Ash or Walnut? Count the Leaflets, Then Crush One

A compound leaf has many small leaflets on one stalk, and from below several unrelated trees look identical. Four common ones share the pattern, and separating them takes two checks.

First: opposite or alternate

Look at where the whole leaf joins the twig, not at the leaflets. Ash leaves grow in pairs directly across from each other. Walnut, hickory, tree of heaven and sumac all grow singly, staggered along the twig.

Very few trees are opposite-leaved, and among common compound-leaved trees ash is essentially the only one. That single check resolves most cases before you have looked at anything else, and it works in winter on bare twigs because bud arrangement follows leaf arrangement. The full logic is in opposite or alternate.

Then: crush a leaflet

Black walnut is unmistakable by smell. Crushed leaves give a strong sharp citrus-and-turpentine scent that stays on your fingers, and the husks around the nuts smell the same, more strongly.

Ash smells of nothing much. Tree of heaven smells distinctly unpleasant, often described as rancid peanut butter, and it is the reason the tree is disliked as much as its invasiveness.

The leaflet count

Ash has few and large: usually five to eleven, each several centimetres long. Walnut has many and small, fifteen to twenty-three, and the whole leaf can be half a metre long.

Walnut frequently loses its terminal leaflet, so the leaf ends in a pair rather than a single point. Ash almost always keeps it. This is a quick check when you cannot smell anything, which happens in cold weather.

Tree of heaven's giveaway

Look at the base of each leaflet. Tree of heaven has one or two blunt teeth near the base, each with a small gland underneath, and the rest of the leaflet edge is smooth. Nothing else common does this, and it is visible with a hand lens or good eyesight.

That matters because tree of heaven is invasive across much of North America and Europe, and it is also the host plant for spotted lanternfly, so identifying it correctly has practical consequences beyond curiosity.

Sumac

Staghorn sumac is smaller, often a large shrub rather than a tree, with velvety fuzzy twigs that give it its name and a dense upright cone of red fruit that persists through winter. Once you have seen the fruit it is never confusing again.

The fuzzy twig is the winter character, and it is distinctive by touch even in the dark.

Where to start

Arrangement first, every time. It is visible from further away than any other character, it works in winter, and it eliminates three of the four possibilities in one look. Then smell, then count. Run anything still unresolved through the tree key, which asks these in the same order.

A note on ash in 2026

Emerald ash borer has killed hundreds of millions of ash trees across North America since 2002, and in much of the affected range a large mature ash is now an unusual sight. Standing dead ash with the bark falling away in sheets, revealing S-shaped galleries beneath, is far more common.

Those galleries are diagnostic in their own right. If you find a dead compound-leaved tree with winding tracks under loose bark, it is an ash and the borer is why.