CO-Horts Blog

Thursday, August 25, 2016

A Tale of Two Honey Locusts

Posted by: Mary Small, CSU Extension State Master Gardener Coordinator
One troubled Honey Locust!
Trekking across campus I came upon these two honey locust trees. They represent the kind of question I really dislike answering - especially when you can’t actually see the plants in question. Why does one look great and the other one, well - awful? They were planted at the same time. They were purchased from the same company. They both looked good the first few years. And so on…..

So let’s go see what’s wrong with the tree in question, even though I want to shout “Hello, it’s trying to grow in a parking lot!”

First I found weak-looking leaves that were minimal in number. Twig growth increments aren’t (and haven't been) very long either, telling me that the plant hasn't been growing well. It makes sense – few leaves produce few carbohydrates resulting in little growth. On top of that, since mid-August, the tree has been experiencing “early fall”. Another bad sign, indicating some kind of stress that has caused photosynthesis to slow or cease.
Gummosis - not good

Uh-oh – there’s some gumming on the trunk and many of the larger branches I can see. Not a good sign. Right off the bat, it means the tree is stressed. The stressors include but are not limited to drought, sunscald, canker diseases and collar rot. 

The lower trunk has no flare and is flattened on a couple sides.  You can see one of those sides in the photo below. That signals girdling roots, so it’s time to do some excavating. Having only a twig for digging, I find one root just under the soil. Although I couldn’t dig deep enough, with the appropriate tools, I’m pretty sure I would have found more girdling roots.
Flat side and sprinkler too close to trunk

Girdling root

I will admit to not being astute when it comes to automatic tree irrigation, but I have a hard time understanding how one emitter adequately irrigates a tree of this size. Having it so close to the trunk of a honey locust can invite root collar rot infection, too, by keeping the collar area moist and conducive to pathogen growth and development. 
Narrowing trunk and slightly excavated trunk flare (right)
I was able to find only the beginning of the trunk flare and it’s at least 3 inches too deep. It's just showing there on the right side of the photo.  This tells me the root system is also planted too deep. Roots need oxygen to carry on various processes and when they can't get the oxygen they need, processes slow or shut down. They can't resist pathogen infection well or at all, either. 

For fun, compare this deeply planted trunk to the healthier tree’s trunk (below)– it has a comparatively good flare. And you can see it without any excavation.
Better trunk flare of healthier tree

So why does the one tree look healthy and the other awful? The nearly dead tree was planted too deep, causing oxygen starvation and a slow decline. I strongly suspect girdling roots, which would restrict water, nutrient and carbohydrate movement, are also at work here. The heat generated by the asphalt parking lot isn’t helping things, either.  Poor tree!

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