Mulch is Your Tree's Best Friend

Picture a new landscape – newly planted trees with a layer of mulch placed on top of the soil in a circle. There are certain things that people do when it comes to their landscape and their trees, but this season we would like to get into the reasons why mulch is used. Some people may apply it because it looks nice, or because it’s just something that is accepted as a component of planting a new tree. Mulch is so much more than that though, and it can greatly benefit newly planted trees as well as established trees throughout urban areas.

Here is something to consider: When walking the trail at Hafer Park or hiking at Arcadia Lake, notice the forest floor surrounding the trees. It is covered by what is known in forestry as a “litter layer” or “duff layer” comprised of fallen leaves and branches, decomposing logs, and other organic material that is breaking down. These materials are one phase of mineral cycling in a forest ecosystem. As they decompose, they are adding minerals back to the soil which provide necessary elements for the surrounding trees. In addition, they prevent moisture from evaporating from the soil, which helps to prevent water stress during the hot, dry summer months. The litter layer is part of a complex ecosystem and plays a major role in the health and sustainability of a forest.
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In the urban environment we have many native trees and many trees that we add to the landscape. One major difference commonly seen between an urban yard and a forest, however, is the absence of the litter layer, with turf grass in its place. In sunny locations Bermuda grass often grows beneath landscape trees, while shady lawns often incorporate low light grasses such as fescue. In these situations, the amount of soil organic matter is significantly reduced, which can impact soil mineral content. Grass competes with tree roots for water, much of which is also lost to evaporation. This is elevated in part due to a lack of the natural litter layer which insulates the soil and keeps it cooler. The amount of water applied to fescue can also cause harm to trees such as native post oaks and blackjack oak, which are very susceptible to stress from too much water.
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What solution do we have in a landscape with turf grass to prevent these conflicts between grass and trees? Mulch. Mulch is your tree’s best friend. This is advice we often provide to Edmond residents when providing consultation for tree concerns. Mulch provides that essential insulation over the soil that conserves moisture and regulates soil temperature. It breaks down over time, adding minerals back to the soil. Mulch reduces competition with weeds and also prevents damage to tree trunks from lawn care equipment. Studies have shown that trees surrounded by mulch have denser root systems and grow larger faster than trees surrounded by turf grass. Essentially, mulch mimics nature’s forest floor – it’s a human-applied litter layer. Trees can survive without it, but when it comes to preventing stress and treating trees that have already come under stress, spreading mulch around a tree is the number one recommendation we regularly make. It's relatively inexpensive and highly effective. Now that we have the reasoning why mulch is so important, what is the best way to apply it?
In order to make sure your tree receives the most benefit from a mulched area, it is important to understand a tree’s root system. The majority of tree roots grow in the top three feet of the soil, with most absorbing roots in the top 6”. The growth habit of a tree and its root system resembles a water goblet on a plate. Tree roots extend far beyond the furthest extent of the branches. As a root system spreads, the majority of the absorptive roots that take in water and minerals are located further from the trunk. The best scenario for a tree in terms of mulch is to apply a 3-4” thick layer across the entire area beneath the drip line of the tree, or the area covered by the tree’s canopy. Understandably, this is a little more surface area than most homeowners would like to spread in their yards. We recommend applying as much as you’re comfortable with, having the knowledge that the larger an area applied beneath the drip line, the more it will benefit a tree. A grove of trees could be converted to a landscape bed with shade plants installed throughout. Creative approaches such as this can benefit the trees as well as landscape aesthetics. Mulched beds may also reduce maintenance and water needs in comparison to fescue, which can further reduce overwatering stress incurred by existing trees.
Here are some important things to remember when mulching trees:
  • Always pull mulch back off the trunk and leave a couple of inches around it mulch free. Volcano mulching, where mulch is just piled around the trunk, has no benefit to a tree and can actually lead to trunk decay since it traps moisture against the bark.
  • Apply mulch in a layer about 3-4” thick, ideally. 6” is the thickest that should be considered – amounts larger than that could impact the tree’s ability to exchange gases through the soil surface and to access water.
  • Mulches made of organic material will have the greatest benefit for a tree. We recommend avoiding inorganic mulch such as rubber pellets.
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Equipped with this knowledge, Edmond residents can create more favorable growing environments for their trees. Please see the links below for more information about proper mulching techniques and the relationship between trees and turf in the landscape. This information originates from the International Society of Arboriculture.

Proper Mulching Technique
Trees and Turf

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