Elm Tree Identification: Lopsided Leaf Bases

Pick an elm leaf and hold it by the stalk. One side of the blade starts noticeably lower than the other, so the leaf looks as if it has been pushed sideways. That asymmetry, plus a doubly serrate margin, identifies the genus faster than anything else.

Purdue Extension describes American elm as easily identified by its simple doubly-serrated oval leaves, which carry a large tooth with smaller teeth along its top edge. Feel the blade too — elm leaves are often rough like fine sandpaper on the upper surface.

The tree shape backs it up. Purdue records an open, spreading crown with drooping limbs forming a vase shape, which is why American elm became the classic American street tree. The silhouette survives in winter, so you can identify a leafless elm across a park.

Elm bark is distinctive twice over. From the outside, Purdue describes it as spongy, with strong crisscrossing ridges. From the inside, a broken piece shows distinct reddish brown and light tan or white colored layers stacked through the bark — a feature few other trees offer and a good confirmation when you have no leaves.

The practical routine: break off a loose piece of ridge from a mature trunk, turn it edge-on and look at the layering. Combine that with the vase-shaped crown and you have an elm.

Purdue gives the range of American elm as the northeastern United States south to central Florida, west to central Texas and north to southeastern Saskatchewan, so in most of the eastern US an elm is a plausible answer. Compare the bark categories on our tree bark identification page to place it among the other species you are sorting through.

If you are wondering why a tree this recognizable is uncommon in town, the answer is Dutch elm disease. University of Minnesota Extension names the cause as two closely related fungi, Ophiostoma ulmi and Ophiostoma novo-ulmi, with the second more aggressive and the common pathogen today.

It moves two ways. Bark beetles carry spores from tree to tree — the native elm bark beetle (Hylurgopinus rufipes), the smaller European elm bark beetle (Scolytus multistriatus) and the banded elm bark beetle (S. schevyrewi) — and the fungus also travels between neighboring elms through root grafts.

The symptom to look for: leaves on one or more branches in the outer crown turn yellow, wilt, then turn brown, usually in late spring or early summer but possibly any time in the growing season. Lift the bark on a wilted branch and brown streaking shows in the sapwood.

All of Minnesota's native elms are susceptible, including American, red and rock elm. If you see this pattern, call a certified arborist or your county extension office promptly — timing matters for the trees nearby.

Run these in order and photograph each one:

• A leaf, flat, stalk in frame. Check the uneven base and the doubly serrate margin.
• The twig. Elm leaves are alternate, one at a time along the twig, which rules out the opposite-branched maples and ashes.
• The trunk at chest height. Spongy, crisscrossing ridges.
• A broken piece of bark, edge-on. Reddish brown and tan layers.
• The whole tree from a distance. Vase-shaped crown with drooping limbs.

Several Asian and hybrid elms are planted as street trees because of disease tolerance, and University of Minnesota Extension notes that individual Siberian elms vary greatly in resistance. So a young elm in a new development is quite likely a selected cultivar rather than a wild American elm.

Photograph the tree and the app identifies it; the leaf base and the bark layers are the two things you can verify with your own hands.