Letter to the Editor

I enjoyed the very thorough article on Fertilization of Trees by Keith Babberney.  I have changed my approach to this practice over the years.  For the first half of my career, I fertilized or recommended fertilization for every tree in my care on an annual programmed schedule.  Carl Pool granular applied in 2 inch augured holes, then Bartlett Boost deep root, Doggit Evergreen deep root, and eventually Bio Pak Plus deep root.  Early on, I even used some RapidGro for super stressed trees needing immediate help.  The last 15 years I have changed to just fertilizing trees that are competing with lawn grass and for injured trees.  Today, I am even changing that approach.

Daniel A. Hermes in his Continuing Education article, Understanding Tree Responses to Abiotic and Biotic Stress Complexes, Arborist News December 2016, talks about the natural defenses of trees to insects and pathogens.  He is referring to things like foliar pubescence, toughened cuticle, indigestible structures of lignin and cellulose, and most important, the toxic and deterrent effects of allelochemicals (secondary metabolites).

He disagrees with the premise that environmental stress decreases a tree’s resistance to insects and disease by weakening natural tree defenses.  He states that many reputable studies have found that concentrations of allelochemicals and insect resistance increase in response to nutrient limitations, drought, defoliation and other stresses.  He also says there are other cases where stress does weaken tree defenses. (My comment: to the practicing arborist’s dismay, there are no definite answers in biology.  Mostly it does, but sometimes it does not).

Then he says, “Numerous studies provide strong evidence that fertilization decreases tree resistance to both chewing and sucking insects.  Reliable studies have shown fertilizer to increase growth and decrease concentrations of defensive chemicals and insect resistance.  Plant resistance to disease is also generally (but not always) decreased by fertilizer.  When nutrient stress is severe, only then can fertilization increase tree resistance to defoliating insects.”

We have equated rapid growth with tree vitality, tree health.  Twig elongation, thick growth rings, large green leaves with good color, have been measures we use to determine a tree in good condition.  We stress maximum growth with many of our maintenance operations.  Rapid growth uses a high proportion of tree resources, diverting them from storage and defense.  In many cases, fast growing trees are more susceptible to stress and less resistant to pests.  Fertilized trees often require regular irrigation and pesticide application.  That’s the formula used by the lawn care companies that we so often criticize for creating the problems they need to treat.  We may be doing the same thing without realizing the consequences of our actions.

Dr. Michael Raupp in his presentation to the ASCA Conference in 2018 stated that much of the nitrogen we apply for trees is taken up by the insects that are feeding on our fertilized trees.  He says we are creating super bugs.  He points out that the nitrogen recycled in a climax forest is a little over one pound nitrogen per thousand square feet per year.  We typically apply 3-6 pounds in this same area with our fertilization programs.

Dr. Linda Chalker-Scott in her presentation at the 2018 Texas Tree Conference talking about Arboriculture Myths said there is no reliable supporting science showing that tree fertilization is beneficial.   She says that because tree fertilization is not science based, it should not be recommended.  She has provided the same message in presentations to the International Tree Conference.

There are no easy answers here.  In my career, I have sold and applied more than a million dollars of fertilizer (plus or minus).  Our clients want their trees fertilized, growing fast.  This practice is very profitable and the companies we work for or own require profit.  It is one of the few maintenance operations we can schedule with an annual program.    One man in a fert rig can often bring in daily the same amount of income generated by a 3-man pruning crew and at a higher percentage of profit.

If our goal is to grow strong, resilient, climate change tolerant, stress tolerant trees, we must rethink our approach to fertilization.  In the long run, are we doing more harm than good?  How do we decide if fertilization is the best approach for a tree when we now have good evidence that it is usually not?  How do we ween our companies off a well-established practice that is so profitable?  How do we convince our clients that some stress is good for a tree?  We experienced this same dilemma when Dr. Shigo let us know we should not be cleaning, filling and reinforcing cavities, a practice that was a major percentage of tree care business at the time.

Personally, I have switched to recommending compost and mulch in almost all cases.  Easy for me to do now that I am just consulting and no longer needing the wonderful profit generated by a tree care fertilization program.

David Vaughan
San Antonio, Texas