part 2 SOIL MANAGEMENT: a. Modification, b. Fertilization and c. Drainage.  After all, mulching your tree, shrubs and other garden vegetation is an activity that specifically addresses soils, the biota within the soils and organic matter. 

The objectives of mulching with woodchips are many;

·       Inhibits weed growth, 

·       conserves soil moisture, 

·       moderates soil temperatures, 

·       prevents the compaction of soil (and tree roots), 

·       prevents erosion, 

·       encourages beneficial micro-organisms, 

·       inhibits certain root pathogens and disease, 

·       improves the vigor and growth of trees.   

Just how much mulch should I be applying around trees?

Here are several pointers and standard recommendations for placing woodchip mulch around trees;

1.     Types of mulch (wood preference and if single or double ground) and the methods of application shall be specified to meet the mulching objective.   

2.     When selecting the type of mulch, consideration should be given to tree species, soil conditions, irrigation practices and the presence of pathogenic fungi. 

3.     Fresh and partially decomposed wood-chip mulch from trees should be preferred when the objective is to improve soil structure and increase organic matter (greater than ¾-inch average wood size).

4.     Pervious sheeting or fabric should not be used under the mulch when the objective is to improve soil structure and increase organic matter. 

5.     Mulch should NOT be place against tree trunks. 

6.     Mulch should be applied over as much of the root zone as practical. 

7.     At its basic application, mulch should be applied and maintained at a depth of 2-4 inches around the tree pit area and a 1-ft radius distance away from and off of the tree bark and tree flare. 

Brought to my attention by the Friends of Crocheron & John Golden Park is the over-mulching of many curbside street trees in their planting pits.  This is not an isolated incident and has been observed in many other communities with newly installed street trees.  Tree and landscapers under contract or in agreement with the NYC Parks Central Forestry and other agencies for street tree installation are required to maintain and deliver a level of care to newly installed trees during their 24-month mandatory warranty period.  Bear in mind, over-mulching of trees is unacceptable and it is not tree care- it is tree abuse.  Over-mulching will kill trees.  It is one of the many forms of abuse and harms that we continue to see perpetrated against young public trees that are attempting to establish themselves in the landscape- even by those we would expect to be in the know- the landscape professionals. 

Oxygen Starvation. 

The root cells of unseen tree roots require oxygen to respire, to develop, grow and to function.  Fine tree roots absorb moisture, acquire soil nutrients and transfer them via xylem tissue to other tree parts.  When mulch is applied too deeply over the tree rooting area, tree roots are known to suffocate because of diminished gas exchange or hypoxia.  Suffocating tree roots leads to a diminishment of overall tree health and even tree death, if uncorrected.  Oxygen starvation can also be attributed when overwatering trees.  A similar consequence can occur when trees are installed in a location that has poor water drainage, forcing trees during rain events to remain submerged under water- also creating a hypoxic condition for the tree. 

Tree Disease. 

Long periods of excessively high moisture level can cause an increase in the presence of wood pathogens such as decay and disease, as well wood boring insects.  Most fungal and bacterial diseases require moisture to spread and reproduce.  Trunk diseases will gain a foothold in the moist, decaying bark tissue under the mulch and occur unseen.  Once established, the disease organisms ultimately invades the inner bark, disrupting the flow and storage of sugars produced during photosynthesis (starving and finally killing the plant).   This is followed by a secondary invasion by bark beetles and borers that are attracted to stressed plants and that expedite tree death.  Many tree health issues such as branch die-back have their origin from problems with tree roots.   

Drought Condition. 

When irrigating a newly installed street tree it is absolutely critical for the first 3 years (known as the Period of Establishment) to ensure adequate moisture gets to the entire tree root ball and not just the wood mulch atop of the root ball.  The failure to ensure adequate moisture gets to the tree root ball may just deliver enough water to soak the wood mulch yet never permeating to the lower soil level and germinating tree roots.   This can and will lead to severe drought-stress conditions for the tree and lead to tree death- even though one believes to have adequately irrigated the tree.   One field study revealed that mulch placed over the root ball intercepted most irrigated water causing a drier root ball and resulting in tree stress and reduced survival.  This was in contrast to light applications of water of  those trees with no mulch covering the root ball with improved tree survivability (E.F. Gilman and J. Grabosky, 2004).   With that, tree watering events especially for the first growing season requires that you saturate the area immediately atop of the buried tree root ball and inside the tree soil saucer during scheduled weekly watering events.   For this reason mulch should be pulled back from the trunk at least 12 inches (24-inch diameter circle) allowing water to freely permeate into the root ball.   

See Figure 3, an example for one of several newly installed NYC street trees in Bayside, New York, where amid the July-August 2022 heat wave and drought, absent follow-up care with irrigation and the application of beneficial mulch forced many of those trees into drought stress and death. 

Excess Heat. 

Wet mulch layers piled up next to the trunk may begin to heat up when the bark begins to decompose. This scenario is similar to composting, where temperatures within inner mulch layers may reach 120° to 140° F. This high heat may directly kill the inner bark/phloem of young plants, or may prevent the natural hardening off period that plants must go through in the fall in preparation for the winter. If the trunk flare tissue is not sufficiently hardened off before freezing weather arrives, the tissue may die, the roots may not receive adequate amounts of photosynthetic sugars, and the plant will ultimately decline in health.

Stem Girdling Roots (SRG). 

Upper secondary roots that develop in reaction to the suffocating lower primary roots will drive havoc on tree health.  Secondary roots trend to grow in multiple directions (up, down and back toward the tree stem) rather than outward and away from the tree root ball.  With that, secondary roots can cross each other, circle back and in the process create a condition known as Stem Girdling Roots (SRG).  It is a defect created principally during tree gorwing and cultural practices.  SGR originate in poor and deficient arboricultural treatments starting from its years in the tree nursery, and continued into tree installation at the planting site.  It is during the planting that presents an opportunity to correct the conditions that lead to SGR before the tree goes into the ground- such as measuring proper tree planting depth and an acceptable wood mulch depth. 



See Figure 4.  SRG is an outcome of several factors- poor nursery culturing practices, improper planting depth and / or excessive wood mulch application over a period of a few years- until tree death. 

Over-mulching remains an attention grabber for tree enthusiasts, because it prompts a greater question about what entity provided the oversight and supervision during tree installation for compliance to the NYC Parks Central Forestry contract specifications- or not at all. The Tree Care Industry ANSI Standards as discussed earlier typically find their way into contract specs to stipulate the Best Management Practices (BMP) for all sorts of tree care, including mulching depth. If oversight was not provided by a Certified Arborist or Forester trained and skilled in the installation and care of young trees, there is a 51% likelihood that tree installation was non- compliant and thereby may have a range of conditions that can lead to poor health and tree defects in later years. 

Intervention by those educated in the Care of Trees, when wrongs are being imposed upon new trees, or blatant abuse- is one way to ensure our communities remain treed and to develop the long-lived beneficial canopies that we seek from them. 

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E.F. Gilman and J. Grabosky 2004. Mulch and Planting Depth Affect Live Oak Establishment Journal of Arboriculture 30: 311-317 

Figure 1.  Proper Mulch Application around tree base.

Figure 2.  Mulch Volcanos

Figure 3.  NYC street tree stressed

Figure 4.  Stem Girdling Roots