Description: Trees or shrubs, winter buds with a single, non-sticky outer scale.
Leaves mostly deciduous, mostly linear to elliptic to oblong, margins mostly toothed; petiole less than a quarter as long as lamina.
Floral bracts entire or finely toothed; flowers with 1 or 2 tongue-like glands or rarely a small cup-like gland. Stamens 1–15, filaments free or fused. Ovary mostly 2-carpellate, ovules 4–many, stigma entire or 2-lobed, sessile or on a slender style.
Capsule 2-valved; seeds with silky hairs.
Distribution and occurrence: World: c. 400 spp., mainly temperate & subarctic regions of N. Hemisphere, a few species extending into the tropical & S. Hemisphere. Australia: 12–14 species and several hybrids (some named) naturalised, and many more cultivated.
Most species were introduced into Australia as potential economic plants, for timber, basketry or soil stabilisation, or as ornamentals. As willow seed is very short-lived, all of the early introductions of willows were made as cuttings or pot plants. This meant that most of the plants of any species, or at least the plantings at any one site, were derived from one or very few individuals. As plants are mostly either male or female this has meant that most of the clones were unisexual and plantings rarely produced seed or seedlings. Seed was only possible from hybridisation of two different species where the correct sexes were present, the flowering times overlapped, and the trees were less than 500 m apart (about the maximum effective distance for the pollinating insects). The sparsity of overlap of flowering times for the different species has meant that seeds were rarely formed. Over the years more species and cultivars have been introduced. Some of these cultivars were hybrids between other species, often very rare hybrids. If such hybrids are planted, and either or both parent species are around, they can overcome the problems of non-overlapping flowering times, and also reduce the inter-species fertility barriers. Simply increasing plantings has allowed a greater opportunity for trees to cross pollinate. With both types of increased plantings there is now an opportunity for fertile and viable seed to be formed. Also, as the trees aged, the longer they tended to flower and the more likely that overlaps occurred. Air travel has also meant viable seed could be imported, producing mixed-sex populations of some species. All such situations have occurred and there are now areas where there has been mass colonisation by seedling wilows.
Identification: For positive identification of species, hybrids and cultivars, it may be necessary to collect from the same tree when flowering in spring and in full leaf in summer. The leaves should be collected from branch systems that have borne flowers the current season and not from the vigorous sterile shoots arsing from near the base.
The ridges in key lead 17, are the ridges that are present on the wood underneath the bark (strip bark to see).
Key based on Meikle, R.D. (1984); Carr G.W. (1996); and Cremer, K.W. (1995) and parts of the original treatment in Flora of N.S.W. (1990).
Text by S.W.L. Jacobs & L. Murray (2000)
Taxon concept: Flora of NSW 1 Suppl. (2000)
| ||Key to the species|| |
|1||Trees narrow-columnar mostly evergreen (usually 30% of leaves retained over winter); male clone, usually only a few catkins produced with little pollen||Salix humboldtiana|
|Trees or shrubs, not columnar; always completely deciduous; male and/or female clones; male catkins usually producing ample fertile pollen||2|
|2||Trees; with single or several trunks, 10–20 m high; leaves emerging with catkins; flower scales pale green or yellow; leaves mainly lanceolate||3|
|Multi-stemmed shrubs or trees with 5–50 or more stems, to 4–9 m high; leaves emerge long after catkins (except Salix purpurea); flower scales dark; leaf shape variable|
Back to 1
|3||Branches and leaves contorted||Salix matsudana|
|Branches and leaves not contorted|
Back to 2
|4||Penultimate branches weeping (ultimate branches are short lateral branches on weeping branches)||5|
|No branches particularly weeping, although some may droop|
Back to 3
|5||Bark of twigs and branches very golden, both sexes present, sometimes on the same catkin||Salix x sepulcralis|
|Bark of twigs and branches not markedly golden; male or female|
Back to 4
|6||Catkins 15–20 mm long; catkin stalks 2–3 mm long; ovary pear shaped; mostly female||Salix babylonica|
|Catkins >20 mm long; catkin stalks >3 mm long; ovary more slender, flask-shaped, gradually tapering towards the top; male or female|
Back to 5
|7||Leaves ± hairy; ovary not much larger than the flower scale; Salix x Salix alba; both sexes present||Salix x sepulcralis|
|Leaves ± hairless; ovary far longer than flower scale; Salix babylonica x Salix fragilis; male or female|
Back to 6
|Salix x pendulina|
|8||Branches yellow to orange for more than 1 m from tips; may weep in lower crown||Salix alba|
|Branches not yellow, not weeping|
Back to 4
|9||Young twigs and branchlets exposed to full light usually a dark reddish brown; catkins of both sexes with flowers not crowded on the rhachis||Salix nigra|
|Young twigs and branchlets greenish even when exposed to full light; usually only catkins of one sex present in a population (unless established from seed), or rarely bisexual catkins, and the flowers crowded on the rhachis of the catkin|
Back to 8
|10||Shoots and leaves sparsely pubescent at first, becoming glabrescent||11|
|Hairs on shoots, twigs and leaves ± persistent|
Back to 9
|11||Twigs break easily with a audible crack; fairly stout and rigid||Salix x fragilis|
|Twigs flexible (do not break readily); slender|
Back to 10
|12||Single prominent stem with a narrow crown when young; catkins usually <5 mm wide and <5 cm long; often cultivated as orchard windbreaks but hybridizes with other species to form seedling arms||Salix matsudana x alba|
|One or more prominent stems with a ± rounded crown; catkins often >5 mm wide and/or >5 cm long; usually growing as seedling swarms from crossing between Salix alba and Salix x fragilis, rarely cultivated|
Back to 11
|Salix x rubens|
|Leaves with hairs|
Back to 2
|14||Leaves and catkins ± opposite||Salix purpurea|
|Leaves and catkins alternate|
Back to 13
|15||Leaves with recurved margins and no apparent toothing||16|
|Leaves not recurved and with margins ± toothed|
Back to 13
|16||Leaves linear or linear-lanceolate, the margins almost parallel to one another||Salix viminalis|
|Leaves lanceolate to ovate-lanceolate, the margins distinctly convex, not parallel|
Back to 15
|Salix x sericans|
|17||Ridges on 1 cm diam. branches strong (typically 1x1x10–40 mm); crown tends to spread from an early age; lower surface of leaf may have some rust coloured hairs||Salix cinerea|
|Ridges weak (0.5x0.5x5–10 mm); crown erect, except when old; no rust-coloured hairs|
Back to 15
|18||Male only; catkins ovoid, 20–35 mm long, spreading; eaves 50–100 mm long, ovoid; widely planted for ornament||Salix x reichardtii|
|Female only (sterile); catkins cylindrical, 30–60 mm long, narrow-ovoid; planted for erosion control|
Back to 17
|Salix x calodendron|