Scientific Name
Ulmus americana
The classic and most widespread native elm of eastern North America, American elm ranges from southern Canada to the Gulf Coast. This tree was once widely planted along boulevards, often as a virtual monoculture, to achieve arching, vase-shaped canopies that would merge overhead. American elm — the state tree of Massachusetts and North Dakota — is native to most of the eastern half of the U.S. and just barely misses being a Colorado native at the northeast corner of the state.
This tree usually grows tall, typically reaching 70 feet or more in height. The two tallest Colorado champions measure 98 and 97 feet, respectively. American elm flowers contain both male and female structures, but the female parts mature before the male, which promotes outcrossing and results in higher seed quality. Few people notice the wind-pollinated flower activity in the early spring, but the litter of notched, flat, light-green elm seeds with their hairy-edged circular wings is much more obvious. While many trees develop some degree of trunk flare with age, American elms often develop some truly dramatic buttresses.
Unfortunately, Dutch elm disease, a deadly fungal pathogen carried from tree to tree by bark beetles, arrived in the northeast U.S. in the late 1920s. Over the following decades, it devastated the species, slowly moving westward until it reached all of the lower 48 states. The disease is generally believed to have originated in Asia, though it is named for the Dutch researcher who first isolated the pathogen. Many cities were left with miles of treeless roadways, a stark reminder of the risks of relying so heavily on a single tree species.
Many of Denver’s American elms were fortunate not to encounter the disease until effective preventive chemical treatments had been developed. These fungicides can protect the elms but must be reapplied indefinitely. Of course, it offers little consolation for the enormous number of American elms living in our native forests. As a result, Colorado has retained many grand mature elms along parkways and in public spaces, while most of the historic elms in the eastern cities were lost. Because few trees match American elm’s highly regarded arching form, much effort has been made to develop disease-resistant (though not completely immune) cultivars and hybrids. Progress since the 1990s has been impressive, but it can take many decades for a promising new variety to truly prove its worth.
Meanwhile, another injurious pest — the European elm scale — has emerged as a potential threat to elms. Given these challenges, it is difficult to estimate the lifespan of this tree, which in the past often lived well beyond 200 years.