When a tree shows signs of uniform decline throughout the canopy, how does an arborist begin to sort out the possible causes? The root collar — the base of the tree where the roots begin to spread out from the trunk — is a good place to start. That is where I have been finding an obvious reason for the decline and death of crape myrtles in local landscapes.
Crape myrtles (Lagerstroemia indica and hybrids), common as street trees all over Napa and the Bay Area, put on a great show of color, especially this month. The tree is so reliable, sidewalk-friendly, adaptable to poor growing conditions, and generally free of pests and disease that it has become the all too common “go-to” tree. When in doubt, plant a crape myrtle.
But, over the past few years, I have noticed that the tree has an Achilles heel — the tender bark at the base of the trunk.
When a property manager asked why a crape myrtle, which should be green, leafy and covered with showy flowers, was bronze and dying, a quick look at the base of the trunk revealed the answer: Located in a lawn area, the tree had a neatly trimmed small circle around the base, where turf grass adjoins the trunk. Unfortunately, the bark was killed by the string trimmers that keep the grass cut short.
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Conveniently, as I looked at this tree and another nearby with similar but not as severe damage, the landscape maintenance crew arrived and fired up the power tools.
Seizing a rare opportunity for direct intervention, I didn’t throw myself in front of the weed whacker, but I had a nice conversation with the foreman. I showed him the damage, suggested that the crew should be made aware of it and instructed to back off from the trees when trimming grass.
This was the third property in recent memory where I had seen this very problem. It is scary to think that these nylon string trimmers are buzzing all over the place, with operators oblivious to the damage they are doing.
Part of the problem is that the nylon line spins so fast it is invisible and the bark at the base of the trunk doesn’t go flying. It just gets bruised or cut enough that, over time, it eventually looks like a rodent has been chewing it away.
Ironically, in some instances, the only crape myrtles that are less damaged are the ones that have an old tree stake, no longer necessary to support the trunk, serving as unintentional guards, deflecting the string trimmer line.
A simple solution is to simply remove the turf to a safer distance from the trunk. Just a foot or two is enough. Even better for tree health, to reduce competition between trees and greedy turf, the distance can be increased much farther. The bare space can be covered with a decorative wood chip mulch. Just don’t pile it up against the root collar. That invites other problems.
Bill Pramuk is a registered consulting arborist. Visit his website, billpramuk.com, email questions to info@billpramuk.com or visit Bill Pramuk, consulting arborist on Facebook.