October 2010

That's Mulch Better!

Author: Robert C. Dismuke, Certified Arborist

When most people think of mulch, they think of those little uniform shreds of cypress which are colored red to look so pretty. In truth, there is a wide variety of mulch to choose from.

The two major forms of mulch are organic and inorganic. Some common forms of inorganic mulch include stone, lava rock, pulverized rubber and geo textile mats. These inorganic mulches benefit trees by retaining moisture and they also do not need to be replenished as often as organic mulch because they do not decompose. These qualities of inorganic mulch can also have negative impacts on the soil, because lack of decomposition also means no nutrient replenishment, no improved soil structure, and no improved organic content. This is why most arborists and horticulturalists choose organic mulch.

Some of the most common organic mulches include wood chips, pine straw, tree bark, and even leaves. Wood mulch is preferred over pine straw or bark due to its faster rate of decomposition. This means the wood mulch will need to be replenished more often, but also means more benefits to soil quality.

When applying mulch, you should take care not to apply too thick. A fine mulch will need to be applied no more than two inches thick. More coarse mulch can be applied up to four inches thick. Mulch applied too thick will cause anaerobic reactions which lead to fermentation. The production of alcohols and organic acids creates a harsh environment for specific decomposer organisms. These organisms are a key element needed to convert raw cellulose into energy-rich compounds which can be readily used by the trees. Over-fertilization can also harm these beneficial micro-organisms due to build up of salts.

As you apply the mulch, remember to cover the root zone all the way to the drip line. Most of the feeder roots or hair roots are contained in this area, although they can grow several times that distance. Also remember not to apply chips directly against the bark of a tree because this can lead to excess moisture and will ultimately cause disease to set in. To be safe, start your mulch about 12 inches from the trunk of the tree.

Mulch can be acquired relatively easily in most locations and is usually inexpensive when purchased in bulk. Another option is to contact your local arborist. Sometimes they will deliver mulch inexpensively or even free. If you use fresh mulch or un-composted mulch, be sure not to spread it more than two inches thick, and avoid spreading around young trees.

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