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Ulmus americana, generally known as the American elm or, less commonly, as the white elm or water elm, [a] is a species of elm native to eastern North America. The trees can live for several hundred years.
The tree can grow to greater than 30 m in height, and is distinguished by its dense, symmetrical, upright form and dark green foliage, ultimately forming a broad umbrella crown. Crotch angles can be acute, with considerable bark inclusion which can later lead to branch breakages.
The American elm cultivar Ulmus americana 'New Harmony' was raised by the Maryland Agricultural Research Service and released by the United States National Arboretum in 1995, along with 'Valley Forge'. 'New Harmony' proved the most successful U. americana cultivar in the US National Elm Trial, averaging a survival rate of 85.5% overall. [1]
Ulmus americana var. floridana, the Florida elm, first described as Ulmus floridana by Alvan Wentworth Chapman in the 1860s, is smaller than the type, and occurs naturally in north and central Florida south to Lake Okeechobee.
It was steadily weakened by viruses in Europe and had all but disappeared by the 1940s. However, the disease had a much greater and longer-lasting impact in North America, owing to the greater susceptibility of the American elm, Ulmus americana, which masked the emergence of the second, far more virulent strain of the disease Ophiostoma novo-ulmi.
The American elm cultivar Ulmus americana 'Lewis & Clark' (trade name Prairie Expedition) is a development from the North Dakota State University (NDSU) Research Foundation breeding programme, released in 2004 to commemorate the 200th anniversary of the eponymous expedition. [1]
Most of North America's notable elms are Ulmus americana, a fast-growing and long-lived species capable of attaining great size in a few centuries, especially when open-grown. [1] American Forests, a non-profit conservation organization, uses the following formula to calculate a point score for each tree to permit comparisons with others:
The American elm cultivar Ulmus americana 'Aurea' was cloned from a tree discovered by F. L. Temple in Vermont at the end of the 19th century. [1] [2] Description