|Ulmus americana subsp. var.||American Elm, White elm|
Ulmus americana, generally known as the American Elm or, less commonly, as the White Elm or Water Elm, is a species native to eastern North America. It is an extremely hardy tree that can withstand winter temperatures as low as −42 °C (−44 °F). Trees in areas unaffected by Dutch elm disease can live for several hundred years.The American Elm is a deciduous tree, which, before the advent of Dutch elm disease, commonly grew to > 30 m (100 ft) tall with a trunk > 1.2 m (4 ft) d.b.h.leaves are alternate, 7–20 cm long, with double-serrate margins and an oblique base. The tree is hermaphroditic, having perfect flowers, (i.e. with both male and female parts) and is therefore capable of self-pollination. The flowers are small, purple-brown, and, being wind-pollinated, are apetalous; they emerge in early spring before the leaves. The fruit is a flat samara 2 cm long and 1.5 cm broad, with a circular wing surrounding the single 4–5 mm seed. As in the closely related European White Elm, U. laevis, the flowers and seeds are borne on 1–3 cm long stems. American Elm is wholly insensitive to daylight length (photoperiod), and will continue to grow well into autumn until injured by frost .
The tree reaches sexual maturity at around 150 years of age and is unique within the genus in being tetraploid, i.e. having double the usual number of chromosomes. However, nowadays it is uncommon for the tree to reach over 100 years of age, such is its susceptibility to Dutch elm disease.
|Standard Cyclopedia of Horticulture|
Ulmus americana, Linn. (U. alba, Raf.). White Elm. Water Elm. American Elm. Tall and wide-spreading tree, attaining to 120 ft., usually with high light gray trunk, limbs gradually outward-curving with pendulous branches: branchlets pubescent when young, glabrous in fall: buds acutish, glabrous: lvs. ovate-oblong, unequal at the base, acuminate, doubly serrate, pubescent when young, at length glabrous and rough above, pubescent or almost glabrous beneath, 3-6 in. long: fls. in many-fld. clusters; stamens 7-8, exserted: fr. oval or elliptic, veined, deeply notched, incision reaching to the nutlet. Newfoundland to Fla., west to the base of the Rocky Mts. —One of the favorite avenue trees in the northeastern states. The elm varies considerably in habit, and the following forms have been distinguished. In the "vase form" the main trunk separates at 15-30 ft. into several almost equal branches, which diverge at first slightly and gradually, but at the height of 50-70 ft. sweep boldly outward and form a broad flat head, with the branches drooping at the extremities. This is the most beautiful and also the commonest form. The "plume form" is much like the foregoing, but the trunk is less divided and the limbs are clothed with short branchlets, thus forming feathery plumes. The "weeping-willow form" usually has a rather short trunk with limbs curving outward more rapidly and with long and very slender pendulous branches, forming usually a broad and round head. The "oak-tree form" is distinguished by its limbs spreading abruptly and in sharp turns and the branches being usually less pendulous. The name "feathery" or "fringed" elm is applied to trees which have the limbs and the main trunk clothed with short somewhat pendent branchlets thrown out usually in clusters at short intervals. This may appear in any of the forms named, but is most conspicuous in trees of the plume form. There are a few named varieties in nurseries: Var. aurea, Temple, with yellow foliage, found in Vt., by F. L. Temple; var. pendula, Ait., with slender pendulous branches; this is the "weeping-willow form" described above. CH
Pests and diseases
The American Elm is highly susceptible to Dutch elm disease (DED) and Elm Yellows; it is also moderately preferred for feeding and reproduction by the adult Elm Leaf Beetle Xanthogaleruca luteola. U. americana is also the most susceptible of all the elms to verticillium wilt
- American Liberty, Ascendens, Augustine, Aurea, Beaverlodge, Beebe's Weeping, Brandon, Burgoyne, College, Columnaris, Deadfree, Delaware, Exhibition, Fiorei, Flick's Spreader, Folia Aurea Variegata, Hines, Incisa, Independence, Iowa State, Jackson, Jefferson, Kimley, Klehmii, Lake City, L'Assomption, Lewis & Clark (Prairie Expedition), Littleford, Maine, Markham, Minneapolis Park, Moline, Morden, New Harmony, Nigricans, Patmore, Pendula, Penn Treaty, Princeton, Pyramidata, Queen City, Sheyenne, Skinner Upright, Star, Valley Forge, Variegata, Vase, Washington
The National Elm Trial, begun in 2005, is currently evaluating 19 cultivars in scientific plantings across the United States to better assess the strengths and weaknesses of leading cultivars.
The few disease-resistant selections that have been made available to the public as yet include 'Valley Forge', 'New Harmony', 'Princeton', 'Jefferson', and a set of six different clones collectively known as 'American Liberty' . The United States National Arboretum released 'Valley Forge' and 'New Harmony' in late 1995, after screening tests performed in 1992–1993 showed both had unusually high levels of resistance to DED. 'Valley Forge' performed especially well in these tests. 'Princeton' has been in occasional cultivation since the 1920s, and gained renewed attention after its performance in the same screening tests showed it also to have a high degree of DED resistance. A later test performed in 2002–2003 confirmed the DED resistance of these same three varieties, and that of 'Jefferson'. 'Jefferson' was released to wholesale nurseries in 2004 and is becoming increasingly available for planting. Thus far, plantings of these four varieties generally appear to be successful.
In 2005, 90 'Princeton' elms were planted along Pennsylvania Avenue near the White House and to date are healthy and thriving. Introduced to the UK in 2001, 'Princeton' was selected by HRH The Prince of Wales to form the Anniversary Avenue from the Orchard Room reception centre to the Golden Bird statue at Highgrove House. In 2007, the Elm Recovery Project from the University of Guelph in Ontario, Canada, reported that cuttings from healthy surviving old elms surveyed across Ontario had been grown to produce a bank of resistant trees, isolated for selective breeding of highly resistant cultivars  .
- ↑ Downs, R. J. & Borthwick, H. A. (1956). Effects of photoperiod on growth of trees. Botanical Gazette, 117, 310-326
- ↑ Costello, L. R. (2004). A 10 -year evaluation of the performance of four elm cultivars in California, U. S. Journal of Arboriculture, March 2004.