In hot and steamy June of 2017, a team of
researchers and arborists from Cornell University's
Urban Horticulture Institute (UHI), headed up by
UHI Director Nina Bassuk, worked dawn to dusk
evaluating the condition of the American elms
and soils on the National Mall in Washington
DC. This iconic landscape is often referred to as
"America's Front Lawn," and the National Mall
turf grass was fully renovated between 2010
and 2016, involving infrastructure upgrades, at a
cost of $40 million dollars. Now, UHI hopes the
Mall trees will get the same level of attention.
Bassuk and then-graduate student Yoshiki Harada
worked together on soil evaluation, taking 108 soil
samples back to Cornell, while ISA Board Certified
Master Arborist Barbara Neal and UHI Visiting Fellow
Bryan Denig performed an ISA Level 2 evaluation of
the National Mall's 550 trees. Bassuk and team also
used ground penetration radar on a sample of 16 of
the trees to find out precisely where the roots are.
A report on the findings of that work was completed
in April of 2018 and presented to the National Park
Service (NPS). "The Mall's sandy soils are fine in terms
of nutrient availability," Bassuk says, "but the big issue
is soil compaction, which has the cascading effect of
destroying soil structure, reducing micro-organism
populations, and decreasing aeration. We couldn't get
a penetrometer more than 3 inches in the ground!"
It's perhaps not surprising, then, that the ground
penetrating radar confirmed that the Mall trees'
roots are shallow at best. A simple visual assessment
shows roots are on the surface and being damaged
by mowers. "There's very little grass under many of
the largest trees," Bassuk says, "but instead of routine
mulching, mowers have been running over the roots."
Because the roots are surface to shallow in depth,
the UHI team recommends that the NPS not attempt
to remediate the soil of existing trees, because that
would be too disruptive to the roots. This goes for
soil aeration techniques as well as soil amendment.
The Urban Horticulture Institute
Produces Plan for a Sustainable
National Mall Treescape
By Michelle Sutton • Photos courtesy of Urban Horticulture Institute
From left: Barbara Neal, Bryan Denig, and Nina Bassuk assess the health of
one of the iconic elms ringing the National Mall. Photo by Yoshiki Harada
32 CityTREES
