Committing Tree-icide: Water & Mulch


(Don’t miss last month’s post about proper tree planting!)

Once we have the new tree in the ground, we want to do our best to help it not only survive but thrive. Knowing how dry our climate is, it’s natural to focus on providing enough water for the tree to become established.

A newly planted tree needs to be watered where its roots are. Those roots will be close to the trunk, which is why the landscapers set up their drip emitters to irrigate that area.

However, assuming the tree was properly planted with its roots emerging like spokes of a wheel, eventually the water will need to be applied further and further from the trunk. Over the next few years, the ring of emitters needs to be repeatedly enlarged and moved to keep up with root growth. Obviously, this didn’t happen in the case of this pine tree:


The water-absorbing roots of a mature tree extend from about mid-way to the drip line (how far the branches extend) to about five times the diameter of the tree! In most cities, that means well into the neighbors’ yards. Most of the roots, however, will be concentrated near the drip line, and that’s where the water should go.


There are roots closer to the trunk, but these are prop roots, which support the tree. They can’t absorb water, so water applied there is wasted. Worse, keeping those roots soggy can lead to rot and disease.

Tree roots_SnohomishCo-WA_LAH_5503

Worst of all is this set-up I noticed on a neighborhood walk. The emitter is actually spraying the trunk! I’ll be surprised if this tree doesn’t succumb to some sort of fungal or bacterial infection in the near future—if it lives that long; notice that it is also planted too deeply!

Tree irrigation_2016-08-24 13.17.58

Once we get that water where it belongs, the next step is to mulch. A 4 inch layer of mulch helps keep that water in the soil, as well as suppress weeds, mitigate soil temperature extremes, and encourage beneficial microbes. However, the last thing  you want to do is create a “mulch volcano” as these homeowners did:

Piling mulch against the trunk is a bad idea for several reasons. Ohio’s Cuyahoga Soil and Water Conservation District is tackling this relatively new issue head on with their “Let the Flare See the Air” campaign.* As they explain, “A covered root flare weakens the trunk of the tree, encourages girdling (choking) roots to grow around the trunk, and leaves the trunk wet so it is vulnerable to infections and diseases.” Mulch is good, but place it over the roots and leave the immediate space around the tree bare.

Make the effort to properly care for your trees. It saves time and money, and allows you to enjoy them for years to come.

* Sadly, I noticed that on their otherwise excellent website, they still don’t recommend root washing to correct underlying root issues, even though the practice is strongly supported by research.

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