Tree care for drought stressed trees

Last year’s record heat and drought compounded the severe impact that the 2009 drought had on trees in Austin, Texas.  In 2009 we saw record levels of dead and declining trees due to drought.  The heavy rains that occurred that fall and into 2010 were not enough to save many of the trees that initially survived.

severe dieback caused by drought

The same thing is happening now.  The heavy rains of this winter were not enough to save many of the trees that survived through 2011’s drought.  Now that the late-blooming species are in leaf, what is dead is apparent, and the phones at tree care services around town are really ringing.

My business is about tree care, not about being a “tree mortician”.   I have focused my 30-plus year career on maintaining trees and advising property owners on what they can do to preserve their trees.  This means pruning, cabling, fertilizing, and other tree care techniques.  I can certainly perform difficult removals, but they are not a satisfying thing for a highly experienced Certified Arborist.

So, what can one do for a drought stressed tree?  Well, the short answer is, unfortunately not much.  If there is substantial dieback, nothing is going to bring those dead limbs back to life.  Now that almost every live tree in Austin is in leaf, if it doesn’t have leaves, it’s very likely dead.  Once you know for certain that a large limb is dead, it should be removed quickly.  This is  for safety purposes.  Some species shed their dead branches much more quickly than others.  Pecan for instance can shed large dead limbs within a year, whereas live oak will often hang on to them for many years.  Dead limbs really aren’t a health issue for trees in the short term (less than a year).

Some tree services recommend fertilization for trees affected by drought.  Although fertilizing has its place, I do not automatically recommend this.  In fact, fertilizing can be harmful to a tree in a severely stressed condition.   Fertilization should only be used when there is a known nutrient deficiency.  This is something I always assess before considering the addition of nutrients.

The best approach to trees stressed by drought is to maintain proper irrigation levels and improve soil quality.  Soil improvement is largely done through the addition of organic materials to existing soils, but more aggressive mechanical means (such as an air spade) are sometimes employed for extremely poor soils.  As with fertilizing, there are right ways and wrong ways to improve soils.

Proper irrigation can be achieved for trees even under most stages of city water restrictions.  I generally recommend a thorough soaking about once a week during the hottest months.  How much time and water that will take will depend on your specific irrigation system, water pressure and soil type.  A heavy soak once a week is far preferable to multiple short waterings per week.  If you have a thick turf like St. Augustine grass, you may not even get much water through it and to the tree roots if you are doing short waterings, so in this situation your tree could still suffer drought stress even if you were watering every other day.  I can discuss the specifics of how to best achieve these things on your property during a consultation.

Another big problem that trees stressed by drought often experience is borer insect infestation.  Although some aggressive species of borers can attack (and kill) healthy trees, most are opportunistic, and are more likely to attack stressed trees, and some species are more susceptible than others.  While chemical treatment of these insects is possible and sometimes necessary, often the infestation is only noticed after the insect has already damaged the tree and is no longer present.  Correcting the stress factor is usually the best approach.

If you have trees that are in need of tree care, you will get the best results from a tree service in which the treatment is personally provided by an owner/operator who is an I.S.A. Certified Arborist with at least 15 years of local experience such as mine.  I have owned and operated Arbor Vitae Tree Care for 29 years.

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Spring is here! (Almost)

Guy LeBlanc

Warm weather will soon be returning to central Texas, which means folks here will soon be thinking about pruning their trees and other aspects of tree care.  A common question that comes up now is, “What about oak wilt?”  For years, it has been recommended by government agencies to avoid pruning oaks in the spring because of a perceived increase in the risk of getting oak wilt.  What is the reality of pruning oaks while minimizing the risk of this deadly fungus?

About a year ago, I chaired a group of arborists from the Texas chapter of the International Society of Arboriculture, the Texas Forest Service, and Texas AgriLife Extension Service which clarified the existing guidelines.  This was to explain to the public that certain kinds of pruning can be done year-round on oaks.  This would include removing dead branches, and branches at risk of being hit by vehicles over streets, and those rubbing against buildings or other branches within the canopy.  For details on this, see my oak wilt page.  You can also see a Central Texas Gardener video of me explaining oak pruning and oak wilt on YouTubeTo read the statement in its entirety, do a web search for “pruning guidelines for prevention of oak wilt in Texas”  and “Guy LeBlanc”.

oak wilt symptoms

Oak wilt has been in the news locally for so long (in Austin it almost created hysteria in the early 80’s) that I sometimes forget that most homeowners don’t really know much about it.  In brief, it is caused by a fungus very closely related to the Dutch Elm disease fungus which many folks from northern U.S. are all too familiar with.  It only affects oaks (and tanoaks).  The disease spreads in two primary ways.  The majority of the spread of the disease (95% of it according to one researcher) occurs underground due to the propensity of live oaks to form grafts (actual vascular connections) between the roots of different trees.  Live oaks also occur in groups called motts, which means that all of the trees in the group have formed from root sprouts.  Motts are also therefore interconnected and the disease spreads quite readily through them.

The disease can also spread above ground, creating new infection centers.  This occurs when vectors (in this case a beetle called the nitidulid is most likely the culprit) visit the fungal spore patches (called mats) on infected trees and then carry those spores to fresh open wounds on healthy trees.  In Texas these mats only form on red oaks, never on live oaks, so it is important to remove dead red oaks immediately- in fact in Austin, there is an ordinance requiring any property owner to do so.

It is believed that the nitidulid is attracted to both the odor of the mats, which have a fruity-licious smell, and the odor of fresh sap from new wounds.  The highest levels of this insect population and the creation of fungal mats coincide only when weather conditions are just right, which is usually in the spring.  This is why government agencies have recommended avoiding pruning in the spring.

However, we also know that very often in spring the weather conditions are not conducive to the development of the fungal spores (too hot and/or dry).  No spores = no above ground spread.  Or it can be too cold for the insects to be active (last year it was in the 20’s for 3 days at the end of Feb).  We also know that painting wounds provides a nearly total barrier against spore invasion.  There is no published scientific study in which painted wounds contracted oak wilt (one study in 2007 seemed to contradict this, but it was found that the trees were becoming infected underground).  We also know that dead tissue cannot be an invasion point for the spores, so removing completely dead branches never exposes a tree to oak wilt; technically, dead tissue doesn’t even have to be painted, however, I usually paint it just in case some live tissue is present on part of the wound face.

The bottom line is that while oak wilt is often fatal, the actual risk of contracting it through open wounds, even in the spring, is minimal.  If it wasn’t we probably wouldn’t have any oaks left, as wind, hail and animals such as squirrels and birds cause extensive wounding, often in the spring.  And this risk can be actually be reduced if the pruning is done by a competent professional – which is of course the only type of person you would ever let touch your trees!


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