7 Reasons You Should Never Mulch Around Your Trees

Mulching your garden beds is one of the most beneficial methods of improving your garden. Organic mulches add structure, nutrients, and living organisms to the soil. They protect sensitive plant roots, and they house thousands of worms that form tunnels underneath the surface. 

So, why is mulch bad for near a tree’s trunk? A little layer of leaves or straw is beneficial for trees, so long as it’s under two or three inches deep. Any depth above three inches begins to cause issues. It drowns roots, causes girdling growth, and exposes your specimens to pests and diseases. 

Avoid adding mulch that touches the tree trunk, keep organic layers under three inches deep, and resist the urge to create a volcano around your tree! Your newly planted specimen will grow a healthy root system that resist droughts, insects, and infections for many years.

Some proposed benefits of mulch mounds are they protect trees from droughts and they help ease transplants into their new homes. These seven reasons provide evidence that these piles are actually harmful to tree health in the long run.

Tree Roots Need to Breathe

Trees must breathe! Like us, they need access to air above and below the soil’s surface. They require access to carbon dioxide and oxygen. When they respire they suck in oxygen, and when they photosynthesize they breathe carbon dioxide. The roots lose access to these gases when mulch is over three inches thick.

When you add the proper amount of organic matter on top of the soil you build its structure. This creates air pockets where moisture or air stays until tree can access them. You want to feed your tree without suffocating it! Think of it in this way and the mantra will help guide you while adding organic matter to the soil.

If there is a tree well around your specimen, simply layer materials like compost or leaves on top. Keep it under three inches, and water the site well. For established specimens, place organic matter like wood chips or straw on top of the ground in a layer three inches thick. Keep these materials away from the base of your tree, leaving a thin layer of un-mulched ground where the roots flare out from the trunk.

Excess Mulch Softens Trunks

One reason mounds of organic matter affect trees is because they target trunk bases. Bark consists of dead, hard, and dry wood that forms a layer between the elements and the inner living parts of the tree. When that dry wood stays wet, dark, and covered for large periods of time, then bark rot and wood decay follow suit.

The best way to avoid harming the base of your tree is to keep leaves and straw away from the trunk’s base. Leave a layer of un-mulched ground around the bark when adding compost or leaf mold. You want wood above where the trunk flares out to be aboveground, and the roots below the flare to be belowground.

In the same way that avoiding excess mulch around the trunk is beneficial, so is planting trees and shrubs at the right level. Ensure you place new specimens in their holes so their root flares, where the roots start growing out from the trunk, are belowground while their upper trunks are above the soil. You want the transplants to be not too high and not too low, but just right in their planting holes.

Dry Mulch is Hydrophobic

Mulch mounds suffocate trees during rainy seasons, and alternatively, they dry out their root system during summer weather. Excessive heat and a lack of rainfall cause mulches and topsoil layers to dry out. When wood chips, peat moss, and compost dry out, they become hydrophobic. Hydrophobic substances repel water, which means the layer will expel moisture rather than soak it up.

It’s easier for mulch mountains to grow hydrophobic because of their shape. Their gentle slope encourages water to drip away from the tree, and dry, hydrophobic conditions accelerate this. When straw or wood chips are in a perfect three-inch thin layer above the ground, they soak water at an even rate throughout, allowing moisture to stay where a tree needs it most.

One way to prevent organic mulches from drying out and becoming hydrophobic is to keep it moist. It’ll soak up water better when moist rather than completely dry. Of course, you should water as often as the specific tree species you’re growing requires. If it tolerates moist conditions, keep the watering regular so it never grows hydrophobic.

Old Roots Girdle The Trunk

Mulch volcanoes may seem harmless at first. And they are! It isn’t until many months pass by that you’ll start to notice the damage they cause. If you see trees around your neighborhood with surface roots that circle the trunk, they probably had a mulch volcano when they were first planted.

Trees use their topmost roots to access nutrients, moisture, and air. As thick layers of organic material sit above these feeder roots, they suffer, drown, and die. New feeders grow above the root flare to compensate for the lost growth. These new roots often circle around the pile, forming constricting woody structures that strangle the tree.

As the mulch decomposes and thins out, these girdling roots expose themselves. Look for trees that sit above the ground on mounds. You’ll notice their thick, woody roots growing erratically on the mound. Prevent these adverse effects by keeping mulch under three inches and placing it away from the trunk’s bark during applications.

Weak Trees Are Vulnerable

After all the effects above occur, your tree may be susceptible to pests, diseases, and adverse physiological conditions. Weak trees struggle to employ natural defenses like sap, healthy wood, or smelly pheromones.

Normally trees use excess sap to block wounds and seal them off. When thirsty they struggle to release this moisture-rich sap, allowing more insects to invade their bark. Weak wood is also more susceptible to tunneling insects. Any open wounds these bugs leave invite fungi and bacteria to colonize the tree’s living cells.

Keep your specimens healthy and strong and they’ll resist these outside pressures. Insects, diseases, and rots will be of no concern! Give new transplants consistent moisture, the right amount of mulch, and the correct sunlight exposures. They’ll excel in no time.

The Trees Can Fall Over

As pests, diseases, and rots creep into weak trees they grow more at risk of toppling over. Their roots are weak and girdling, and their root flares are too deep to provide proper structural support. All it takes is a strong, windy storm or heavy rainfall to cause them to fall over.

Not only is this a bummer because it typically kills the tree, but it’s also cause for concern. A falling tree is a large hazard that can damage structures and harm gardeners! Keep fresh transplants happy, healthy, and without mulch mountains to ensure they grow a strong root system as soon as they’re in the ground.

After strong storms affect your garden take a walk around your local town. Look for toppled trees and inspect their roots. They form girdling circles or grow short and erratic in tight spaces. On the other hand, healthy roots resist fast winds and grow stronger after storms push and pull on them.

It’s A Waste!

My favorite reason for avoiding mulch mounds is that you conserve your organic materials! Why waste this expensive and extremely beneficial natural resource? Use the extra mulch you save to create compost, living soil, or for your other garden beds. Adding a healthy dose annually keeps soils protected, moist, and drought-resistant.