Siberian elm is a fast-growing tree in the elm family (Ulmaceae) distinguished by small toothed leaves about 1-2½ in (3-7 cm) long and half as wide, and pointed at the tip. Unlike other elms, the leaf base is usually symmetrical, forming a nearly even "V". Leaves are smooth and dark green above, paler and nearly hairless beneath, and alternate from side to side along twigs. Mature trees reach a height of 50-70 ft. (16-22 m.), with a round crown of slender, spreading branches. The bark is rough, gray or brown, and shallowly furrowed at maturity. Twigs are nearly hairless with small, blunt buds. Flowering occurs in the springtime. The small greenish flowers lack petals and occur in drooping clusters of 2 to 5. After flowering, a single seed forms in the center of each smooth, flattened, circular, ½ in (10-15 mm) wide fruit.
Other species of elms (Ulmus) and the close relative Zelkova, especially younger plants, look similar to Siberian elm. Some may even confuse it with choke-cherry (Prunus serotina) and hackberry (Celtis sp.). The native slippery elm and American elm typically have leaves that are greater than 3 in (7.3 cm) long, with unequal heart-shaped leaf bases, and leaf margins with double teeth.
Dry to mesic prairies and stream banks are vulnerable to Siberian elm invasion. Thickets of seedlings soon form around seed-producing trees, bare ground areas, animal and insect mounds, and other disturbed areas. Wind carries seed to distant areas where new colonies can form. This tough exotic survives under conditions not easily tolerated by other species, allowing it to take advantage of open ground and resources otherwise used by native plants. Fast growing seedlings of Siberian elm quickly overtake native vegetation, especially shade-intolerant species. This often leads to invasion by additional weedy species, compounding the problem.
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. [4] 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.[4] 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.[5][6][7] 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 [5][6].
Fruits (and larva of Satyrium w-album)
Cultivated specimen of Ulmus pumila, Morton Arboretum
The tree has considerable variability in resistance to Dutch elm disease.[8][9] 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,[10]powdery mildew, cankers,[3]aphids, and leaf spot. In the NetherlandsU. pumila was also found to be susceptible to coral spot fungus Nectria cinnabarina,[11] moreover its flowers, emerging in early February, were often damaged by frost, consequently the species was dropped from the Dutch elm breeding programme.[12] In Italy, the species was also found to have a slight to moderate susceptibility to Elm Yellows.[13] However, U. pumila is the most resistant of all the elms to verticillium wilt.[14]
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,[17] renders it vulnerable to frost damage.
As an ornamentalU. 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".[18] 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.[16] It is the superior 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. [7] In the UK, 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[19] northward across the eastern and central United States to Ontario, Canada.[20] It also hybridizes in the wild with the native U. rubra (Slippery Elm) in the central United States.[21] In South America, the tree has spread across much of the Argentinian pampas[22][23]
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.[24][25] Research is ongoing into the extent of hybridisation with U. minor in Italy.[26]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,[27][28] 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.[29]
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:
The USA National Champion, measuring 33.5 m high in 2011, grows in Berrien County, Michigan.[31][9]