Leaves

Simple or compound

A simple leaf has one blade on its stalk. A compound leaf is divided into separate leaflets. The test is the bud: a bud sits where a leaf stalk joins the twig, and never at the base of a leaflet. If the bud is at the base of the whole structure, the whole structure is one leaf.

Pinnate leaves have leaflets set along a central stalk, like a feather. Palmate leaves have leaflets that spread from a single point, like the fingers of a hand. Bipinnate leaves are divided twice, so that each leaflet is itself a feather; honey locust and Kentucky coffeetree are examples.

Opposite or alternate

Look at how leaves, or the buds where they were, sit on the twig. Opposite leaves come in pairs at the same point, facing each other. Alternate leaves come one per point, staggered up the twig. Whorled leaves come three or more at a point, as on catalpa.

Most broadleaf trees are alternate. The common opposite ones are easy to remember with the phrase MAD Horse: maple, ash, dogwood, horse chestnut. Buckeyes, which are close relatives of horse chestnut, are opposite too.

Edge

A smooth edge is called entire. Small sharp points along the edge are teeth, and teeth that carry smaller teeth are double-toothed. Lobes are deeper cuts that divide the blade, and they may be rounded or pointed. Some trees have all three kinds on one branch, so look at several leaves.

Shape, base and veins

Note the overall outline (oval, heart-shaped, triangular, narrow), the tip, and the base. A base where one side of the leaf starts lower than the other is a quick clue to elms. The leaf shapes page has drawings of the main shapes.

Pinnate veining has one midrib with side veins running off it. Palmate veining has several main veins fanning from the base of the blade, as in maples.

Surface, smell and feel

Rub a leaf between your fingers. Slippery elm leaves are rough like sandpaper. Silver maple leaves are silvery white beneath. Sassafras leaves smell spicy when crushed. Hairs along the veins underneath can separate close relatives such as white and green ash. Never taste a leaf you have not identified.

Choosing a leaf to look at

Pick a leaf from a sunny outer twig on a mature part of the tree. Leaves on shaded lower branches, on young shoots and on stump sprouts are often larger or more deeply cut than the typical leaf. In autumn, colour changes, but shape does not.

Deciduous or evergreen

Deciduous trees drop all their leaves in one season, usually autumn. Evergreens replace leaves gradually and always carry some. Evergreen broadleaf trees such as live oak, holly and magnolia are common in the south; most northern evergreens are conifers.