Emerald Ash Borer and the Gateway Arch Grounds

The renewed grounds of Gateway Arch National Park. NPS.

The original landscape

The grounds were designed with the Arch itself: architect Eero Saarinen worked with landscape architect Dan Kiley from the start. The Arch was finished in 1965, but the landscape waited for funding and went in in phases through the 1970s and early 1980s — departing from Kiley's plans. The walkways to the park's north and south ends were meant to be lined with tulip poplars, but they weren't available locally and horticulturists doubted they'd thrive in the city, so the single-species rows became pin oak, then Rosehill ash.

A landmark landscape

The grounds are celebrated as a triumph of landscape architecture — a National Historic Landmark under National Register Criteria A and C, and among the world's finest examples of mid-century modern landscape design. The NPS must preserve it as a cultural resource. Its key features include the landform and layout, designed views, the walks, the single-species allées, the ponds, overlooks, the railroad cuts and tunnels, and the baldcypress circles. More: the landscape's significance.

The grand staircase, one of the landscape's significant features. NPS.

The emerald ash borer

Rosehill ash was chosen partly because it's hardy and tolerant of pests, pollution and disease. No one foresaw the emerald ash borer (EAB), a small green beetle whose larvae eat the spongy, nutrient-rich layer just under ash bark.

• At home in China, EAB has predators and ash trees have defenses, so it mainly kills sick trees.
• In North America, trees lack those defenses: nearly 100% of healthy infested ash die within a few years. Downy and hairy woodpeckers eat some, but not enough.

Its spread: first confirmed in Detroit in 2002 (perhaps arriving earlier); Ohio by 2003, Indiana by 2004, Illinois in 2006, Wisconsin and Missouri in 2008. By June 2021, Missouri had it in 89 of 114 counties, and the USDA had confirmed it in 35 states and D.C. Federal domestic quarantines have ended.

The mature ash trees along the walkways in the 1990s. In time, their replacements will grow as tall and shady. NPS.

Removing the ash

In 2010 the NPS decided to remove all the Rosehill ash before EAB arrived. Pesticide protection exists but is costly and must be repeated every couple of years, and some trees were already declining with age — so replacement made more sense. Because the single-species walkways are a character-defining feature of the landmark, they had to stay single-species.

A different choice in the city: after Dutch elm disease, St. Louis had replaced most dead elms with ash, leaving streets lined with nothing else and the urban forest especially exposed. The city now stresses diversity, replacing removed ash with 40 different species.

Choosing a replacement

The priority was Kiley's design intent, with a tree that resists pests and disease, suits the grounds' alkaline soil, tolerates air and light pollution, grows fast, and is sturdy enough not to drop branches on hundreds of thousands of visitors. The NPS held 11 public meetings with nearly 1,000 people and consulted the CityArchRiver design team, the Missouri Department of Conservation, the Missouri Botanical Garden, local arborists and nurseries.

• Eight semifinalists: Miyabe maple, Freeman maple, ginkgo, tulip poplar, London planetree, Trazel filbert, American linden and silver linden.
• Three finalists: ginkgo, tulip poplar and London planetree.
• Chosen: the London planetree (Platanus × acerifolia), which best met the criteria — and had been Kiley and Saarinen's second choice.

When the CityArchRiver renovation was done, the park had 4,129 trees, more than double the number before. Outside the allées, plantings use far more species.

The park's 74 acres of ornamental turf need mowing once or twice a week in the growing season. NPS.

Keeping it healthy

Grounds crews prune, mow, water, fertilize and control insects and weeds. Besides Midwestern temperature and rainfall extremes, the city adds heat islands, poor soil, tight space and air pollution (though it has improved). As a National Historic Landmark, the landscape and its significant features will be kept for the future.

Sources

Based on "Emerald Ash Borer at Gateway Arch National Park," National Park Service; a work of the United States government in the public domain. A photograph of the beetle, licensed for non-commercial use only, is left out; the original's botanical names are spelled as Gingko, Lilodendron tulipiferous and Platanus acerfolia.