Scientific Name
Fraxinus pennsylvanica
Green ash has a very large native range covering the eastern U.S. and much of the Midwest and Great Plains. It’s been widely used as a shade tree along streets, on residential properties and parkland because of its fast growth and adaptability to drought and city conditions. Ash and maple are the main genera of trees in Colorado whose leaves (and thus buds and twigs) are arranged opposite each other. Green ash twigs are rather stout and have compound leaves with five to nine leaflets. In the fall, green ash is one of the first trees to color, with the foliage usually turning bright yellow. Ash trees are early to leaf out in spring, so the tender young foliage is sometimes killed off by late freezes, requiring the trees to grow a replacement set of leaves. Ash trees are either male or female, and the latter will often produce a large quantity of winged seeds that somewhat resemble the bottom part of a canoe paddle. The flowers on the male trees have their own problems, as a mite often converts them into clumpy galls that somewhat resemble broccoli heads, and they can remain on the branches through the winter. To avoid seed litter from ash landscape plantings, grafted male cultivars have become the standard choice over the past forty years, but there are still females present among older trees. The large green ash in the O’Connell Grotto is a female, and it often retains its seed crop on its branches for many months before they eventually drop.
Green ash trees generally grow to around 60 feet tall here, though the Colorado champions are typically in the 90-foot range. Ash bark is usually a light gray with a tight ridge/furrow pattern. It can be difficult to distinguish between the region’s two common ash species, the green and the white, and it can be necessary to examine minute features such as leaf scars to do so.
All North American ash species are threatened with extinction in their native habitats by an invasive insect pest that arrived accidentally in Michigan in the late 1990s and spread. This is the emerald ash borer which appeared in Boulder in 2013 and in Denver in 2024. So far, no natural resistance to the beetle has been observed in any North American ash species, so every tree attacked by the beetle has died within a few years. This has caused a large problem in urban forests, both in and out of Colorado, because ash constituted such a large percentage of the trees. An effective insecticide treatment that must be repeated every few years prevents beetle attacks. Unfortunately, the treatment is costly and impractical given the magnitude of the urban ash population and the 7.5 billion ash trees growing in native forests before the beetle’s arrival. In urban areas, only the healthiest well-established ash trees are selected for preventive insecticide treatment, while both very young and declining older trees are sacrificed. This also describes the situation at Regis, which had an unfortunately large complement of ash trees planted on campus before the threat of the emerald ash borer was recognized. This is the third wholesale die-off of North American shade trees in the past 100 years, following the American chestnut and the American elm, which has emphasized the need to diversify our urban tree plantings.