Ash Tree Identification: Opposite Branches, Diamond Bark

Ash identification is a short sequence, and each step throws out most of what is left.

1. Is the branching opposite? On an ash, branches, buds and leaves sit directly across from each other rather than staggered. Very few trees do this, so a staggered twig ends the question immediately.

2. Are the leaves compound? Ash carries pinnately compound leaves with 5 to 11 leaflets. Purdue Extension records white ash specifically as opposite with seven to nine leaflets.

3. Does mature bark form diamonds? Purdue describes white ash bark as gray, corky to the touch, with an interlacing network of ridges forming a diamond shape. Young trees are smoother, so judge a full-grown trunk.

One more confirmation for winter: Purdue notes the squatty terminal buds and a bud that dips down into the leaf scar so the scar resembles a smiley face.

Opposite branching plus compound leaves is a narrow combination. Michigan State University Extension's key makes the point that the only other oppositely branched tree with compound leaves is boxelder (Acer negundo), which almost always has three to five leaflets. So leaflet count settles that pair: three to five, boxelder; seven to eleven, ash.

Walnut and hickory also carry long pinnately compound leaves, but both are alternate, so step one removes them before you ever count leaflets. If you are holding a single compound leaf with no twig attached, go back and look at the branch — that is the check that matters.

Mountain ash is not an ash at all despite the name; if the branching is alternate, you are not in the Fraxinus group regardless of how the leaf looks. See tree leaf identification for the arrangement test in full.

Most ash identification requests come from one worry: emerald ash borer. Penn State Extension describes the adult as bright metallic green, slender and bullet-shaped, and 1/3 of an inch long, and the insect as using green, white and black ash to complete its life cycle.

The signs Penn State lists, roughly in the order people notice them:

• Woodpecker damage — jagged holes where birds dig for larvae, often the first sign
• Canopy thinning — foliage wilts and branches die back, working down from the top
• Epicormic shoots — new sprouts pushing out of the trunk
• D-shaped exit holes — left by emerging adults from late May into early June
• S-shaped galleries — winding larval tunnels in the phloem, widening as the larva grows, visible when bark is lifted

Penn State reports that heavy infestations can kill trees in as little as three years, with trees typically dying after three to four years of heavy infestation.

If the tree is an ash and it looks healthy, that is worth knowing rather than acting on immediately. If it shows the signs above, the decision — treat, remove, or monitor — depends on the size of the tree, how much canopy it has already lost, and what your state and municipality require. Several states and counties regulate movement of ash firewood and wood waste.

Sensible next steps:

• Photograph the trunk, the canopy from a distance, and a branch if you can reach one
• Look for the D-shaped holes on the trunk and larger limbs rather than on twigs
• Contact a certified arborist or your county extension office before treating
• Do not move ash firewood between counties or states

Photograph the tree with the app and it identifies it; the leaflet count and bark pattern above are how you check that answer against the tree in front of you.