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Whooah with that Mulch around your trees!

As an arborist I see an excessive use of mulch and it leads to numerous long-term problems for trees. Especially susceptible are newly landscaped areas where folks, including the landscaping team are over mulching.

My main concern is that often trees are packed with mulch many inches up the side of the tree –especially young trees. The main rule to follow is leave the root flare of the tree free of mulch.

Volcano mulching is a common term used in arboriculture describing mulch piled up against the tree trunk that is so thick and conically piled that it actually looks like a volcano. There is no positive use for this display of mulch, in fact it is highly destructive to trees, especially young ones.

"Volcano" mulching is harmful to trees

Mulch piled up against tree trunk causes:

  • Excess moisture to build up on tree trunk, inviting mold, mildew, insects and fungus to damage the trunk, weakening the tree and inviting decay.
  • Stimulation of feeder roots to grow upwards through the new layers of mulch which inevitably become girdling roots, stunting the growth of the tree, choking off nutrient flow, and eventually killing it.
  • Deprivation of gas exchange in the feeder roots. In addition to water and nutrient uptake, roots take in and release gases! A pile of mulch on the roots is just the same as burying the roots too deeply. Certain death for a tree!
  • Does not effectively slow moisture loss; the delicate feeder roots are located further away from the trunk under the leafy canopy, not at the base of the tree where mulch is usually laid.

How to Mulch a Tree Correctly

  • Identify the root flare of the tree—where the trunk begins to flare out, even just slightly in a smaller tree. Can’t find it?—the tree is probably planted too deeply already, or has a large volume of mulch hiding the root flare. Dig away the mulch. This may seem like a new project, but you may just be embarking on saving your tree. If you have this problem you already have enough mulch to redistribute.
  • Pull grass or herbaceous plants from the area around the roots. Where the roots descend into the soil is where the mulch can begin. Two or three inches deep of mulch here is fine. It will compact down over time and with rain.
  • Place mulch out from the root flare to the drip line, and even further if you are designing a landscape bed or you want to establish a mowing boundary around the tree. (We advise this!) Out at this distance from the trunk, three or four inches is enough. Again, it will compact down.
  • Make it look natural, spreading away from the tree.
  • Be at peace with mulch for a few years. Applying mulch every year will only be a disservice to your trees as it piles up, so if you want to renew the look of mulch, just get a rake and fluff it up. You'll save yourself some time & money.

Know a bit about Tree Biology so you’ll mulch better

  • Trees appear to push themselves up out of the ground, letting soil, leaves, and eventual compost to fall away from the trunk.
  • Trunk bark is not the same as root bark. Roots and the root bark can withstand soil and moisture; Trunk bark can withstand sun, wind, and air.
  • Trees have stabilization roots which grow away from the trunk, usually within the upper foot of soil around the tree—extending out from the tree roughly one or two times the height of the tree!
  • Trees have feeder roots with tiny hairs, the sum total of the surface area of a trees feeder roots are greater than the surface are of its leaves! That’s a lot of surface area providing the tree with:
    -gas exchange—occurs through the soil
    -water uptake-from ground sources and rain
    -starch storage for next year’s growth –thanks to this year’s photosynthesis
    -the uptake of minerals often with the assistance of beneficial fungi, called mycorrhizae
  • Feeder roots compete with grass especially for moisture and nutrients

Why mulch?

In the forest, a tree has the forest litter to cover the ground. In your suburban or urban yard here are some good reasons to mulch correctly:

  • Create a no-mow, no-weedwacker zone. Repeated injury to the roots and bark can kill a tree of any age. Weed around the trunk by hand, and extend a mulch bed out past any roots that rise above the soil.
  • Reduce the nutrient and water competition with grasses and vigorously growing groundcovers.
  • Conserve soil moisture by reducing evaporation
  • Can control soil erosion
  • Promotes healthier soil ecology by bringing in and providing food and cover for microorganisms and insects
  • Cools soil surface temperatures from 15 to 30 degrees!

Using fresh mulch chips from your recent tree job?

If you are using chipped wood and debris from recent tree care, let the chips compost in a pile for several months to several years before applying them.

  • The raw chips have a high carbon content and draw nitrogen from surrounding soil on which they lie. Continue to add appropriate fertilizers to your plants to augment nitrogen loss. A clue will be yellowing leaves in an otherwise healthy plant.
  • A pile of raw chips immediately shifts into a literal hot bed of activity where bacteria begin breaking down the chips. This heating process gets the entire pile started with decomposing. Again, leave the chips in a pile to assist with this process.
  • Pine chips are acidic; use for acid loving Azaleas, Hollys, Rhodies etc…
  • Black Walnut chips are best not used for planting beds due to strong biochemical constituents that inhibit plants from growing. Use for pathways instead.
  • Keep large piles of chips away from the side of buildings during the decomposition phase, as the pile is host to numerous insects who are breaking down the pile.