What’s Wrong with My Tree?
PART 2: Oozing Stains on Tree Trunk
May 2, 2020
My husband and I just bought a house with a beautiful cottonwood tree in the back yard. We noticed a gooey, rotting section in the crook of the trunk. We certainly don't want to lose this gorgeous tree! Do you know what this is and how we can treat it?
Yes, I know what it is and what caused it. But no, I don’t have easy answers for how to treat it. What you’re seeing is called slimeflux, aka bacterial wetwood. If you were to smell it, you’d never forget it. It smells to me like nail polish remover that’s gone rancid. My uncle, also an arborist, said it smells like gym socks. So basically it smells like what it looks like: gross.
There is no treatment recommended for this unfortunate condition. Hosing the goo away with water doesn’t really help anything since the bacteria are living in the trunk tissue, and the stain won’t go away either.
The reason there’s no quick fix or long-term treatment is that the slimeflux is not the problem. It’s a smelly symptom that there’s an underlying issue: decaying tissue between the two trunks. This brings up a major reason why structural pruning should be done when trees are very small: to keep two branches from becoming co-dominant in the first place. When the branches were small, that narrow crotch wasn’t a problem. But because of co-dominance, both grew at the same steady rate; eventually, as each trunk became wider and wider, they started pushing against each other. Over time, if they continue to grow at the same rate, an included bark junction or “bark inclusion” can form at the base of the branches, and this creates a weak union that’s a perfect place for decay. As Dr. Ed Gilman, tree researcher and Professor Emeritus at the University of Florida, says, “Branches with bark inclusions split easily.” For more on avoiding co-dominant stems and how to prune them (when they’re skinny), visit the blog version of my column and search “Gilman.”
Woody decomposition is caused by the activity of microorganisms, including bacteria and fungi. These organisms are all around in our environment and are unavoidable. Plus, they’re extremely important for normal decay. They aren’t responsible for killing plant tissue; they’re responsible for breaking up and breaking down tissue that’s already dead, especially in the case of fungal decay. And get this, there is even new evidence from recently published research that wetwood-causing bacteria may actually protect included bark against fungal decay! Stay tuned on that doozy. (I’ve added a link to several papers on the blog in case you’re burning for more info.)
OK, so now what? Like I said, I knew the cause, but not the cure. I reached out to several Certified Arborists, landscapers, landscape designers, and urban foresters across the state for their input. I depend on these long-time professionals and their expertise all the time. It was good to hear the same feedback from different angles. To sum it up, this is a major bummer of a question. The options are few and each is risky.
You can live with the aesthetic issue and try to ignore the slimeflux. Pulling back those larger rocks at the base of the trunk, confirming that there’s no plastic under the surrounding rock, adding a thick layer of fibrous or woody mulch, and confirming that irrigation water can reach the very wide root system to a depth of 2–3 feet are tactics that will help your odds. But eventually this tree is going to fail. We already know that cottonwoods are shorter-lived trees. Average lifespans may be around 40–50 years, and even shorter during droughts combined with warmer temperatures. They grow so fast they don’t have time to create very strong structures—or immune systems. Even if your tree was healthy, it may be on the way out. Depending on how bad the bark inclusion is, or how bad it gets, one of those two branches could fall and do some serious damage to surrounding structures, other plants, or people. I’m cringing just thinking of a big wind gust that whips around your yard and torques one of those huge trunks in the wrong direction.
Over-pruning of lower limbs over the years just exacerbated the problem. The branch tips are carrying too much weight, and this will be even more true once your tree leafs out completely. The natural branching structure of trees is such that the weight is distributed evenly among upper and lower branches so that a gust of wind doesn’t become a hazardous situation.
I specifically asked if one of the trunks could be removed by a certified professional, and if the rest of the tree would survive. Unfortunately, these trunks are too big at this point, and the remaining trunk might still topple over if given the opportunity.
Again, Dr. Gilman, “Drastic measures such as cabling and bracing can be taken to help hold trees with these defects together.” But these techniques require skilled, licensed professionals, and will not prolong the life of this tree indefinitely. I personally do not have experience with cabling or bracing. I’m afraid on a tree this size that, even if it’s done correctly, the risks associated with the cabling failing are too high. One local expert pointed out that most of the research that’s been done around the world on cabling and bracing has been done on trees that were healthy, not old cottonwoods that were already pushing their life expectancy. In case you go this route, I’ll share resources on these techniques and terminology on the blog too.
Several of the experts I consulted suggested you do a real risk assessment of the surrounding area, and if the risks are too high, consider having this beautiful tree removed. If you end up doing this, consider replanting with a medley of native and adapted tree—and shrub—species that have a chance at living longer lives and can also put up with our increasing average temperatures. Just to name a few: netleaf hackberry, desert willow, Osage orange, Texas red oak, Chinese pistache, lacebark elm, and honey mesquite. Through The Nature Conservancy’s Urban Conservation Program, I had the privilege of working with tree and climate experts on a year-long project called “Climate Ready Trees for Albuquerque’s Community Forest.” The final report is due to come out any day now, and you can all be sure I’ll write about it as soon as I can. Leaf it to me.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: email@example.com, office: 505-865-7340, ext. 113.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!