by Eric Meier
The most basic division of elm species is between hard and soft elm. The wood of the hard elms (sometimes referred to as rock elm) generally range from 41 to 47 lbs/ft3, while soft elms typically have a density from 35 to 38 lbs/ft3.
The primary element for distinguishing elm types is found in the earlywood pores. For elms in North America, hard elms are characterized by smaller earlywood pores that are closer in size to the latewood pores. The earlywood is generally in a single, broken row.
By contrast, soft elms tend to have larger earlywood pores. The earlywood may be one or two rows wide, as in American Elm (Ulmus americana), or two to four pores wide, as in Red Elm (U. rubra).
However, elm species from Europe and Asia do not always follow the same earlywood patterns as the North American species. Most notably, English Elm (U. procera) and Wych Elm (U. glabra) both have single, intermittent rows of smaller earlywood pores, but are considered soft elms.
With the exception of the multiple earlywood rows found in Red Elm, individual species of both hard and soft elms cannot be further separated down to a species level.
Dutch Elm Disease
This fungal disease is spread by elm bark beetles, and has been responsible for the demise of tens of millions of elm trees in North America and Europe. As a result, disease-resistant cultivars and hybrids have been sought out. Hybrid elm trees may have characteristics from either of the parent trees, confounding identification.
Elm is sometimes confused with ash (Fraxinus spp.), though the two can be separated based on elm’s wavy latewood bands.
This zig-zag pattern, also called ulmiform, is even visible on flatsawn surfaces as minute jagged lines.
Hackberry (Celtis occidentalis) is anatomically very similar to elm, though its wide sapwood and grey or yellowish color usually serve to differentiate it from elm.