Turf Reduction - Problems For Some?

Young Ash tree displaying symptoms
of severe drought stress due to a
poorly designed irrigation system.

Turf Reduction  -  Problems For Some

In December of 2004 at the conference of the Colorado River Water Users Association a $200 million conservation initiative was announced to assist public agencies to save water through turf-reduction programs.  Southern Nevada Water Authority’s ‘Water Smart Landscape Rebate Program’ has since helped both homeowners and businesses in reducing landscape water use while creating aesthetically pleasing desert landscapes. 

In general, the rebate program has been a tremendous success.  However, there have been a few problems associated with the conversion of turf to desert landscape.  The most common problem involves maintaining the health of trees that once grew in large turf areas where they enjoyed high soil moisture levels, cooler soil temperatures and higher humidity and now find themselves in an arid environment.  Trees are one of the strongest and most important elements of a landscape and during any landscape conversion process their preservation should have a high priority. 

During the conversion process not only is the turfgrass removed, but so is the overhead irrigation system.  With the overhead turf irrigation system maintaining high soil moisture levels over large areas, mature trees could often expand their root systems beyond their dripline.  Conversions called for the replacement of overhead irrigation systems with a more efficient surface or subsurface drip-type system.  In most cases, the new drip-type irrigation system would not be designed to continue to provide coverage to 100% of a large or mature trees existing root system.  The new irrigation design concept would attempt to meet a minimum of 70% coverage of the existing root system.  Obviously, a 30% loss of irrigated root zone, especially the perimeter feeder roots, is going to cause stress to trees.  However, with care and proper cultural practices most trees will adjust and will continue to thrive.  However, when drip-type irrigation systems are not properly designed slow decline and loss of established trees may occur. 

Poorly designed irrigation
system caused drought
stress predisposing tree to
Sooty Canker (Fungus)

This past summer I consulted on several projects involving tree decline and death on HOA properties that had undergone a prior turf-reduction and landscape conversion.  Poorly designed drip-type irrigation systems were most often responsible.  Tree conditions ranged from healthy to severely stressed or even death.  In general, the healthier trees were also the smaller trees.  This was due to the trees having smaller root systems thus being less impacted by the reduced size of the new irrigation system as well as less root disruption or damage during installation.

Bark of dead branches
split open revealing a black
dusty mass of Sooty Canker
fungal spores 

Trees that are stressed due to insufficient irrigation are weakened and can become subject to a whole new set of problems.  Environmental factors such as sunscald or insect and diseases that previously were not an issue now become major problems.  A good example can be seen in the attached photographs of a Las Vegas townhome complex that had a few years earlier undergone a landscape conversion involving extensive turf reduction and the installation of a poorly designed irrigation system.   The photographs were taken late summer.  The previous fall and late winter we received rain and conditions were fairly moist.  This was followed by a hot windy summer.  During this time the fungus ‘Sooty Canker’ (Hendersonula toruloides) easily established itself in the weakened trees.  The disease in one season had severely damaged or killed most of the large ash trees. 

Throughout many areas of the Southwest we are seeing the use of turfgrass being reduced in landscapes with landscape designers now placing more emphasis on turf functionality.  Aesthetics is still important; however, we can still create attractive landscapes with less turf.  It’s also important that we continue to reduce the amount of established turf in an effort to conserve one of our most precious natural resources.  However, it’s also important to maintain the health of our large and mature trees during the process of turf reduction.