DendroTech California Oaks and Associated Wildlife
DendroTechCalifornia Oaks and Associated Wildlife

Innovations and Research

Throughout his career, Rob Gross has been on the forefront of innovation and an array of research related to arboriculture, oaks and wildlife.  A few are listed below.

Super-sonic Soil Excavation

Rob Gross worked directly with Concept Engineering Group (Pittsburgh, PA) to bring super-sonic air jets to the Horticultural market.  Healdsburg City Arborist, Mathew Thompson, is shown holding the first "Air-Spade" soil excavation jet in California.  This prototype was succeeded with the "yellow stick", and a few generations later, the tool is now widely used throughout the World and marketed as "Air-Spade".


Gross, Rob and Michelle Julene. 2002. Supersonic air jets preserve tree roots in underground pipeline installation. Proceedings of the Fifth Symposium on Oak Woodlands: Oaks in California’s Changing Landscape. Gen. Tech. Rep. PSW-GTR 184. Albany, CA: Pacific Southwest Research Station, Forest Service. US Department of Agriculture; 381-386.

Hydraulic Soil Excavation 

The use of water to erode soils becomes a fast and effective means to excavate roots efficiently, when done properly.  Hydraulically excavated soil becomes slurry which, with the addition of water to the excavated soil, increases the amount of waste produced.  Water is heavy to move and requires a source of adequate volume for the job.

DendroTech worked with Taske Force Tips, Inc, a fire apparatus company in Valparaiso, Indiana, to establish and fine tune many useful aspect of Hydraulic Soil Excavation for the most efficient and predictable root excavation method using hand-held water delivery.  Per unit of energy expended, water is more efficient at excavating soils than air jets.  This method leaves a washed root surface which is easily observed in fine detail. 


Rizzo, David; and Gross, Rob. 2000. Distribution of Armillaria on pear root systems and a comparison of root excavation techniques. In: Stokes, Alexia, editor. Proceedings: The supporting roots of trees and woody plants: form, function, and physiology. Kluer Academic Press; 305-311.

Root collar of large California Black Walnut excavated with Hydrauic Soil Excavation

In this image Dr. Bob Raabe (UCB) and Rob Gross inspect and discuss this excavation.  This procedure exposed the root collar and upper roots of this massive black walnut.  This specimen was found to have been doomed by repeated agricultural disking which apparently tore away the bark girdling this old giant.  The tree responded by producing downright epicormic roots over the section of the root collar that was completely missing the essential conductive bark.  Using water alone this excavation took about one hour.

Fertilizer and Soil Moisture Influence on Root Development

Dr. Ken Shackle, Tree Physiologist at UCD, needed to see what happened to the roots of his experimental peach tree root systems.   Using Hydraulic Soil Excavation he was able to examine Nitrogen and irrigation effects on root occurrence and root volume.  Once these trees were excavated, the scientists were stunned by the quality, speed, and detail of the root system exposed using Hydraulic Soil Excavation.  We had a hard time getting the nozzle away from Dr. Shackle that day - "new toy".  This excavation method was a first for UC Davis.  Dr. Shackle was able to quickly and accurately establish the root system responces which resulted from the different irrigation and Nitrogen regimes.

Hydraulic Soil Excavation progresses faster and excavates deeper with a mobile vacuum

Then we added the Vacuum truck to our soil excavation techniques and the excavation rate increased again and the depth of excavation was also increased.  Water works better than air when the vacuum is used. 


Gross, Rob. 1993. Hydraulic vacuum excavation: getting down to the roots, Arbor Age. July.

Root Collar Excavation

Root Collar Excavation is extremely valuable for exploring existing tree structure and health conditions.

This is a procedure which removes the soil from the lower trunk and upper root area often known as the "root collar".  This technique can expose fill soils, root disease(s), gopher damage, structural issues and sometimes formerly unknown factors.  Root Collar Excavation was first demonstrated to reduce root rot impacts on Avocado trees in Riverside decades ago.  This technique was not well known back then and this treatment fell from use and awareness.

In the mid-1970's, Rob Gross teamed with Dr. Bob Raabe, horticultural plant pathologist at UCB.  They performed root collar excavations on many Coast Live Oaks in Sausalito to better observe and reduce the advance of the common oak root fungus (Armillaria mellea) which was widespread at the site.  After the root collars had been excavated manually  a far greater amount the pathogenic oak root fungus damage was exposed than previously visible or even thought to be present.  The excavations were left open and the advance of the fungus was halted. No more oaks died there for 17 years! Tree care in Marin County, California changed then with the investigative power of exposing root collars.  Before excavating root collars was conducted more commonly, Oak Root Fungus was then treated only from the soil surface upward.  When diseased coast live oak root collars were finally excavated the insights where powerful and often showed that they were damaged, girdled, or killed by this common root disease.  The most devastating impacts were found associated with raised grades that were irrigated routinely.  The irrigation was then provided to the entire oak root area where azaleas and camilias and rhododendrons were popular.  We now know better than to mix water needy species like azaleas in the dry summer adapted oaks.  Rob's has worked with Marin Arborists, then Sohner Tree Service (Phillips family) and John Britton Tree Service in St. Helena, who also helped to popularize this procedure.  Today it is done on valued trees everywhere when root disease is suspected or structural evaluation is needed.

This technique has been researched and successfully used to control Armillaria Root Rot on wine grapes in California.

Midday Stem Water Potential 

A new, refined method using an old device: quick, easy and accurate

Root impacts are common, especially in developed landscapes.  Root system damage and loss debilitates and kills trees.  This investigation shows how little irrigation water could be applied to root damaged valley oaks (Napa County, CA) and still maintain sufficient tree hydration.  DendroTech collaborated with Dr. Ken Shackle at UCD using the stem water-potential metering technique. This method is a quick and accurate way to appraise internal water status of valley oaks, therefore guiding the minimization of irrigation application. 


Shackel, Ken A. and Rob Gross. 2002. Using midday stem water potential to assess irrigation needs of landscape valley oaks. In: Proceedings of the Fifth Symposium on Oak Woodlands: Oaks in California's Changing Landscape. Gen. Tech. Rep. PSW-GTR 184. Albany, CA: Pacific Southwest Research Station, Forest Service. US Department of Agriculture; 397-402.

Pneumatic Soil Excavation for high volume settings 

This mature tree stand contains about 100 large eucalyptus trees.  The root collars were excavated using two 330cfs super-sonic soil excavation tools simultanously.  This system is powered by an 825 cfm tow behind air-compressor.  All of these large tree root collars were excavated in detail, for hazard appraisal, in just one crew day.

Super Sonic Soil Excavation at Extremely High Flow Rate

The supersonic air jet delivery shown in this image is delivering 625 cfm air traveling at twice the speed of sound.  This is the largest handheld, engineered air jet known for excavating roots.  WARNING - This may be dangerous to the operator's health, give a feeling of lightness to the user, and excavates soil and roots at a stunning rate!

Where are the roots of that tree anyway?

Five trenches were excavated (1' W, 10' L, 2' D) from the root area of cork oaks on central UC Davis campus using pressurized water.  See - Hydraulic Soil Excavation. Few roots were found under the asphalt pavement.  A limited number of small sized roots were found in the two trenches under the decomposed granite sidewalk.  The vast majority of roots were in the irrigated and fertilized turf area.  This investigative approach removes speculation and generalizations by allowing a close and detailed opportunity to directly observe the exposed roots.  This research was performed using hydraulic soil excavation with a mobile vacuum conveyor ("Vac truck").  The soil is first saturated.  A template is layed on soil surface and a fire hose was used to liquify the soil into slurry.  The vacuum removed the slurry from the trenches and the roots remained.  After analysis the trenches were filled with fresh imported soil.


Lindsay, Patricia; Gross, Rob, and Bob Milano. 1995. An investigation to assess the impact of street infrastructure improvements on the roots of adjacent cork oak trees. In: Proceedings; Trees and Building Sites. International Society of Arboriculture. Savoy, IL. 22-32.

Hydraulic Soil Excavation delivering 1250 gpm from a remote controlled stream

Digging roots with water at 1,250 gallon per minute at UCD!  This is a Fire Truck (manufactured by Snorkel Fire Apparatus Company) with a pump and remote controlled, articulated, extension tower flowing high rate of water through a sophisticated nozzle.  This is the highest flow rate delivery known for root exposure.  Placer mining used even higher flow rates, and was known for destroying everything to get the gold!  The delivery rate (water over time) influences the movement of soil particles, while water pressure or rate of water travel (particle speed) drives the degree of impact or excavation rate of a water stream.  Saturated soil conditions speed excavtion considerably.  So the challenge would be to find a nozzle which produced a well defined and adjustable stream shape at lower pressures of 60 to 80 psi.  At higher pressures the droplet size diminishes and more air becomes part of the fluid delivery stream.  Fire fighters usually operate attack hoses around 125 psi.  Most nozzles are designed for these higher pressures too. DendroTech experimentation has established flow rates and pressures to most efficiently excavate tree roots.


Crown Gall Control on Grafted Walnuts

This bacterial disease causes galling and seriously reduces nut production.  The use of air jets to clear the root collar and treat the exposed galls is detailed.  See - UCCE Video - Crown Gall Control on Grafted Walnuts.

Growth of Blue Oaks in California  

Image to the right shows four blue oak cross sections.  The growth increments of these were microscopically measured.  The number of annual growth increments for each tree cross section varies from 81 years (upper left), 229 (lower left), 290 (upper right), and 356 years of growth for the half section (lower right).  These are from different parts of California.


A dendroecology study of blue oak growth from five sites in California:  Jane Kurtis, Rob Gross, Dave Peterson. 1993. Growth trends of blue oak (Quercus douglasii) in California. Canadian Journal of Forest Research, August. 1720-1724

Oak Root Fungus (Armillaria mellea) on Landscape Trees

Oak Root Fungus (Armillaria mellea)

This common, widespread, native soil fungus can become a root pathogen under certain conditions and is responsible for damaging or killing a number of woody plant species.  There has been a lot of research conducted on this fungus because it is responsible for so much damage.  This image shows Colorado State Forest Pathologist and Vail City Arborist inspecting a damaged fir root collar with oak root fungus.  That was part of the soil excavation training for American Society of Consulting Arborists by Rob in Vail, Colorado.


See: Armillaria Melles:Native Soil Fungus Causing Root Rot

Soil Replacement 

Using fluids of either accelerated water or pressurized air, soil is removed from parts of the rhizosphere, to the depth of existing root penetration and out beyond the canopy drip-line.  The excavated soil voids are then filled with humus, which is worked into existing surrounding soils.  This provides volumes of new soil for "root breakout areas". This results in the tree growing faster and longer into the dry season.  Soil replacement using a back-hoe has also been demonstrated by others to increase tree growth.  Both hydraulic or supersonic air excavation avoids the root damage usually associated with heavy machinery.


This image of a grafted walnut was taken at a meeting of the International Society of Arboriculture in Santa Rosa, where DendroTech demonstrated  this valuable technique of removing and replacing damaged soils in the existing root area of living trees.


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