Salix Linnaeus. Willow.

Contributed by Scott G. Ward and Alan S. Weakley

Key to Salix

A genus of about 400 species, trees, shrubs, and subshrubs, mostly north temperate and boreal.

ID notes:Plants of Salix are dioecious; flowers are rather simplified, lack a perianth, and are aggregated into structures called aments or catkins and are either sessile on branches or borne on short shoots. Each flower is subtended by a floral bract; the female flowers bearing a single pistil; the male flowers bearing 2 or more stamens. Some species produce catkins just before or as leaves emerge (aments coetaneous) while others flower before any sign of leaf emergence (aments precocious). Many keys are written with vegetative priority due to the relatively short-lived flowering period in many species. Pussy willow is a broad, collective vernacular that has been used to refer to early (spring) flowering shrubby Salix with pubescent floral buds (pussies) and conspicuous catkins; the term catkin derives from the Dutch word for kitten, katteken. In general, pussy willow has referred to a number of species sharing the above characteristics. In our area, Salix discolor is the most commonly accepted native "pussy willow"; however, other species now occurring in our area have historically and culturally been referred to in the same manner (e.g. Salix caprea, Salix cinerea). These willows have been used in a variety of cultural traditions and are in many ways a harbinger of spring in more northern latitudes. Some of the weeping willows (see notes under S. babylonica) are commonly cultivated and used in the horticultural trade throughout the e. US.

Key advice:Some species can contain both glaucous or non-glaucous leaves and can bear either serrate, crenate, or entire margins; these two character groups (leaf undersurface and leaf margins) are essential within this key, and the four primariy references used for this treatment (Argus 2005, Argus 2010, Haines 2011, Werier 2014). Species with irregular, somewhat sharpened marginal teeth are distinct from those with consistently sharp teeth throughout (e.g. S. eriocephala vs. S. discolor). Branchlets refer to stems produced within the year; branches are stems older than one year. Stipules are conspicuous leaf-like appendages sitting at the base of petioles on twigs, often encircling the stems, and are absent or late to develop on some species, or persistent and foliaceous on others. Three major bud types of twigs have been described: alpha-type (buds similar in size and shape along branchlets), arctica-type (branchlets with few buds, the terminal few largest), and caprea-type (floral buds strikingly different than vegetative buds) [Argus 2005]. Another important vegetative distinction concerns the bud-scales; those species in subgenus Protitea having bud-scales with distinct, overlapping margins compared to subgenus Salix, which contains bud-scales with connate margins (Argus 2005). Some keys are optimized for the southeastern United States, which narrowly covers fewer taxa than a broader eastern regional flora (see Argus 1986).

Ref: Argus (1986); Argus (1997); Argus (2010) In Flora of North America Editorial Committee (2010); Belyaeva (2009); Chen et al. (2010); Dorn (1995); Dorn (1998); Marchenko & Kuzovkina (2022). Show full citations.

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