Rock elm (Ulmus thomasii): what the agencies record

This page reports what US federal publications say about one tree. It does not identify trees from descriptions or photos and does not say what should be done with any tree. The family overview, tree species in the agency records, explains the sources and holds the comparison table.

At a glance

Item	What the source records
Scientific name	Ulmus thomasii Sarg. in USDA PLANTS, elm family
Other names	Cork elm (Silvics)
Native status, USDA PLANTS	Native in Canada and the lower 48 states
Size and age, Silvics	On good sites it may reach 30 m (100 ft) in height and 300 years of age; on average to better sites mature trees reach 27 m (90 ft) and 61 cm (24 in) in diameter, and virgin stands in the east and north have recorded heights of 27 to 30 m (90 to 100 ft) and diameters of 91 to 152 cm (36 to 60 in)
Corky wings, Silvics	Called cork elm because of the irregular thick corky wings on older branches; the handbook says the conspicuous ridges of corky bark on branches and older twigs are one of its identifying features
Shade, Silvics	Shade tolerant in the seedling and sapling stage and able to recover from long suppression, but apparently more light demanding with age; overall classed as intermediate
Green weight of the wood, NRS-38	54 lb per cubic foot, without bark

Native range and sites

The Silvics of North America handbook (Agriculture Handbook 654, volume 2, 1990) says rock elm is most common in the Upper Mississippi Valley and lower Great Lakes region. The native range includes portions of New Hampshire, Vermont, New York and extreme southern Quebec; west to Ontario, Michigan and northern Minnesota; south to southeastern South Dakota, northeastern Kansas and northern Arkansas; and east to Tennessee, southwestern Virginia and southwestern Pennsylvania, with some growth in northern New Jersey. The handbook describes the climate across the range as continental, with cold winters and warm summers.

It says rock elm grows best on moist loamy soils in southern Ontario, lower Michigan and Wisconsin, and may also be found on dry uplands, rocky ridges, limestone outcrops and bluffs, with the highest quality sawtimber on deeper loamy soils. It is always associated with other hardwoods and is a minor component in two forest cover types.

Flowers, seed and growth

Silvics says the perfect flowers appear about 2 weeks before the leaves, any time from March to May. The hairy fruit has a broad wing 13 to 25 mm (0.5 to 1 in) long and matures in May or June. Trees 20 years old produce viable seed, with maximum yields from trees 45 to 125 years old and good crops every 3 or 4 years. Seeds germinate within a week or two after dispersal if moisture allows, but the handbook says the species regenerates poorly despite high viability.

On growth, the handbook says few species recover from prolonged suppression as well as rock elm: an analysis of 153 trees found a large share had survived under suppression for 50 years or more, which makes diameter hard to relate to age. It gives about 50 rings per 2.5 cm (1 in) of radius for suppressed trees and 10 to 20 for dominants.

Wood

Silvics says rock elm wood has long been valued for its exceptional strength and superior quality, that the tree has been drastically overcut in many localities, and that much of the old-growth was exported for ship timbers. It describes the wood as stronger, harder and stiffer than any of the other commercial elms, highly shock resistant, with excellent bending qualities for bent parts of furniture, crates and containers, and a base for veneer. The Wood Handbook (FPL-GTR-282, 2021) lists rock elm with the hard elms, somewhat heavier than the soft elms, in its elm entry. Table 5-3b has a separate rock elm row, shown below. Soft elm is described on the slippery elm page.

Wood Handbook Table 5-3b	Specific gravity	Modulus of rupture (lbf per square inch)	Side hardness (lbf)
Green	0.57	9,500	940
At 12% moisture content	0.63	14,800	1,320

The Wood Handbook states that specific gravity is based on oven-dry weight and the volume when green or at 12% moisture content. The green weight of the wood without bark is in the NRS-38 table, the same figure used in the log weight calculator.

Diseases, insects and damage the handbook names

Silvics (1990) names few agents for rock elm specifically.

Dutch elm disease. The handbook says nearly all native North American elm species are susceptible, that the causal fungus (Ceratocystis ulmi) has been isolated from rock elm logs, and that the disease will likely greatly reduce the number of rock elm. See Dutch elm disease.
Seed fungus. A seed-borne fungus (Gleosporium ulmicolum) has been reported on the fruit; the handbook says few such fungi are thought to reduce germination or weaken seedlings.
Insects. Not listed as a particular host for specific insects, but the handbook says it is undoubtedly host to the borers, defoliators and sucking insects that attack American elm.
Frost. Killing frosts are common during the flowering period and subfreezing temperatures may prevent seed development in some years.

The guide on reporting a pest in the United States and the Canada reporting page give the offices that take sightings of regulated pests. Who may apply pesticides is in pesticide licences by country.