New York Natural Heritage Program
Cork Elm
Ulmus thomasii Sarg.

General Description [-]
Ulmus thomasii is a medium-sized tree, commonly reaching up to 70 to 80 feet in height, and occasionally up to 100 feet (Burns and Honkala 1990), and may live for up to 300 years. It has a strongly upright form and a narrow crown, markedly different from the spreading shape of American Elm (Ulmus americana). The bark of the trunk is furrowed with flattened, spongy ridges, similar to that of American elm. Young twigs are covered in short hairs, and have reddish buds much like those of American Elm, but twigs a year or more old become covered in the distinctive corky ridges that give the plant its name. The leaves are alternate, with doubly-toothed margins and asymmetrical bases, and are smooth to only slightly pubescent. They also tend to be somewhat shiney and papery in feel, unlike those of American Elm or Slippery Elm. The flowers are small and lack petals, occur in racemes up to 4 cm long and appear in early spring before the leaves. The fruit are flattened, round samaras, notched at the top, and covered with soft hairs.

Best Life Stage for Proper Identification [-]
Mature Cork Elm can be identified at any time of year.

Similar Species [-]
Ulmus thomasii is the only elm species native to New York which has corky wings on the older twigs and branches. Not every twig develops the corky wings, however, so it may be necessary to look at several. The other two common elm species in New York, Ulmus americana and U. rubra, also both have smooth fruit, more pubescent, softer leaves (unlike Cork Elm's papery leaves), and are not typically found on the dry, limestone ridges and outcrops favored by Ulmus thomasii.

Cork Elm is very similar to Ulmus alata, a southern species which in New York is known only from cultivation. Ulmus alata has smaller leaves, the largest 4-7 cm long, and both young and old branches may have corky bark.
Cork Elm Images
click to enlarge
The Best Time to See
This woody plant may be identified year-round using the very unique and characteristic corky bark. Fruits may be present late April through May.
Vegetative Flowering Fruiting
The time of year you would expect to find Cork Elm vegetative (blue shading), flowering (green shading) and fruiting (orange shading) in New York.