Mulch Madness: How to Properly Mulch a Tree!
What is mulch?
The most common type of mulch is recycled wood waste in the form of wood chips. However, there are many varieties of mulch, both organic and inorganic. We recommend sticking with natural, organic mulch derived from plants such as hardwood and softwood bark, leaves, and cocoa hulls. Mulch decomposes at different rates depending on specific material, the climate and the microorganisms present.
A note on inorganic and dyed mulches:
Inorganic mulches, such as pulverized rubber, gravel, and landscape fabric, cause environmental problems and are not recommended for use. These materials do not decompose or improve soil structure. They wash away and become litter in our waterways. Dyed mulches (red, black, green etc.) leach toxic chemicals that harm beneficial soil bacteria, insects and even plants. Not only are the dyes harmful, but the wood sources are questionable. The types of wood that can absorb these dyes often come from pallets or reclaimed treated wood from construction or demolition sites. Dyed mulch contains chemicals such as creosote and CCA (Chromated Copper Arsenate, used in the manufacturing of pressure-treated wood). Not something you want to put in your garden!
Photo: Blue Water Baltimore staff and volunteers caring for young trees at one of our largest planting sites, US Route 40 aka “The Highway to Nowhere.”
Mulching mimics some of the benefits usually enjoyed by trees in a forest setting where leaves and other organic materials break down into soil. While not as rich in nutrients, mulch can help improve soil conditions as it decomposes.
The main benefit of mulch is that it acts like a sponge and holds moisture. This extra layer of insulation is helpful during hot summer months when trees are stressed by drought. Mulch is especially valuable for newly planted trees that do not have extensive root systems. For some shallow-rooted species, such as river birch, mulching protects against root sunburn. Mulch is also popular because it looks nice and provides areas with a uniform, well-manicured look.
Mulch suppresses grass and weeds, which compete with young trees for much-needed water and nutrients. Mulch rings can also reduce trunk damage caused by lawn mowers and weed wackers. If grass grows close to the tree trunk, lawn & maintenance crews inadvertently get too close with their power tools and create wounds in the bark at the base of the tree. Weed wacker damage is one of the main causes of young tree mortality!
A note on tree biology: plants never heal from wounds. They simply block off the damaged area through a process called compartmentalization. Damaging the bark, specifically the cambium – a paper-thin layer just under the bark, halts the transportation of nutrients and water between leaves and roots. If this occurs around the entire circumference of the trunk (called girdling) the tree will die. In addition to mulch, young trees should also always be protected by plastic mower guards for their first several years in the ground.
Photos: (L) A young river birch that was girdled by intense weed wacker damage around its trunk.
(R ) One of our dedicated volunteers, Nancy, places a mower guard around the base of a young willow oak.
Incorrect Mulching Practices
Unfortunately, improper mulching is an all too common sight in landscaped areas. The dreaded mulch volcano is created when mulch is piled in a tall mound against the tree trunk, burying the root flare. The root flare is the area at the base of the tree trunk where it flares out, or grows wider. This flare is easier to discern on some species more than others but can always be found if you look just above where the topmost root emerges from the trunk. (Note: Improper mulching can also occur as a result of improper tree planting. When measuring the depth of your planting hole, always make sure the root flare is level with, or slightly above ground level!)
Photos: (L) An Eastern redbud that arrived from the nursery with its root flare buried by 3 inches of soil.
(R) Where is my root flare? An American sycamore buried with more than 7 inches of mulch volcano.
If mulch is piled against the tree trunk and covers the root flare for a prolonged period, it will greatly reduce the tree’s lifespan. First, microbes in the damp mulch will cause the bark to rot, which can lead to insect damage and disease. It will also restrict important gas exchange of carbon dioxide and oxygen between the living bark (phloem) and the air. It’s like wearing wet socks indefinitely!
Excessively piled mulch causes the tree roots to begin to grow above ground level, rather than in the soil underneath. Over time, these roots will run out of space and begin to circle around the trunk. They can eventually girdle or choke the tree. Mulch volcanoes can also lead to desiccation, or extreme dryness, of these roots. Finally, a thick mat of mulch may become impenetrable and block water from thirsty roots at ground level.
Photo: Mulch mountains, while rare, are also to be avoided! A truly impressive and horrifying sight!
How do you properly apply mulch?
Do you like donuts? Not as much as trees do! Make a large ring, or mulch donut, 2-4 inches deep around the trunk of the tree. Keep the area in the center clear, so that you can see the root flare and the soil beneath. Proper seasonal mulching twice a year in the spring and fall is recommended for most landscaped trees and shrubs.
Tree roots are not a mirror image of the canopy above. They can extend far beyond the reaches of branches and they do not go nearly as deep. A thin layer of mulch, applied as widely as possible, can improve soil structure, oxygen levels, temperature and moisture availability where these roots grow. Most mulch rings reach 2-3 feet out from the trunk, but the tree would prefer mulch to extend to the drip line (edge of the canopy spread) and beyond! Again, this mimics a forest setting where leaves and other organic materials are always cyclically growing, dying and decomposing. (A side note: Consider leaving your leaves alone! Not raking in the areas under trees can really improve soil content. Plus, you will have less grass to mow! Did you know, in one hour of operation, a gas powered lawn mower produces the same amount of volatile organic compounds and nitrogen oxides emissions as eleven cars?)
Where to buy mulch?
We sell mulch at our very own Herring Run Nursery! (Currently closed due to COVID-19. Keep an eye on our website for updates)
Also, Baltimore City’s Department of Recreation & Parks, under the TreeBaltimore Partnership, runs Camp Small, a wood re-utilization facility. Camp small sometimes hosts free mulch giveaways for residents. Keep an eye on their social media page for upcoming events!
Happy mulch madness from your local forestry team at Blue Water Baltimore!
Zoe is Blue Water Baltimore’s Community Forestry Coordinator. In this role, she leads community tree plantings and maintenance with our lovely volunteers. She also organizes our free tree giveaway program and assists with ecoliteracy and educational activities.