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.

Don't Take It Out on Your Trees!

     The big wind and rainstorm that hit California this week will be fodder for newspaper photographers and TV cameramen for several days. Pictures and video of large, downed trees, teetering against rooftops, has frightened homeowners. They may be asking themselves, "Will that large oak in our front yard come tumbling down in the next storm?" Their first line of response may be the wrong ones: topping the tree so that it stands lower than the house; or, removing the tree from the landscape.

The correct immediate response: inspect those trees (a subject tackled back in October in this FF Rant posting).

And when you see those fallen trees in the paper or on the TV news, your first question should be, "Why did those trees fail?"

And don't take the word of a reporter. Today's front page picture of a fallen tree in the Sacramento Bee was described as "uprooted". A casual reader might then infer that the tree fell over due to saturated soil. But take a close look at that picture. Do you see roots? Nope. That oak had snapped at the base. Take a look at the interior of that trunk. See all that black area? That's a sign of rot.

"It looks like a typical case of crown rot," says Consulting Arborist Analisa Stewart of Sacramento-based Arbor Entities. "And it was certainly not 'uprooted'; if it had been 'uprooted' you'd see a large root plate - usually twice the diameter of the trunk, hanging out all over the place. I'd suspect fungal decay, which could have been detected with a routine root crown exam."

     After seeing those news reports, a homeowner might think that the wise move may be to remove a large tree looming over a house as soon as possible. But you better think again, says Stewart.

     "A downed tree makes an exciting visual manifestation of a storm - so I think they're disproportionately covered in the media," she says. "The most expensive time to remove a tree is during or immediately after a storm event - because companies are paying overtime, and are paying emergency and hazard wages to their climbers and staff. People seem surprised by that. "
     "Trees have evolved to withstand wind. In a native forest environment, it's natural and normal for trees to fall apart in pieces. That's not desirable in an urban situation, which is why it's necessary to have a knowledgeable person responsible for maintaining trees.
     "Some tree species are more prone to wind failure than others. Down on the valley floor, we see more ash trees, elm trees and the occasional oak fail. In the foothills, pines are more likely to fail. Grey pine in particular is more susceptible to wind throw than other species according to the International Tree Failure Database.

     "It's a good idea to have large trees looked at on a routine schedule. A typical tree maintenance rotation for a homeowner would have a tree pruned about every three years. There are some trees that can go 5, 7 and in some cases even 10 years between pruning cycles. If a tree has been cabled, the cables should be examined every year by a certified arborist. It can't be done from the ground - the arborist actually needs to examine the cable attachments and make sure the cable is under tension. Cables don't eliminate tree limb failures - they do reduce limb failure by reducing the amount of weight and stress on a particular limb. If that limb does fail, the cable helps pull the limb back into the tree, instead of letting it fall away from a tree and onto a target, like a roof.
     "One of the things a certified arborist can do is evaluate a tree's hazard potential and recommend things to mitigate the potential damages. Remedies can range from routine pruning, limb cabling and targeting (what the tree or limb would land on if it failed) removal, not total tree removal. In some cases, there are very minimal risks of failure or damages and nothing at all needs to be done. In any event, if an arborist assesses a tree and completes a written report on it and the tree fails - the arborist bears the liability and has insurance to that end."

     "Things that increase the risk of tree failure during a storm include root pruning within the last 10 years (did you have to replace a water main? Were lots of roots cut on one side of the tree when you did?), incorrect pruning of the canopy (like topping), lack of maintenance (no pruning is just as bad as incorrect pruning), construction near the tree within the last 10 years, a history of surface watering, location of the tree (on a cliff that's been eroding for the last 7 years), changes in the tree's environment (a sheltering tree within 30 feet was removed, altering wind movement in the remaining tree) and overall age, condition and species of a given tree. Those are lots of variables, which is another reason to get a certified arborist out to look if a person has any concerns.

     There are tremendous benefits to trees here in Sacramento - and large canopy shade trees have exponentially larger benefits, according to the Sacramento Tree Foundation.

     "After our storm in January 2008 I saw a lot of gratuitious tree removals," says Stewart. "Large trees came down because people were scared, not because their trees posed a serious risk. Given my vocation its probably not surprising that I was saddened to see so many stately mature trees go, and am worried about the potential for a repeat. Mature trees simply give too much back to remove them needlessly. I'm not advocating people keep risky trees - I am advocating that they get some good solid advice from a certified arborist before they start a chain saw."

     The other urge worried homeowners need to control: tree topping. Dramatically reducing the overall height of a tree to lessen its threat to a roof may instead increase the risk to people and cars down below.

     According to the Arbor Day Foundation:  "Never cut main branches back to stubs. Many people mistakenly 'top' trees because they grow into utility wires, interfere with views or sunlight, or simply grow so large that they worry the landowner. Unfortunately, the topping process is often self-defeating. Ugly, bushy, weakly attached limbs usually grow back higher than the original branches. Proper pruning can remove excessive growth without the problems topping creates. In addition, many arborists say that topping is the worst thing you can do for the health of a tree. It starves the tree by drastically reducing its food-making ability and makes the tree more susceptible to insects and disease. The appearance of a properly pruned tree is like a good haircut: hardly noticeable at first glance."

     

Here are seven reasons from the Arbor Day Foundation why you shouldn't "top" trees:

1. Topping removes so much of the tree's crown that it temporarily cuts off its food-making ability. Good pruning practices rarely remove more than one-third of the crown.

2. Topping suddenly exposes the tree bark to the sun, possibly resulting in scalding to the tree trunk and any surrounding (and formerly shaded) shrubs or lawn.

3. The large stubs that are left after topping expose the tree to an insect or disease invasion.

4. Any new limbs that sprout after removing a larger limb will be more weakly attached. When the next wind and rain storm hits, it will be these branches that may land on your car or house.

5. Trying to control a tree's height by topping? Those new limbs that sprout will be more numerous than normal, so the tree returns to its original height in a short time with a far denser crown. Again, storm damage may result.

6. A topped tree is an ugly tree that will never regain its original grace and character, robbing you and your neighbors of a valuable asset.

7. Topping may be easier and cheaper than applying the skills and judgment necessary involved in good pruning. However, the hidden costs of topping include reduced property value, the expense of tree removal if the tree dies, the risk of liability from weakened branches and the loss of other trees, shrubs and lawn if they succumb to the dramatically increased sunlight.

Bottom line?

Trees: good.

Taking care of trees: better.

Having a professional (certified, bonded, insured) arborist take care of your trees: best.