Critical Root Zone

Laytonsville, MD

Central Maryland is in a drought and as of 10/5/07 BWI Airport states that we are -9.21 inches below the normal precipitation amounts for this growing season.  I was going to a job site today and noticed a hedgerow of non-native sawtooth oak in a lawn area that distinctively showed drought stress and subsequent root zone.

In order for these trees to respire (evapotranspiration) they require the uptake of water from the soil.  As turf competes within the trees root zone, it does not fare as well as the adjacent turf beyond the trees root zone.  The color change in the turf was quite obvious.  In the first photo, I’ve highlighted in yellow, the extent of the fibrous tree roots that are more or less consistent with the canopy drip line of the tree, and which is competing with the turf.

drip line

Each tree in the hedgerow had a ring more or less the same size as the trees drip line.


Most all arboricultural books say that if you want to protect a tree during construction (excavation, fill and/or compaction), you need to protect the “critical root zone.”  The critical root zone is defined as being the tree diameter x 1.5 = critical root zone.  An example would be a 10-inch diameter tree (at chest height) would have a critical root zone radius of 15-feet from the trunk out.

If you protect this area from all impact, then the tree should not ail or fail from nearby impacts, in that, it’s shallow, fibrous uptake roots would not be compromised.  Another way to visualize the critical root zone, would be to take the canopy drip line and add one-third, for it’s protection area.


This set of photos is interesting because you rarely see visual evidence of the root zone.


I thought that you might like this basic lesson in silvics.  The sawtooth oak (Quercus acutissima), also know as gobbler oak, is a USDA introduction, as it produces hard mast (nut food for wildlife) at a young age, and is a native of eastern Asia.

This blog entry reminds me of a story/observation.  I went to the University of California at Davis for a specialty class, while I was in the National Park Service.  Every morning at where I parked my car, I would walk past a flowing creek bed.  Every afternoon when I walked back to my car the creek bed was dry.  It became obvious to me that the shallow groundwater table of stream discharge was being taken-up by the plants.  At night, when photosynthesis stopped, the riparian plants released water back into the creek.  I suspect that these sawtooth oak may do the same, especially taking away water from the turf grass during the day.

All the reason why streams flow at their highest base flows at the end of the non-growing season (March, April), and their least, at the end of the growing season, September and October, as trees suck-down the water table.