Don't Water a Tree's Trunk

As gardeners, we usually don't think twice if sprinkler water is hitting a tree trunk. As long as the grass, flowers and shrubs that we want irrigated are getting water, who cares? Arborists care.

Analisa Stewart, consulting arborist for Sacramento-based Arbor Entities, cringes at the thought of what might happen to the long-term health of that tree with the trunk that's getting doused several times a week.
"Trunks don’t absorb water," explains Stewart. "Water on tree trunks is wasted irrigation."      

"A tree's main stems were not designed to come into regular contact with water," says Stewart. "More so when the outside temperature is hot. The only time main stems should be in contact with water is when it’s raining outside."

Armillaria root rot due to drip system touching tree trunk

"Water droplets are vectors for pathogens," says Stewart, who also serves on the Tree Advisory Committee for the Sacramento Tree Foundation. "The bacteria travel in water droplets. The same is true for many fungi, like anthracnose."

Water dripping off main stems to the ground aren’t readily used by trees either. "Most water-absorbing roots are at the end of the roots, farther outside the canopy," she says.

 Avoiding tree trunks with an automatic garden watering system, such as in-ground sprinklers on a timer, may be difficult. Try to adjust the sprinkler spray to that tree trunks are spared.
  

However, if you are watering a tree with a hose-end sprinkler or drip sprayers, position them to point away from the tree, not towards it.

Two Mini-Sprayers at the edge of the tree canopy, spraying outward.

 Place the sprinkler away from the trunk, at least halfway between the trunk and the dripline (outer canopy) of the tree. As Stewart says, the tree's dripline and beyond is where the water will be absorbed by the tree roots.
 
Stewart goes on: "There is some controversy about how pathogens gain access. The late Dr. Alex Shigo (the father of modern arboriculture) researched extensively and put forth the idea that pathogens could only gain access through a wound, be it a pruning cut or mechanical damage caused by rodents. Wounds were the entry point. Since his death there has been extensive research on both Pseudomonas and Phythopthora (especially P. ramorum, aka Sudden Oak Death) and early results seem to indicate that pathogens could be gaining access through lenticels, which are small openings in bark.
 
"Depending on the force of the spray, tree species, and overall health of the specimen I routinely see trees that have actually been bruised by water from sprinklers on thin barked trees. Often this leads to the formation of a canker, or sunken depressed area of bark. That area loses vascular flow and  becomes a site for other potential problems. Best case scenario is the tree has to expend resources sealing the wound, meaning those resources aren’t available for say, growth, anchorage, storage or other kinds of defense. I’m thinking specifically of a private country club that had a number of Japanese Maples all in a row, all beat to death by their irrigation system." 

Stewart concedes: "I don’t know how much a homeowner cares about all this, but she does care about my ability to manage the pathogen in the landscape."

And successful management may begin by just redirecting the water away from the tree trunk.