Over this past week, the fall weather has brought bare branches to the previously color-filled canopies of so many trees in the landscape. As the autumn leaf drop has progressed, there are several elm trees that I pass each day which have held onto their fall color and really look spectacular as they grasp their foliage late into the season.
“Frontier” elm (Ulmus “Frontier”) is one of many cultivars and hybrids that were developed in response to the virtual elimination of the American elm by Dutch elm disease. “Frontier” has proven to host one of the more spectacular foliage displays among its cohort of hybrids.
The ornamental appeal begins early in spring as leaves emerge with a reddish tint that transitions to lustrous, dark green for the growing season. Fall color is spectacular, ranging from a bright red to a deep purple that remains a beacon of color late into the fall season, with leaves hanging on longer than many other deciduous species.
The impetus for development of hybrid elms began as early as the 1930s, as the U.S. watched Dutch elm disease eliminate the American elm (Ulmus americana) from urban areas and forests. The fungal pathogen (Ophiostoma ulmi), spread by elm bark beetles, had already infected elm trees in Europe after its introduction from Asia in the 1920s. Asian elms developed resistance to the fungus over millions of years, while European and American elms had little resistance having not been exposed to this exotic fungus until its sudden, human-caused appearance.
Elm bark beetles from Europe harboring the pathogen were inadvertently introduced to the U.S. elm population in the early 1930s. By the 1970s, Dutch elm disease had swept across the vast native range of American elms, eliminating nearly all of them. Today, younger American elms can be found in the landscape as they seem to have some resistance early in life, but mature elms are quite rare.
Scientists and plant breeders have searched for an American elm replacement since we first started to lose this valuable urban tree. Many view it as the nearly “perfect” urban tree. Its vase shape and arching growth habit created a cathedral-like canopy over city streets and provided shade for homes, parks and other areas of human habitation.
In nature, the American elm is adapted to a wide range of conditions, making it equally adaptable to urban environments. This adaptability and relatively quick growth made for a fast-growing and long-lived shade tree that was extensively planted in the early 20th century.
In the quest for a replacement, breeders have attempted various hybrids between Asian, American and European elm species. Many of these hybrid selections have created unique combinations. Most are virtually free of Dutch elm disease with the inclusion of Asian genetics for natural resistance, but few offer the same tall, vase-shaped habit and adaptability as our native species.
Introduced in 1990, “Frontier” elm is a hybrid between a European elm (U. minor) and an Asian elm (U. parvifolia), representing the first commercially available hybrid of a spring-flowering European elm crossed with a fall-flowering Asian elm. The resulting plant rarely flowers or produces seed, although I have observed fall flowers on certain individuals.
It does have the characteristic vase-shaped habitat and is quite fast growing but matures to much shorter stature, topping out around 30 or 40 feet. It has been proven to handle urban conditions well and makes an excellent medium-sized street tree with remarkable fall color.
Breeders have also looked for American …….