CO-Horts Blog

Tuesday, April 19, 2022

Old Mulching Technique

Old Mulching Technique
By CSU Horticulture Agent, Linda Langelo

                                  Liquidambar styraciflua 'Volcano'

                                             Photo credit: Anonymous

You might ask yourself, "Is the author for real?" This is not a cultivated variety. Unfortunately, the technique of volcano mulch practice is still alive and well. This practice was considered the best practice during my parent's lifetimes in the 1940's. As Maya Angelou has said, "When we know better, we do better." Hopefully.

Liquidambar styraciflua is commonly called Sweetgum. It grows in zones 5-9 on wet river bottoms, swamps that frequently flood and sometimes on drier uplands. According to the Colorado Tree Coalition there is a sweetgum that is The Colorado State Champion Sweetgum (DBH 17"/Height 57') in Fort Collins. Sweetgum can grow to be 80-120 feet tall and 4 feet wide. But this depends on many environmental factors such as growing it in a moist soil with no high winds, hail damage or temperature extremes from 75 degrees Fahrenheit to -5 within in a few hours. It is a medium to fast grower. In aklaline soils iron chlorosis is a problem along with webworms, scale, bleeding necrosis and leaf spots.

As a tree hugger, seeing the volcano style mulch I should have titlted this blog article, How to Kill a Tree in Two Years or Less. I might add the person who sent this to me is not a tree hugger but sensed that this was not the right thing to do. Knowing that we already have Sweetgum as a Champion Tree why not give Sweetgum Trees everywhere the same opportunity? Or for that matter any tree the opportunity to become a Champion Tree or just to be an old tree?

Here is what can happen when trees are mulched this deeply:

1)the foliage starts to yellow or become off-color

2)there are abnormally small leaves

3)poor twig growth and dieback of limbs

What happens when you bury the root flare? Because the root flare has different tissues than other roots, piling mulch heavily decreases the gas exchange stressing the inner bark. These are lenticels which are pores the tree uses for gas exchange. Blocking them or waterlogging them with a barrier of mulch begins to affect the health of the tree. When the inner bark dies, roots become malnourished and weak. It would be no different than planting the tree too deep. 

By having a volcano of mulch around the trunk, this encourages diseases. Bacterial and fungal diseases require moisture. These diseases are opportunistic and mulching like this gives them the conditions they need to survive and prosper. These diseases can stop both the flow and storage of sugars produced in photosynthesis placing the tree in decline. Many borers are known to be attracted to trees in decline.

As if oxygen deprivation and starvation are not enough in over-mulching a tree, excess heat is another issue caused as the mulch decomposes. The high temperatures again affect the inner bark. With young trees if the trunk flare is not hardened off before a hard freeze, this results in damaged tissues.

Lastly, rodents and field voles like cover whether it is tall grass or deep mulch. Add girdling the trunk to the list of possibilities. There is no way of saving the tree when this happens. 

What is the solution? Leave the root flare free of mulch. Mulch 2-3 inches on the edge of the shoulders of the root ball. Here are some links to proper tree planting and mulching:

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