Illustrated Key to Massachusetts Willows

This key is arranged by sections; however, as this is a simplified version for just Massachusetts common willows, it cannot serve a key to willow sections. Hybrids and those introduced species that are rarely found are not included, and therefore some sections are missing. Section names follow the Flora of North America (Argus 2010). Corresponding section names used by Skvortsov (1999) are shown after '=' sign.
1. All buds uniform (floriferous buds similar to vegetative ones), small (up to 3 mm long), conical (not compressed), positioned at acute angle to shoot, bud scale margins free, overlapping [1], [2].
Sect. Humboldtianae: S. nigra — black willow
Tree to 20 m tall [3], [4], [5], [6], sometimes even taller; may be frequently recognized from the distance by its characteristic habit produced by long and narrow, sickle-shaped leaves [7]. Bark rough, forming wide, thick, irregular plates [8] [9]. Branchlets highly brittle at base. Leaves lanceolate to sublinear, flat, 6-13 times as long as wide, 6–25 mm broad; in mature leaves pubescence usually inconspicuous or absent; leaf margins serrulate [10]; lower (abaxial) leaf surface green (not glaucous); leaf texture very delicate, lateral veins entirely submersed in parenchyma, except for the very origins of lateral veins; upper surface with or without stomata; stipules present at least on vigorous shoots, broad, frequently rounded [11], [12]. Flowering late; catkins serotinous, on leafy stalks. Staminate flowers with two nectaries and numerous, usually 6, stamens [13], [14]. Pistillate catkins with one nectary; ovaries pyriform, glabrous, on short stipes and with short styles, floral bracts small, pale, short pubescent, at least partially deciduous after flowering [15], [16]. — [all gallery photos]
Buds not necessarily uniform, of different shape and structure, bud scale cap-like, always with connate margins.
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2. Stamens 4 to 10; young leaves glandular, producing pitch, like leaves in poplars.
Sect. Salicaster = Pentandrae [3]
Large or small trees or shrubs; floriferous buds similar to vegetative ones; petioles always with distinct glands [cf. [17], sometimes foliaceous. Leaves highly lustrous above, densely glandular dentate at margin. Flowering late; catkins serotinous, on leafy stalks, dense, rather stout; ovaries stipitate, styles short; staminate and pistillate flowers with two nectaries which can form cup-like structure (around stamens).
Characters not as above. Stamens 2, usually free; in introduced S. purpurea connate in 1 [18]; young leaves not producing pitch.
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3. Fruit ripening and seed dispersal from late fall into winter [19], [20]. Outermost primordial leaves (cataphylls) in bud nearly round, embracing the entire bud content, frequently persistent in spring on developing shoots [21], [22], [23]. Bud scale glabrous and shiny, usually dying and hardening (though not blackening) at the start of winter, with just a narrow stripe of live tissue left at the very base [24]. Leaves always completely glabrous.
pentandra-group: S. serissima — autumn willow
All references to S. pentandra from New England are to be attributed to introduced hybrids: S. pentandra x (S. x fragilis) or S. pentandra x S. euxina (= S. x meyeriana). More about this group: [Skvortsov 2014, translation of 1960 article], [Zinovjev 2011], [Salicicola note], [Salicicola note], [photo gallery: S. serissima], [photo gallery: hybrids]
Fruit ripening and seed dispersal in mid-summer (as in all other willows); buds different [25], cataphylls elongate [26]. At least the youngest leaves usually with some reddish hairs.
lucida-group: S. lucida — shining willow
Shrub or small tree up to 6 m tall. Bud scales remain alive during the winter, their inner membranaceous layer (second prophyll) free, separated from outer layer, yet connected to base of developing shoot. It partially embraces and covers the developing cataphylls together with long silvery tufted trichomes that are spreading from the shoot origin and from medial parts of cataphylls [27]. More: [Salicicola note], [all gallery photos]
4. Bracts in pistillate catkins deciduous after flowering (all or some fall off when capsules ripen), pale, rarely brown, never black [28]. Outermost leaf primordia in floriferous buds at least as long as catkin primordium, either broad or narrow.
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Bracts persistent, in most species not entirely pale; leaf primordia in floriferous buds much shorter than catkin primordium (except for S. pedicellaris).
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5. Colonial shrubs producing root shoots; leaf blade very narrow, typically linear, with unusually distant dentation; petiole not glandular [29], [30]. Buds with elongate leaf primordia, sometimes with branched catkin primordia [31], [32]. Pistillate floral bracts deciduous in fruit; stigmas deciduous after flowering.
Sect. Longifoliae: S. interior — sandbar willow
A small section endemic to North America. All species are shrubs adapted to arid climate, their leaves small and narrow, of xeromorphic structure. S. interior is the only representative of the section in New England. This is a counterpart of S. exigua, a western North American species. The two of them may be considered subspecies of a single species. Both are typical alluvial willows; natural populations of S. interior in Massachusetts are confined to the Connecticut River. — [all gallery photos]
Characters not as above. Trees not colonial by producing root shoots [33], [34]. Leaf blade not linear [35], petiole glandular. Buds with large and broad outer leaf primordia similar to those in pentandra-group but silky on outer surface [36].
Sect. Salix
Trees, sometimes large. Floriferous buds similar to vegetative ones. Petioles with a pair of glands near blade base. Catkin stalks leafy. Bracts pale, deciduous in pistillate catkins. Nectaries 2 in staminate flowers and mostly 1 in pistillate. Stamens 2 (but very rarely up to 4 or even 8). Capsules glabrous, stipitate, styles short. In North America includes introduced S. alba, S. euxina and S. x fragilis (S. alba x S. euxina). See [article], [S. x fragilis], gallery: [S. euxina (or hybrids most similar to S. euxina)], [S. alba]
6. Large trees with weeping branches [37]. Floriferous buds 4-7 mm long, floral bracts pale, capsules sessile to subsessile.
Sect. Subalbae — introduced weeping clones of S. babylonica and its hybrids
An East Asian section, a counterpart of the European-West Asian section Salix, sometimes included with the latter. Differently from those of sect. Salix, willows from sect. Subalbae don't produce conspicuous petiolar glands; besides, their floriferous and vegetative buds considerably differ by the size. Other characteristics of the section: persistent floral bracts; one or two nectaries in pistillate flowers and two in staminate; stamens with short filaments; capsules small, sessile, ovoid, glabrous or pubescent. In North America, the section is mostly represented by introduced hybrids of weeping S. babylonica clones with S. alba or S. x fragilis (or else with S. euxina). In cultivation, one may also occasionally find non-weeping clones of S. babylonica (including S. matsudana) and even more rarely yet another species from East Asia, S. pierotii. — [all gallery photos]
Characters not as above: shrubs or small trees, not weeping/pendulous (except for cultivated S. caprea 'Pendula').
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7. Catkins very densely flowered; ovaries sessile to short-stipitate, tomentose or woolly [38], [39]. Leaves narrowly elliptic to oblanceolate, with dense white tangled hairs (woolly or tomentose) beneath, rugose (wrinkled) on the upper side [40], [41] [42].
Sect. Candidae: S. candida — hoary willow
S. candida is a wide-ranging North American willow, which is, however, restricted to calcareous substrates, thus remaing uncommon within that large area. In the North American tradition, it has been placed in its own section; Skvortsov included it in Sect. Villosae. This low to medium-sized shrub can be found at calcareous bogs, fens, and meadows, often forming thickets. Branches and branchlets densely woolly. Leaves narrowly elliptic, with revolute margin, entire or undulate, densely dull woolly on underside (abaxially), dark green or brownish adaxially, with dull white, floccose hairs. Specimens with glabrescent leaves (named S. candida forma denudata) may be of hybrid origin). Stipules foliaceous (on cataphylls either foliaceous or minute rudiments [43]). Catkins coetaneous (flowering as or just before leaves emerge), densely flowered. Ovaries tomentose or woolly; styles 0.3-1.9 mm long. — [all gallery photos]
Leaf pubescence not as above, never woolly.
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8. Low shrubs with slender shoots up to 1.5 (1.7) mm in diameter [44], [45]. Leaves on short (2-8 mm) petioles, relatively small, broad, usually entire, with veins not prominent beneath, somewhat glaucous (with bluish tint), at least mature leaves glabrous; stipules absent or sometimes rudimentary [46], [47], [48]. Catkins late (serotinous), on leafy branchlets [49]. Floral bracts in staminate catkins pale, with rose tint [50]. Ovaries long stipitate, glabrous, often glaucous, frequently reddish [51], [52]; styles and stigmas short [53]. Floriferous buds ovate to lanceloate, faintly pointed, not at all or slightly compressed, up to 8 (rarely 10) mm long [54]; unfolding leaves (primordia) longer than catkin [55] (not as broad as in section Salix or S. serissima).
Sect. Myrtilloides: S. pedicellaris — bog willow
S. pedicellaris is the only representative of sect. Myrtilloides as well as the entire subgenus Chamaetia in southern New England. The species ranges across nearly all of northern North America, absent only from Alaska. A northern plant in Massachusetts, S. pedicellaris occurs here in fens and on open sphagnum and forested black spruce bogs, frequently producing a decumbent habit (having prostrate branches with ascending tips) and featuring leathery, glabrous leaves that are glaucous on both sides, loosely flowered catkins, and reddish, glabrous, often glaucous ovaries on long (2.1-3.2 mm) stipes. — [all gallery photos]
Characters not as above; either ovaries/capsules hairy, or stipules well developed and leaves densely denticulate, or else leaves with pronounced pubescence.
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9. Ovaries and capsules on distinct stipes, acute at apex, glabrous [56], [57]. Stipules usually present (at least on vigorous shoots), well developed, frequently equilateral or subequilateral [58], [59], [60]. Mature leaves almost invariably glabrous or glabrate, green or glaucous beneath; commonly cordate at base but sometimes cuneate; venation distinct, yet not forming reticulum [61]. Inner membrane of bud scale completely free ("loose"), falling off as translucent "additional cap" when shoot starts to grow [62], [63], [64].
Sect. Cordatae = Hastatae in part: S. eriocephala — heartleaf willow
A largely wetland shrub (0.2-6.0 m tall) [65], not necessarily alluvial (water may be running or stagnate), occuring on gravelly or rocky river and stream banks, in mixed mesophytic woods on alluvium, at pond margins, in marshy fields and wet thickets; also at roadsides, especially if there is a ditch along a road. S. eriocephala is one of the most common and easily recognized willows in New England. In spring, immature leaves pinkish or yellowish, translucent [66], [67]; in summer, large leaves on upper (distal) parts of vigorous shoots form characteristic regular "ladder-like" pattern [68], [69]. More about this species: html [70] — [all gallery photos]
Characters not as above. Ovaries and capsules hairy; stipules, if present, of different shape, or else leaves with different type of dentation, shape, or venation.
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10. Leaves narrow, lanceolate or oblong, serrulate or serrate; glabrous or silky sericeous beneath, veins of third order never prominent, i.e., never forming reticulate pattern. Floriferous buds not very large.
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Leaves different: if narrow, then with maximal width above middle, crenate or entire at margin; pubescence, if present, different; veins of third order prominent, reticulate pattern pronounced beneath (except for S. discolor, but in this species leaves crenate, floriferous buds very large, strikingly different from vegetative).
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Sect. Vetrix in the broad sense (Skvortsov 1999). A very large section that can be divided in a few subsections; contains at least 30 species widely distributed across the boreal forest and the temperate belt; at the same time, entirely missing from subtropical and arid-climate areas. The section is much more diverse in the Old World; only a few species are native to North America. Members of the section are small to medium-sized trees or typically rather large shrubs. Wood under bark usually with abundant longitudinal ridges (striae). Floriferous and vegetative buds in many species extremely different; bud apices typically recurved. Leaves mostly broad, entire or coarsely and irregularly dentate, leaf veins usually conspicuously prominent beneath; stipules mostly pronounced, inequilateral; petioles convex above. Catkins precocious or subprecocious. Nectary solitary, short. Ovaries stipitate; stipes may elongate when capsules ripen. Styles mostly short (seldom more than 0.5-0.6 mm long); stigmas nearly as long as styles.
11. Branchlets brittle at base. Pistillate catkins very dense, with very short stipes, ovary ovate, abruptly tapering to short/indistinct style [71]. Leaf blades beneath densely short-silky, with age becoming glabrate, very rarely completely glabrous; hairs white (rarely some rusty hairs present); leaves on average broader than in S. petiolaris (up to 2.5 cm broad, length/width ratio mostly 4-6) [72].
Sect. Griseae: S. sericea — silky willow
See: [html] — [all gallery photos]
Branchlets flexible at base. Pistillate catkins loosely flowered, stipes long (1.5-4 mm); ovary pyriform, gradually tapering to distinct style [73], [74]. Leaf blades 5-9 times as long as wide, more or less glabrous or silky sericeous when young [75], with white and usually at least some rusty (ferrugineous) hairs [76], [77].
Sect. Geyerianae: S. petiolaris — meadow willow
See: [html] — [all gallery photos]
12. Floriferous buds not essentially different from vegetative ones: elongate, lanceolate, flattened, ribbed laterally, attenuate into slightly recurved beak [78], [79], [80]. Flowering subprecocious (nearly simultaneous with leaf development) [81]. Capsules narrowly lanceolate or sublinear, stipitate; stipes considerably elongating after flowering [82], [83], [84], [85], [86]. Leaves elliptic, rarely oblanceolate, usually conspicuously pubescent beneath; veins of third order prominent, forming distinct reticulate pattern on blade surface beneath and impressed above (leaves rugose above) [87], [88], [89], [90], [91], [92]. Shrubs or small trees with short trunks [93], [94]. Wood usually with short, scattered ridges (striae). Bark on old branches and trunks with characteristic "woven" pattern [95], [96].
Sect. Fulvae = Vetrix subsect. Substriatae: S. bebbiana — Bebb's willow
Section Vetrix subsect. Substriatae, according to Skvortsov, includes at least 5 species, all but one from the Old World. S. bebbiana is that species with a unique range. This circumboreal willow occurs across North America and nearly all of Eurasia. — [all gallery photos]
Floriferous buds large, drastically different from vegetative ones; groups of floriferous and vegetative buds alternating (following in turn) along shoots. Flowering precocious (before leaves emerge). Pistillate catkins dense, stipes never elongate after flowering to such extent as in S. bebbiana. Leaf shape different from that in S. bebbiana, venation less prominent; bark different.
Sect. Cinerella = Vetrix (in part) [13]
All species included here, except for S. discolor, would run to subsect. Laeves of sect. Vetrix as delineated by Skvortsov: "Trees or shrubs, usually large [the American S. humilis and especially S. occidentalis are exceptions as far as their size]; leaves mostly large, with prominent reticulation beneath, clothed with deviating or rumpled trichomes. Catkins precocious, mostly large, densely pubescent, usually with large black bracts. Capsules rather short-pubescent, their stipes not very much elongating on flowering."
13. Leaf veins of third order not prominent beneath, reticulate pattern absent [97], [98], [99], [100]; upper leaf surface shiny, without impressed veins [101], [102], [103], [104]. Floriferous buds very large, with beak parallel to shoot, blackening in winter [105], [106], [107], [108]. Catkins very large and initially very dense, then becoming slightly more lax [109], [110], [111]. Styles and stigmas longer than in other species of the section [112], [113].
S. discolor — pussy willow
The species has a very broad range across North America, occuring at marshy margins of ponds, in alluvial woods, fens, frequently on peaty substrates. A tall shrub (2-8 m), which may grow as a tree only as rare exception. Peeled wood smooth or striate. Stipules on mature leaves typically foliaceous. Leaves vary from narrowly elliptic to oblanceolate or obovate, 2.3 to 4.5 times as long as broad; blade base convex or cuneate, margin flat, crenate, irregularly toothed, sinuate, or entire; lower (abaxial) surface glaucous, glabrous, pilose, sparsely pubescent or long-silky; hairs (if present) white and/or rusty (ferrugineous). May hybridize with S. humilis in areas where they grow in proximity (ecology of these two species is different: S. discolor occurs in wetlands, S. humilis in comparatively dry sandy upland forests). Hybrids have been named S. x conifera and characterized by tomentose leaves (like those in S. humilis) and large long catkins and long styles (like those in S. discolor). These hybrids are fertile; however, "large [hybrid] swarms have not been observed" (Argus 2010). — [all gallery photos]
Leaves conspicuously reticulate beneath (veins of third order distinct). Floriferous buds of different shape, not blackening in winter. Catkins smaller, styles and stigmas short.
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14. Native shrubs of mostly sandy substrates, medium-sized to low, with numerous successive floriferous buds in upper (distal) parts of shoots uninterrupted by vegetative buds. Floriferous buds differ from vegetative ones, though not as drastically as in preceding species [114]; staminate and pistillate catkins frequently arching, unless very few flowered [115], [116].
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Introduced and/or cultivated trees and shrubs. Combination of characters not as above. Floriferous buds extremely large, drastically different from vegetative ones; groups of floriferous and vegetative buds alternating (following in turn) along shoots.
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15. Low shrub (to 1 m) with bluish foliage [117], [118], [119], [120] of open sandy habitats, such as treeless frost pockets in pine woodlands or sandplain grasslands; sometimes at roadsides [121], [122], [123]. Branched near ground, producing thin, nearly parallel twigs (a habit similar to that of Cytisus). Buds small and broad [124]. Leaves elongate, with revolute margins; leaf shape only insignificantly changing along shoots [125]; stipules usually absent or rudimentary, sometimes developed on vigorous plants (narrow, subequilateral). Catkins, especially staminate ones, few flowered [126], [127], [128], [129].
S. occidentalis (S. humilis var. tristis) — dwarf upland willow
Taller, medium-sized shrub (1 to 3 m) tolerating some shade. Foliage green without bluish tint [130], [131]. Branching pattern unremarkable. Buds usually elongate [132], [133], [134]. Leaf shape changes gradually but drastically along shoots: lower (proximal) leaves broad elliptic, broadest near blade middle; upper (distal) ones narrow (relatively to their length), broadest near apex [135], [136]; stipules usually present, typically equilateral, more rarely subequilateral [137]; catkins flowered more profusely than in previous species [138], [139], [140], [141].
S. humilis — upland willow
Produces hybrids with non-native invasive willows (S. atrocinerea and probably S. cinerea). — [all gallery photos]
16. Pubescence on leaf underside of rumpled hairs, conspicuously denser on veins than in-between, typically with some rusty (ferrugineous) hairs—hence the common name 'rusty willow.' According to Skvortsov (1999), leaves frequently absolutely glabrous, but such specimens are rare, if at all present, in eastern MA. Upper leaf surface bright green, shiny; leaf shape variable, with some leaves broadest at about blade middle, others above middle [142], [143]. Single-trunk trees to 10-15 m tall (if not damaged) or tall shrubs [144], [145], [146], [147]. Bark on branches and limbs smooth, without cracks (except for senescent specimens), fluted, looking similar to that in Carpinus caroliniana; sometimes pitted [148], [149]. Peeled wood with long dense ridges (striae) [150], [151]. Floriferous buds "clog-shaped" (with distinct, recurved beaks), glabrous, in autumn and winter usually red [152], [153], [154]. Branchlets and young branches slenderer than in gray willow, glabrous, frequently with reddish coloration.
S. atrocinerea (S. cinerea ssp. oleifolia) — rusty willow
Native to Atlantic Europe and northern Africa. Together with S. cinerea (the two wollows have been sometimes considered subspecies of a single species), it has been deemed invasive in Massachusetts. — [all gallery photos]
Leaf pubescence on veins of second and third order not more dense than between veins, uniform across leaf surface; rusty hairs absent; upper leaf surface not bright green, grayish or somewhat bluish (not yellowish). Branchlets and young branches more stout than in previous species.
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17. A large (to 5 m) spreading shrub that never grows as a tree. Similar to S. atrocinerea in having smooth, light gray, fluted bark, wood with long dense ridges (striae), and clog-shaped floriferous buds. Mature leaves dull green to ash gray above; leaves broadest near apex, gradually attenuating to base. Branches and buds gray, coated with dense persistent pubescence [155], [156], [157], [158].
S. cinerea — gray or ash willow
Native to West Siberia and most of Europe except the Atlantic coast. — [all gallery photos]
Plant typically grows as a tree [159], [160]. Peeled wood with long dense ridges (striae), yet bark not at all or only indistinctly fluted [161]. Floriferous buds in plants introduced to North America not clog-shaped, with indistinct beak parallel to branchlet [162], [163], similar to floriferous buds in S. discolor, but not blackening in winter, with a free inner bud scale membrane. Leaves broad (20-50 mm, 1.25-3.5 times as long as broad), medial and upper (distal) leaves on shoots mostly broadest at about blade middle [164]. All leaves pubescent beneath, at least along midribs (rarely completely glabrous); hairs white or grayish, deviating off leaf blade surface, pubescence on veins of second and third order, if any, not more dense than between veins [165].
S. aegyptiaca (S. medemii) — Egyptian willow
A willow originating from West Asia and widely cultivated. (Plants from Egypt, after which the species was named, represent cultivated material.) Due to tree habit and broad hairy leaves, S. aegyptiaca resembles the European goat willow S. caprea; however, unlike S. caprea, it is easily propagated from cuttings and thus has been favored for cultivation (Skvortsov 1999: 178). According to Rehder (1954), it has been in cultivation in the United States since 1888 (under the name S. medemii). As its naturalization is not yet positively demonstrated, for the time being we prefer to treat any findings as waifs. All examined herbarium samples that we could confidently identify as S. aegyptiaca as well as living plants found in Berkshire County might represent planted willows. Plants found in natural settings, deviating from S. atrocinerea or S. cinerea by somewhat bluish foliage, more densely pubescent leaves, and buds without recurved beak may belong to this species. Alternatively, these plants may constitute hybrids whose parents not necessarily include S. aegyptiaca. — [all gallery photos]

Sources and References

Argus G.W. 1986. The genus Salix (Salicaceae) in the Southeastern United States. Systematic Botany Monographs 9. 170 pp.

Argus G.W. 2006. Guide to the Identification of Salix (Willow) in Illinois, Indiana, Ohio, and Pennsylvania. https://www.researchgate.net/publication/242148532_Guide_to_the_Identification_of_Salix_Willow_in_Illinois_Indiana_Ohio_and_Pennsylvania

Argus G.W. 2010. Salix L. in Flora of North America North of Mexico, Volume 7. Magnoliophyta: Salicaceae to Brassicaceae, pages 23-162. Eds. Flora of North America Editorial Committee. Oxford and New York: Oxford University Press. Online version: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=129059

Belyaeva I.V. 2009. Nomenclature of Salix fragilis and a new species. Taxon 58: 1344-1348.

Rehder A. 1954. Manual of cultivated trees and shrubs hardy in North America. The Macmillan Company, New York, 996 pp.

Skvortsov A.K. 1955. The willows of Central European Russia and their identification during the wintertime. Translation: http://salicicola.com/translations/Skvortsov1955.html

Skvortsov A.K. 1960 (English Translation 2014). Bay-leaved willow (Salix pentandra L.) and related species: Taxonomic and geographic overview. Skvortsovia: 1(2): 187-206. PDF available on line: http://skvortsovia.uran.ru/2014/1206.pdf

Skvortsov A.K. 1968 (English Translation 1999). Willows of Russia and adjacent countries. Taxonomical and geographical revision. Univ. Joensuu, Faculty of Mathematics and Natural Sciences Report Ser. 39. Joensuu, Finland. Original title: Ivy SSSR. Sistematicheskiy i geograficheskiy obzor [Willows of the USSR. Taxonomic and geographic revision.] Nauka, Moscow. 262 pp. PDF available on line: http://www.salicicola.com/announcements/skv/skvortsov.pdf

Skvortsov A.K. 1973. Present distribution and probable primary range of brittle willow (Salix fragilis L.). Translation: http://salicicola.com/translations/Skv1973SF.html [Note: The plant called in this article 'the true S. fragilis' has been newly described under the name 'S. euxina' (see Belyaeva 2009).]

Zinovjev A.G. 2010. Identification of Salix lucida Muhl. http://www.salicicola.com/notes/salix_lucida/

Zinovjev A.G. 2011. Salix × meyeriana (= Salix pentandra × S. euxina) — a forgotten willow in Eastern North America. Phytotaxa 22: 57-60. PDF available on line: http://www.mapress.com/phytotaxa/content/2011/f/pt00022p060.pdf

Zinovjev A., Kadis I. 2009. Salix atrocinerea and related willows in eastern Massachusetts. 34 pp.


Draft: 2016-04-26T02:47:36-07:00 (I. Kadis & A. Zinovjev)