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Home , Elm, Ulmus thomasii

Ulmus thomasii: The Hard That’s Hard to Find

Brian Pruka

f you hike Path up the steep slope One of the best traits for identifying a rock from Valley Road and continue to the point elm—not often listed in identification books— Iwhere a footpath branches off to the right, is its form. The is typically tall and you will find a slender, stately next to the slender, with a single bole that gets remarkably trail. It is a rock elm, , one of tall before it splits into a narrow crown. Rock only three native to northeastern North elms growing in crowded forest situations also America. This particular specimen, accession usually have small corky branches that droop number 444-88-A, is the only rock elm cur- downward from the middle third of the main rently in the Arnold ’s living collec- bole. The Arboretum’s specimen has grown out tions. It is well worth seeing. in the open all its life and does not have droop- Rock elm was originally named Ulmus rac­ ing branches at mid-bole, though it does have emosa in 1831 by its discoverer, American civil a strikingly straight main trunk and currently engineer David Thomas of New York. It was measures 44.24 feet (13.48 meters) tall with a renamed Ulmus thomasii in 1902 by Arbore- dbh (diameter at breast height) of 18.31 inches tum director Charles Sprague Sargent when (46.5 centimeters). he determined that another elm already had Rock elm can look much like American the name Ulmus racemosa. Rock elm is most elm leaves. Tree identification books generally common in the northeastern and north-central list three identifier traits for rock elm: branches states, with the core of its range stretching from with 3 to 5 irregular corky wings; inflorescences north-central Wisconsin to southern Michi- of 7 to 13 arranged in a long, pendulous gan and southern . Populations exist raceme; and (samaras) covered with tiny as far south as , but it is primarily a hairs and an inflated paper wing that is not dis- cold-weather tree, not often found in regions tinct from the seed case. Unfortunately those warmer than USDA Hardiness Zone 5 unique traits are not always present. Some rock (average annual minimum temperature -10 to elms, including our specimen, lack corky twigs. -20°F [-23.3 to -28.9°C]). Rock elm is rarely Rock elms don’t reproduce until about age 20, encountered in New England, likely because it don’t produce full seed crops until age 45, and has a strong preference for limestone substrates, produce bountiful seed crops only once every 3 which are not common here. to 4 years. Seeds drop from the tree as soon as Back in the 1910s to 1930s there were as they ripen, so from May to February there are many as twelve rock elms growing on the Arbo- no reproductive structures to aid identification. retum grounds, all procured from well known The timber of rock elm is especially prized for plant nurseries of the era. Most of these its hardness. It has interlaced fibers that make eventually died of (DED), it almost impossible to split, yet easy to bend. a devastating fungal vascular wilt. Four suc- It is especially durable underwater. In past cen- cumbed to ’s first big DED epidemic in turies, much rock elm was cut and shipped to 1946. Two died of DED in 1987, another three in to build wooden battleships. Rock 1989. Specimen 17925-B was recorded as being elm is also highly regarded for its beautiful gold in “excellent health” on May 5, 1989. It was cut fall foliage color, so consider a hike up Beech down 75 days later, on July 19, dead from DED. Path this autumn. A tall, handsome native elm Our current living specimen was propagated is awaiting you. in 1988 as a from a then 102-year-old tree (accession 17926-A) that was planted at the Brian Pruka is a 2016 Isabella Welles Hunnewell Intern Arboretum in 1886. at the .