How to identify an ash tree

Why this matters first

Every decision about emerald ash borer starts with whether the tree is actually an ash. EAB attacks true ash — the genus Fraxinus — and nothing else. People remove healthy trees over an EAB scare on a species the beetle does not touch, and they miss real ash because they assumed a tree was something else.

Three checks settle it, and you can do all three from the ground.

1. Opposite branching

The most useful single test. On an ash, branches, buds and leaves emerge in pairs directly across from each other on the stem. Most trees are alternate — branches stagger along the stem one at a time.

Look at the smaller twigs rather than the main structure, because large limbs get broken and pruned and the pattern gets obscured. Opposite branching immediately narrows the field to a short list: ash, maple, dogwood, horsechestnut and a few others.

2. Compound leaves with five to nine leaflets

An ash leaf is not a single blade. It is a compound leaf made of five to nine leaflets arranged along a central stalk, with one leaflet at the tip. Green ash typically carries seven.

The mistake is counting each leaflet as a leaf. Trace back to where the whole structure attaches to the twig — that whole thing is one leaf.

Combined with opposite branching, compound leaves rule out maple, which is opposite but has simple lobed leaves.

3. Diamond-patterned bark on mature trunks

Mature ash bark forms tight, interlacing ridges in a distinct diamond or lattice pattern. It is regular in a way that most bark is not, and once you have seen it a few times it is recognisable from across a street.

Young ash bark is smoother and does not show the pattern yet, so this check works on established trees rather than saplings.

The lookalikes

Mountain ash

The most common confusion, and the most consequential, because people panic unnecessarily. Mountain ash is Sorbus, not Fraxinus, and emerald ash borer does not attack it. It has alternate branching and produces clusters of orange-red berries, which no true ash does. If your "ash" has berries, it is not an ash.

Boxelder

Boxelder is a maple with compound leaves, which makes it the trickiest lookalike. Opposite branching, compound leaves — so far it matches. The difference is leaflet count: boxelder usually has three to five, ash has five to nine. Boxelder leaflets are also coarsely and irregularly toothed, and the twigs are often distinctly green or purple-green.

Walnut and hickory

Both have compound leaves but alternate branching, so the first check separates them immediately.

Tree of heaven

Compound leaves with many more leaflets, alternate branching, and a strong unpleasant smell when a leaf is crushed.

Which ash species are in Greeley

Green ash is by far the most planted across Northern Colorado, followed by white ash including the purple-leaved autumn purple cultivar. Both are susceptible. So is every other true ash species. There is no ash variety that resists EAB.

Ash represents roughly 15% of Greeley's canopy, with the City owning around 5% of those trees and the remainder on private property — which is why identification matters to individual homeowners rather than only to the City.

In winter, with no leaves

Use branching and bark. Opposite twig arrangement is visible year-round, and diamond bark does not change with the season. Ash buds are also distinctive: a large, blunt, dark brown terminal bud sitting directly above a crescent-shaped leaf scar.

Related

• Signs of emerald ash borer — once you have confirmed it is an ash
• Emerald ash borer in Greeley
• What to plant instead of ash

Questions we get about this

What is the fastest way to tell if a tree is an ash?

Look at the small twigs. If branches and buds come off directly across from each other rather than staggered, and the leaves are compound with five to nine leaflets, it is very likely an ash. Mature ash also has distinctive diamond-patterned bark.

Is mountain ash affected by emerald ash borer?

No. Mountain ash is a different genus entirely and EAB does not attack it. It has alternate branching and orange-red berry clusters, neither of which a true ash has.

How do I tell ash from boxelder?

Leaflet count and twig colour. Boxelder usually has three to five coarsely toothed leaflets and often green or purple-green twigs. Ash has five to nine more finely toothed leaflets on grey-brown twigs.

Are some ash varieties resistant to EAB?

No. All true ash species and cultivars are susceptible, including green ash, white ash and the popular autumn purple cultivar.

Can I identify an ash in winter?

Yes. Opposite twig arrangement and diamond-patterned bark are both visible year-round, and ash has a distinctive large blunt dark terminal bud sitting above a crescent-shaped leaf scar.