Scientific Name
Fraxinus americana ‘Rosehill’
White ash, like green ash, has long been a common tree in the forests of the eastern United States and a frequent choice for urban planting. Unfortunately, that may be changing. The emerald ash borer, an invasive insect first detected in North America around 2002, reached Boulder in 2013 and Denver in 2024. Its larvae tunnel beneath the bark, disrupting the tree’s vascular system and ultimately killing it.
There is a real concern that ash species native to North America could be largely eliminated from natural forest settings, as they have little inherent resistance to this pest. Some Asian ash species tolerate the insect due to co-evolution, but their suitability as replacements here remains uncertain. In cultivated landscapes, however, chemical treatments can protect trees if applied regularly every few years. Even so, ash trees may persist mainly where people actively choose to preserve them.
White ash can be difficult to distinguish from green ash, another widely planted species in our region. Both have opposite branching and compound leaves with five to nine leaflets. The most reliable difference is fall color. Green ash typically turns a consistent bright yellow, while white ash displays a mix of yellow, red and purple.
Because ash trees are either male or female, male cultivars are often planted to avoid the heavy production of winged seeds. One drawback is that male flowers can be affected by ash flower gall mites, which deform them into small, dark, broccoli-like clusters that remain on the tree. The cultivar ‘Autumn Purple’ is the most widely planted seedless white ash and is valued for its strong fall color. The ‘Rosehill’ cultivar at Regis is also seedless and typically turns a rich brick red in autumn.
In our region, white ash trees usually mature to around 40 feet in height.