Which tree has compound leaves? Count the leaflets, then check the twig

The leaf-or-leaflet test

The University of Illinois Woody Plants Tutorial glossary gives the terms field guides use. A pinnate leaf has leaflets along both sides of a central stalk, the rachis. A palmate leaf has its parts radiating from a common point. A bipinnate leaf has leaflets that are themselves divided again, which gives the feathery, fern-like look. An odd-pinnate leaf ends in a single leaflet; an even-pinnate one ends in a pair.

The quickest way to tell a leaf from a leaflet is the bud. A bud sits in the angle where a whole leaf meets the twig, and never at the base of a leaflet. Follow a "leaf" back: if you reach a bud and a twig, it was a whole simple leaf; if you reach a long shared stalk, it was a leaflet. The leaf identification guide shows the same test with drawings of the terms.

Alternate and pinnate: walnut, hickory, locust, mountain-ash

Black walnut has the longest leaf of the common trees here. NC State gives 12 to 24 inches with 15 to 23 finely toothed, lance-shaped leaflets. The fruit is a round nut 1 to 3 inches across inside a yellow-green husk that does not split and stains skin and clothing, ready in October. Cut a twig lengthwise: the pith is chambered, with thin buff-colored walls. Walnut is native from northeastern Canada west to North Dakota and south to Texas and Florida. The walnut page separates black walnut, butternut and English walnut.

Shagbark hickory has fewer, bigger leaflets. NC State describes leaves 8 to 14 inches long with 5 to 7 finely toothed leaflets, the one at the tip the largest, and leaflets 3 to 7 inches long. On older trees, narrow plates of bark 1 to 3 feet long peel away from the trunk at both ends. The husk splits into four parts when the nut is ripe in October. Five to seven leaflets with a big end leaflet means hickory; fifteen or more small leaflets means walnut. The hickory page covers the other hickories.

Black locust has smooth-edged, oval leaflets, which sets it apart from the toothed walnut and hickory. NC State gives 7 to 23 leaflets on leaves 8 to 14 inches long, short paired spines up to about 1.25 inches beside each leaf scar, fragrant white pea-like flowers in hanging clusters from April to June, and flat purple-brown pods 2 to 4 inches long. It rates the bark, leaves and seeds as highly toxic, and notes the tree has become invasive in the Midwest. The locust page compares it with honey locust.

Mountain-ash is not an ash: its leaves are alternate. NC State describes American mountain-ash with 9 to 17 toothed leaflets, flat clusters of small white flowers in June and July, and bright red-orange berries about 1/4 inch across from September, often lasting into winter. It grows from Newfoundland to Georgia, mostly in the mountains in the South. The European rowan, planted widely, looks almost the same; see the rowan page.

Opposite compound leaves: ash, boxelder, buckeye

If the leaves sit in pairs on the twig, the list shrinks to a few groups; trees with opposite leaves has the full list.

Ashes have a feather-like leaf. For white ash, NC State gives leaves 8 to 12 inches long with usually 7 leaflets, sometimes 5 or 9, and bark with deep furrows in a net of diamond-shaped ridges. The seeds are single wings up to 2 inches long in drooping clusters. Emerald ash borer, which NC State says usually kills an ash within 3 to 5 years, has left many dead ashes along streets. The ash page separates white, green and European ash.

Boxelder is a maple with a compound leaf. NC State gives usually 3 to 5 leaflets, occasionally 7, green new twigs and paired samaras that hang into winter. Three leaflets on a young boxelder sometimes get it mistaken for poison ivy; boxelder is a tree or sapling with paired leaves, so check the twig.

Buckeyes and horse chestnut are the palmate ones. For Ohio buckeye, NC State describes 5 to 7 smooth leaflets spread from one point and a warty or prickly leathery husk with shiny brown seeds in October, and lists the seeds, leaves and bark as poisonous. Horse chestnut, planted in parks, has the same hand-shaped leaf. The chestnut vs buckeye page explains why neither is a true chestnut.

Fern-like and other look-alikes

• Twice-divided leaves. If the side stalks carry their own rows of small leaflets, you have a bipinnate leaf: honey locust, mimosa, Kentucky coffeetree or jacaranda. The fern-like leaves guide takes that list apart.
• Very long leaves on a weedy tree. A fast-growing tree on a vacant lot with very long leaves, whose leaflets are smooth-edged except for a few teeth near the base, may be tree of heaven rather than a walnut or sumac; the tree of heaven page has the leaflet test.
• Deeply lobed simple leaves. Oak and maple leaves can be cut almost to the midrib, but they are still one blade with one bud.

Using a photo

Lay one whole compound leaf flat, from the point where it left the twig to the tip, and photograph it with the twig if you can: that one picture shows the leaflet count and the arrangement. Tree Identifier names a tree from one photo, and black walnut, English walnut, black locust, European ash, rowan, horse chestnut and honey locust are among the species in its Collection. It shows no confidence score and no list of alternatives, so check the answer against the table above. The follow-up chat under each result can answer questions such as how a black walnut leaf differs from a butternut leaf, and the Tree Map remembers where the tree stands so you can come back for the nuts or flowers.