Ulmus pumila L., the Siberian elm, is native to Central Asia, eastern Siberia, Mongolia, Xizang (Tibet), northern China, India (northern Kashmir) and Korea. It is also known as the Asiatic elm and dwarf elm. It is the last tree species encountered in the semi-desert regions of central Asia. Introduced to the USA in 1905 by Prof. J. G. Jack,Ulmus pumila has been widely cultivated throughout the Americas, Asia and, to a lesser extent, southern Europe.
The Siberian elm is usually a small to medium-sized, often bushy, tree growing to 10-20 metres (33-66 ft) tall, with a trunk up to 80 centimetres (31 in) d.b.h. The leaves are deciduous in cold areas, but semi-evergreen in warmer climates, < 7 cm long and < 3 cm broad, with an oblique base and a coarsely serrated margin, changing from dark green to yellow in autumn. The perfect, apetalous wind-pollinated flowers emerge in early spring, before the leaves; unlike most elms, U. pumila is able to self-pollinate successfully. The wind-dispersed fruit develops in a flat, oval membranous wing (samara) 1-1.5 centimetres (0.39-0.59 in) long and notched at the outer end. The tree is short-lived in temperate climates, rarely reaching more than 60 years of age, but in its native environment may live to between 100 and 150 years .
The tree has considerable variability in resistance to Dutch elm disease. Moreover, like many other elms in North America, it is highly susceptible to damage from many insects and parasites, including the elm leaf beetle Xanthogaleruca luteola,powdery mildew, cankers,aphids, and leaf spot. In the Netherlands U. pumila was also found to be susceptible to coral spot fungus Nectria cinnabarina, moreover its flowers, emerging in early February, were often damaged by frost, consequently the species was dropped from the Dutch elm breeding programme. In Italy, the species was also found to have a slight to moderate susceptibility to Elm Yellows. However, U. pumila is the most resistant of all the elms to verticillium wilt.
U. pumila was introduced into Spain in the 16th century, and from the 1930s into Italy. In these countries it has naturally hybridized with the Field Elm U. minor (see below). In Italy it was widely used in viniculture, notably in the Po valley, to support the grape vines until the 1950s, when the demands of mechanization made it unsuitable. Introduced to the USA by Prof. J. G. Jack in 1905, and later by Meyer, the tree was initially cultivated at the USDA Experimental Station at Mandan, North Dakota, where it flourished. It was consequently selected by the USDA for planting in shelter belts across the prairies in the aftermath of the Dustbowl disasters, where its rapid growth and tolerance for drought and cold initially made it a great success. However, the species later proved susceptible to numerous maladies. Attempts to find a more suitable cultivar were initiated in 1997 by the Plant Materials Center of the USDA, which established experimental plantations at Akron, Colorado and Sidney, Nebraska. The study, no. 201041K, will conclude in 2020.
The seeds lose their viability rapidly after maturity unless placed on suitable germination conditions or dried and placed at low temperatures. The species has a high sunlight requirement and is not shade-tolerant; with adequate light it exhibits rapid growth. The tree is also fairly intolerant of wet ground conditions, growing better on well-drained soils. While it is very resistant to drought and severe cold, and able to grow on poor soils, its short period of dormancy, flowering early in spring followed by continuous growth until the first frosts of autumn, renders it vulnerable to frost damage.
As an ornamental U. pumila is a very poor tree, tending to be short-lived, with brittle wood and poor crown shape, but it has nevertheless enjoyed some popularity owing to its rapid growth and provision of shade. The Siberian Elm has been described as "one of [the world's worst], if not the world's worst trees...a poor ornamental that does not deserve to be planted anywhere". Yet in the US during the 1950s, the tree was also widely promoted as a fast-growing hedging substitute for privet, and as a consequence is now commonly found in nearly all states. A better cultivar, the Turkestan Elm, that is seen more often in gardens, and referred to as the 'wonder hedge' (Ulmus pumila celer), being both dense and fast-growing, taking as little as two years to reach fence height.
In the Great Britain, the popularity of U. pumila has been almost exclusively as a bonsai subject, and mature trees are largely restricted to arboreta.
In North America, Ulmus pumila has become an invasive species in much of the region from central Mexico northward across the eastern and central United States to Ontario, Canada. It also hybridizes in the wild with the native U. rubra (Slippery Elm) in the central United States. In South America, the tree has spread across much of the Argentinian pampas
In Europe it has spread widely in Spain, and hybridizes extensively there with the native U. minor, contributing to conservation concerns for the latter species. Research is ongoing into the extent of hybridisation with U. minor in Italy.
Ulmus pumila is often found in abundance along railroads and in abandoned lots and on disturbed ground. The gravel along railroad beds provides ideal conditions for its growth: well-drained, nutrient poor soil, and high light conditions; these beds provide corridors which facilitate its spread. Owing to its high sunlight requirements, it seldom invades mature forests, and is primarily a problem in cities and open areas, as well as along transportation corridors.
The species is now listed in Japan as an alien species recognized as established in Japan or found in the Japanese wild.
Two varieties were traditionally recognized: var. pumila and var. arborea, however the latter has now been sunk as the cultivar 'Turkestan'.
Valued for the high resistance of some clones to Dutch elm disease, over a dozen selections have been made to produce hardy ornamental cultivars, although several may no longer be in cultivation:
The species has been widely hybridized in the USA and Italy to create robust trees of more native appearance with high levels of resistance to Dutch elm disease:
Roerich describes a specimen discovered on his travels through Mongolia:-
The USA National Champion, measuring 33.5 m high in 2011, grows in Berrien County, Michigan.