The number one rule of willow identification is: never base species determination on a single character! Most single characters that are useful for species-level identification are present in many Salix species, but combinations of traits differ across species. Therefore, when considering a particular willow individual, a quick scan of 3–4 traits will in most cases lead to a reliable identification. Particularly useful characters are the shape and size of leaves, color and hairiness or leaf upper and lower surfaces, color and hairiness of shoots, and the presence or absence of stipules on the shoots at petiole bases. In the spring and summer, the shape and size of inflorescences and catkins will also provide useful information. Large plants are small when they are young, so size per se is usually an unreliable basis for identification, but may help to exclude possibilities (some species never grow very tall, or do not flower when they are small).
One should remember, however, that individuals within species will in most cases vary in many traits, and that the level of variation differs across species, so that some species are more uniform with respect to morphology than are others. Looking through numerous willow individuals will help in finding out the limits between species.
Leaves and shoots
Leaf size, shape, color, and texture usually provide the best characters for species determination. In addition to general size and shape, traits to inspect include color and shininess of upper and lower surfaces, the amount and length of hairs (if present), whether leaf veins are elevated from or sunken into the leaf blade, and the presence of serration along the leaf edge. In most species, leaf shape and color will change though the growing season, and will vary markedly across different parts of the current-year shoot. Therefore, checking multiple leaves on shoots is advisable.
The color and hairiness of current-year shoots likewise varies markedly among species, as does the presence and shape of stipules (small additional leaves) on the shoot at petiole bases. Both traits are variable within species in some specific cases, but often can be used at least for excluding potential species during identification.
Willow "flowers" are strictly speaking inflorescences, i.e., collections numerous tiny flowers arranged around a central axis. Male inflorescences can be identified as such based on the presence of stamens consisting of pollen-producing anthers at the tip of long filaments, but male inflorescences are typically present only for a few weeks during the spring or summer before withering and falling off the plants. Characters useful for species determination include, in addition to inflorescence location on shoots and general size and shape, the color of anthers (red/orange/yellow), filaments, and bracts (small scale-like leaves at the base of each stamen, together with a nectar gland), and the number and size of leaves along the inflorescence petiole. The color and amount of hairs in inflorescences can helpful in some cases (for example, newly-opened catkins of Salix lanata look like furry balls made up of long yellow hairs). A complication follows from the dramatic changes that occur during inflorescence development, as stamens mature and open up in sequence as the inflorescence extends in length.
Like in the males, female "flowers" are in fact inflorescences made up of dozens of small flowers that look like small pointy bottles. Each individual flower (carpel) in an inflorescence consists of a rounded ovary that extends up to form a style having a stigma (pollen receptacle) at the tip. In addition, a small supporting leaf (bract) and a nectar gland are located at the base of each individual flower. The size, structure, location, hairiness, and coloration of female inflorescences varies across species, but unfortunately also within species and especially through the flowering season. A main complication is presented by the gradual change of female inflorescences to seed-bearing catkins (see below). Nevertheless, the size and shape of female inflorescences are informative for species deternation, and this is especially true for stigma color, which can be red, green, or yellow depending on the species. Like for males, the size and number of leaves on the petiole of each inflorescence is also a useful character.
The hairiness and coloration of seed-bearing catkins differs widely among willow species and species-groups, and their overall structure ranges from short and sparse (for example, S. starkeana) to long and dense (for example, S. phylicifolia and S. lanata). In some cases, catkins provide the easiest means for separating otherwise similar-looking species; for example, Salix lapponum and S. glauca, which have rather similar leaves, are easy to distinguish based on their entirely different catkins, which are also produced much later in the summer in S. glauca. However, as mentioned above, catkins develop gradually from female flowers, so that the stigmas wither and the ovaries thicken as seeds and silky dispersal hairs grow within the tiny individual capsules (fruits) along the catkin. Eventually, the capsules are split open lengthwise and release the hairy seed masses for dispersal by the wind.
Northern willows are deciduous, so species determination during the winter poses severe but by no means insurmountable challenges. The characters to pay attention to are plant size, shoot thickness, color, and hairiness, and size and shape of buds. For example, S. lapponum is identifiable based on its large flower buds with pointy tips. Additional cues for species determination are provided by the angle between shoots (which may, however, vary also within species) and dry leaves that may remain on plants long into the winter. Leaf retention is a regular feature especially in S. myrsinites, in which dead previous-year leaves are present on shoots even the next summer. Seed dispersal in Salix pentandra occurs in the late winter and spring, so catkin-bearing females of this species can be spotted from far away in snowy landscapes. The glabrous shoots with regularly-shaped reddish-brown buds are characteristic also for the males.