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. Indeed, 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.
So how should the NPS approach remediation of these highly compacted soils? “They lose 3 to 5 American elms each year to Dutch elm disease,” Bassuk says. “When they replace the trees, that’s an opportunity to do soil remediation over the area before a new tree is planted.” She recommended using the “Scoop and Dump” technique, a method which has been validated over a twelve-year study at Cornell University. A video about the technique can be seen here.