When some people visit my yard or see photos of my trees, they spot a violation of the rules: Aren’t you supposed to keep mulch away from the trunks of your trees?
(That’s my rule-breaking lime tree in the photo above.)
It’s true, there’s no real benefit to keeping mulch, such as leaves and wood chips, right next to the trunk of a tree. The main reasons to mulch are to prevent weeds, conserve soil moisture, and feed the life in the soil below. Little of that is needed right beside the trunk.
No benefit but some risk
On the other hand, mulch touching the trunk has a potential negative consequence. I’ll let this be stated by the University of California publication, “Fruit Trees: Planting and Care of Young Trees.” It says, “Mulch should be kept several inches away from the trunk to minimize the occurrence of crown rot.”
It’s helpful here to learn more about crown rot. Then we can make sense of why this mulching “rule” can be bent, of why I do bend it.
Crown rot is a disease said to be caused by the Armillaria mellea fungus and especially different kinds of Phytophthora fungi (actually, recently reclassified as oomycetes rather than fungi — but that’s of no practical importance here). The symptoms of the disease can look like drought stress and can show as darkened, even oozing areas at the base of the trunk. (Read more at this UC Integrated Pest Management page, “Phytophthora Root and Crown Rot in the Garden.“)
Why the risk?
But what’s most important is to know why it’s happening.
It so happened that the other day I spoke with Pat Nolan, Plant Pathologist for San Diego County, and this topic was on my mind. So I asked her about the conditions needed for crown rot. Constantly moist, she said.
And so this is the reason for the advice to keep mulch away from trunks: Mulch retains moisture. This is a great thing when it’s conserving moisture in the soil where the tree’s roots are. It’s dangerous when it’s keeping the base of the trunk moist.
It’s all about the watering
When applying mulch, I never deliberately pile it up against the trunk. That’s both wasteful and daring. But neither do I scrape it several inches away from the trunk. Not during application and not later. I just don’t bother.
I can get away with this for two reasons.
One, I don’t water my trees right at the base of the trunk — except during the first month for newly planted citrus or avocados. I apply water out near the edge of the tree’s canopy. So the trunks of my trees stay generally dry because of this.
Two, I water as infrequently as possible. Remember, as Pat Nolan said, it’s constant moisture that is required, not just any moisture at any one time. (Obviously! All trees get wet trunks during a heavy rain.) This is crucial because I use sprinklers and sometimes they get knocked and end up spraying the trunk for one or more irrigations.
No big deal. As long as the irrigations are far enough apart such that the soil at the base of the trunk has dried out in between, Phytophthora won’t find the environment hospitable.
If you use drip emitters on your trees, then make sure they are at least a foot away from the trunk — again, except for the first month or so after a new tree is planted.
So what’s the answer? Should we keep mulch away from tree trunks? Sure. But don’t stress about it. The stress should be more on watering trees as infrequently as possible and watering them where they prefer to grow their roots, which is away from their trunks.
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