Where you find a willow tree, you will typically find some water close by. The willows of North America are common trees along brooks, streams, rivers and near wetlands such as marshes and swamps. The approximately 40 types of American willows have subtle differences you can use to identify them from one another, with many of these dissimilarities occurring between the leaves.
Look at the shape of the willow leaves. Black willow features long narrow foliage that tapers to a long point. The leaves are somewhat curved to one side, notes the "National Audubon Society Field Guide to Trees: Eastern Region." Other willows have leaves with an oval shape, such as hooker willow. Lanceolate leaves, with a shape resembling a spear point, are traits of the pussy willow and Mackenzie willow.
Measure the length and width of willow leaves to narrow down their identities. Measure about 10 leaves on each willow so that you have a representative number to gauge. The willow leaf width is particularly important and can help you pinpoint a species. For example, sandbar willow's leaves are less than ½ inch wide, while those of Scouler willow typically measure a full inch wider.
Examine the margins of willow tree leaves, looking to see if they are smooth or if they possess serrations resembling teeth. Inspect the edges with a magnifying lens if the serrations are not obvious. Some willows will have wide spaces between teeth, such as the pussy willow, while other willow species will have rounded marginal teeth, including the Mackenzie willow.
Look at the colour of the willow leaves. Note any differences in the colours on the upper leaf surface from the hues on the underside. Some willows have a shiny green upper surface, but the lower surface is slightly paler or duller. Black willow falls into this category. Others have more easily noticeable colour disparity between upper and lower surfaces. The balsam willow leaf, for instance, has a dark green upper surface, but the underside is whitish and the main vein bisecting the leaf has a yellow tint.
Inspect the leaf surface and leaf stalks of willow foliage for the presence of tiny fine hairs. Some will have no hairs; the leaves of peachleaf willow will lack hairs when the tree is mature. Others have just a slight coating of hairs on the leaves, including Bebb willow. Feltleaf willow takes its name from the many hairs on the lower side of the leaves.
Use a field guide to gain a much better understanding of the characteristics of willow tree leaves among various species.