5. Ulmus pumila L. (Siberian elm, dwarf elm)

Pl. 570 f, g; Map 2667

Plants trees to 25(–35) m tall but often much shorter. Twigs tan to yellowish gray, glabrous or hairy, never with corky outgrowths or wings. Winter buds 3–4 mm long, ovoid to subglobose, rounded to bluntly pointed, dark brown to reddish brown, glabrous or minutely pubescent with white hairs. Petioles 4–10 mm long. Leaf blades 2–8 cm long, 1.2–3.5 cm wide, elliptic-ovate to elliptic-lanceolate, the base symmetric or only weakly asymmetric, angled or gradually tapered to a usually sharply pointed tip, the major marginal teeth 0.5–1.0 mm deep, sharp, simple or often many of them with 1 smaller secondary tooth, the upper surface smooth, the undersurface glabrous or sparsely to moderately hairy along the main veins, sometimes also tufted in the vein axils, the secondary veins 9–16 on each side of the midvein, those of many leaves with 1–3 secondary veins on each side of the midvein forked toward their tips; juvenile leaves never lobed. Inflorescences dense clusters appearing before the leaves develop in the spring on second-year twigs. Flowers sessile or nearly so, the calyces shallowly 4- or 5-lobed, the tube glandular, the lobes broadly rounded, glabrous or the margin sparsely hairy. Fruits 1–2 cm long, 1.0–1.5 cm wide, nearly circular to occasionally broadly obovate to broadly elliptic, pale tan, the body glabrous, the wings glabrous on the surfaces and margins (except for hairs on the stigmatic surface in the apical notch). 2n=28. March.

Introduced, sporadic in the state, but relatively common in southwesternmost and northeasternmost Missouri and the St. Louis metropolitan region (native of Europe, Asia; introduced nearly throughout the U.S., Canada). Mesic upland forests, upland prairies, sand prairies, and banks of streams; also old fields, fencerows, cemeteries, gardens, railroads, roadsides, and disturbed areas.

Ulmus pumila has a remarkable ability to tolerate extreme heat, cold, and drought, and it thrives in difficult sites. It was introduced to the United States to provide windbreaks and wood for fuel and construction on farms in the Great Plains and arid interior of the western United States, and it was widely planted in the early twentieth century. It has become an aggressive invader in many areas in the western part of the U. S., but in Missouri it is primarily found in areas of moderate to severe human disturbance. It is known to hybridize with U. rubra where they occur together (for further discussion, see the treatment of that species).

The Chinese elm, U. parvifolia Jacq., is widely cultivated in Missouri. It is rather weedy in cultivated situations, and it is known to be invasive in disturbed habitats in several states, including Arkansas and Kentucky (Medley and Thieret, 1991, Sherman-Broyles, 1997), so it should be watched for in Missouri. Ulmus parvifolia has foliage very similar to that of U. pumila; in fact, they often cannot be distinguished confidently based on vegetative twigs. Ulmus parvifolia can be recognized by its distinctive bark, which is not furrowed, but exfoliates as irregular woody scales 2–8 cm across, and is orange-brown where freshly exposed and soon weathering ashy gray, and because it flowers and fruits in autumn rather than spring.