How to identify ash trees (and EAB risk)

Ash trees (*Fraxinus* spp.) are among the most recognizable and -- currently -- the most threatened trees in North American urban and suburban landscapes. Emerald ash borer (EAB), *Agrilus planipennis*, has killed hundreds of millions of ash trees since its arrival in Michigan around 2002. Per USDA.

—- title: "How to identify ash trees (and EAB risk)" slug: how-to-identify-ash-trees hub: plants category: "Identification guide" description: "Identify ash trees by their opposite compound leaves and diamond-patterned bark. Learn the emerald ash borer signs that predict tree death before visible canopy decline." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-

Ash trees (Fraxinus spp.) are among the most recognizable and — currently — the most threatened trees in North American urban and suburban landscapes. Emerald ash borer (EAB), Agrilus planipennis, has killed hundreds of millions of ash trees since its arrival in Michigan around 2002. Per USDA Animal and Plant Health Inspection Service (APHIS), EAB has spread to over 35 states and several Canadian provinces and is present in virtually all ash-growing regions of eastern North America.

Knowing how to identify an ash tree — and how to spot EAB damage early — determines whether you can act while management options remain viable.

Genus-level identification

Ash shares the "MAD Cap Horse" opposite-leaf mnemonic (Maple, Ash, Dogwood, Catalpa, Horse-chestnut) — opposite leaves, paired at each node. The combination of:

1. Opposite leaf arrangement
2. Compound leaves (multiple leaflets per leaf)

immediately narrows to ash, boxelder maple, or elderberry among common eastern trees. Boxelder maple leaflets come in groups of 3–5 and the tree produces maple samaras. Elderberry (Sambucus) is a shrub, rarely a tree. Ash is the most likely identification when a large tree has opposite compound leaves.

Per Penn State Extension, a third separator: ash produces paddle-shaped, single-winged seeds called samaras (not paired like maple samaras), typically in dense hanging clusters.

Leaf features by species

**White ash (Fraxinus americana):** The largest native ash, reaching 60–80 feet. Per NC State Extension, leaves are 8–15 inches long with 5–9 leaflets (usually 7). Leaflets are 3–5 inches, oval, with a slightly stalked base. The underside of the leaflet is distinctly whitish, especially on white ash — this contrast between green upper surface and pale lower surface is visible when wind moves the leaves.

**Green ash (Fraxinus pennsylvanica):** The most widely distributed ash and the most common in urban landscapes. Per USDA NRCS, green ash grows in zones 2–9, the widest range of native ash. Leaves are 6–12 inches with 5–9 leaflets. The underside is green to slightly pale — not the stark white of white ash. Green ash tolerates wet soils, poor soils, and urban conditions, which is why it was planted extensively as a street tree.

**Black ash (Fraxinus nigra):** A wetland species of the upper Midwest and Northeast, zones 2–5. Per UMN Extension, black ash leaflets are directly attached (sessile) to the rachis without an individual stalk — a useful separator from white and green ash, which have stalked leaflets. Black ash grows in swamps, bogs, and floodplains.

**Blue ash (Fraxinus quadrangulata):** The only ash with distinctly 4-angled, often slightly winged twigs. Per Missouri Botanical Garden, this square twig cross-section is diagnostic. Blue ash is native to the Midwest limestone regions in zones 4–7.

Bark identification

Young ash bark is smooth and grayish. As trees mature, the bark develops a distinctive diamond-shaped, interlacing ridge pattern — the "diamond bark" is one of the fastest field IDs for medium-to-mature ash trees. Per Penn State Extension, the diamond pattern is more regular and deeply cut on white ash, with a softer, less angular version on green ash.

On old white ash, the ridges become thick and very blocky; on old green ash, the pattern is similar but slightly less pronounced.

Emerald ash borer identification

What is EAB?

Agrilus planipennis is a small (0.3–0.5 inch), metallic-green beetle native to Asia. It was accidentally introduced to southeastern Michigan in the early 1990s via wood packing materials. Per USDA APHIS, EAB larvae feed in the phloem layer just under the bark, cutting off the tree's ability to transport nutrients and water. All native ash species are susceptible; blue ash has slightly better tolerance.

Early signs of EAB

S-shaped galleries under bark: The larvae create winding, S-shaped tunnels (galleries) between the bark and wood. Per Penn State Extension, galleries are 4–10 inches long and packed with frass (sawdust-like material mixed with excrement). The galleries disrupt the phloem, eventually girdling branches and the trunk. Peel back loose or cracked bark on a declining ash to look for these galleries.

D-shaped exit holes: Adult beetles exit through D-shaped holes 1/8 inch wide in the bark in late spring and early summer. Per NC State Extension, these flat-sided, D-shaped holes are unique to Agrilus beetles and are the most widely known EAB sign. They appear in the trunk and major branches.

Crown dieback: Per UMN Extension, visible canopy thinning and dieback begin in the upper crown in years 2–3 of infestation. By year 3–4, typically more than half the canopy is dead. Dieback starting at the top and progressing downward is the pattern.

Epicormic sprouting: The stressed tree sends up water sprouts from the base of the trunk and along the lower trunk — a stress response to declining canopy. Per Penn State Extension, this sprouting from the lower trunk and root flare is a consistent EAB sign on ash.

Woodpecker activity: Woodpeckers preferentially forage on EAB-infested ash. Per Penn State Extension, heavy blonding (bark stripped by woodpeckers) on the upper trunk in an ash tree is strongly associated with EAB infestation.

Management options

Per Penn State Extension, treatment options depend on the stage of infestation:

• Preventive treatment (before infestation or early stage): Systemic insecticide trunk injections with emamectin benzoate (TREE-äge) provide 2–3 years protection per application. Soil drenches with imidacloprid provide 1–2 years. Per UMN Extension, treatment is cost-effective for healthy trees with good structure and significant landscape value.
• Advanced infestation (>50% canopy dieback): Tree cannot be saved. Removal is recommended before structural failure creates a hazard.
• EAB-resistant cultivars: Research on EAB resistance is ongoing. No commercially available ash cultivar has confirmed high EAB resistance as of 2025. Per USDA Forest Service, several Manchurian ash (Fraxinus mandshurica) selections show moderate resistance.

Species comparison table

Recommended gear: Best dogwood cultivars (Cornus) — our buyer's guide covering picks for every budget, ranked by Extension publication consensus and personal use.

Frequently asked questions

How do I tell white ash from green ash? Leaf underside color is the most practical separator: white ash leaflets are distinctly pale or whitish beneath; green ash leaflets are green to only slightly pale. Per Penn State Extension, the two species hybridize in zones of overlap, so some trees are intermediate. For management purposes, both species are equally susceptible to EAB and the treatment response is the same.

What is the D-shaped exit hole size for EAB specifically? Per Penn State Extension, EAB exit holes are approximately 1/8 inch (3–4 mm) wide, with a flat base (the "D" shape). They are smaller than native borer exit holes, which tend to be round. The flat-sided D is unique to Agrilus beetles; round holes of similar size could be from other insects.

If I treat my ash tree, how long will it survive? Per UMN Extension, trees treated preventively before canopy dieback begins have a good prognosis with ongoing treatments. Treatment does not remove existing infestation but reduces larval populations sufficiently for the tree to recover. Trees with less than 30–50% canopy dieback are good treatment candidates.

Should I remove my ash and replace it now? This is a site-specific decision. Per Penn State Extension, the cost of ongoing treatments needs to be weighed against the tree's structural value, aesthetic importance, and replacement cost. For a large, healthy ash in a visible location, treatment is often worth the cost. For a poorly placed, structurally compromised tree, removal and replacement with a non-ash species is usually the more practical choice.

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Sources:

1. Penn State Extension — Ash trees
2. Penn State Extension — Emerald ash borer
3. NC State Extension — Fraxinus americana
4. UMN Extension — Emerald ash borer
5. USDA APHIS — Emerald ash borer
6. USDA NRCS — Fraxinus pennsylvanica
7. Missouri Botanical Garden — Fraxinus quadrangulata