Salix × fragilis is a medium-sized to large deciduoustree, which grows rapidly to 10–20 m (33–66 ft) (rarely to 29 m (95 ft)) tall, with a trunk up to 1 m (3.3 ft) diameter, often multi-trunked, and an irregular, often leaning crown. The bark is dark grey-brown, coarsely fissured in older trees. The lanceolateleaves are bright green, 9–15 cm long and 1.5–3 cm wide, with a finely serrated margin; they are very finely hairy at first in spring, but soon become hairless.[4]
The flowers are produced in catkins in early spring, and pollinated by insects. They are dioecious, with male and female catkins on separate trees; the male catkins are 4–6 cm long, the female catkins are also 4–6 cm long, with the individual flowers having either one or two nectaries.[2][3][6] In late spring fruit capsules release numerous small cotton-tufted seeds. They are easily distributed by wind and moving water, and germinate immediately after soil contact.[4]
Taxonomy
Carl Linnaeus first described a willow species as "Salix fragilis" in 1753.[7] It was later discovered that Linnaeus was actually describing a species he had also called Salix pentandra. At least since the 1920s, botanists applied Linnaeus's name "Salix fragilis" both to a pure species and to its hybrid with Salix alba.[8][9][10] In 2005, it was proposed that "Salix fragilis" should be conserved for the pure species, with the hybrid called "Salix × rubens". The alternative was to conserve "Salix fragilis" for the hybrid, with the pure species requiring a new name.[10] After discussion, the decision was made by the Nomenclature Committee for Vascular Plants in 2009 to reject the first proposal and conserve "Salix fragilis" for the hybrid.[11] Irina V. Belyaeva then described the previously unnamed parent species as Salix euxina,[8][9][12] and designated a lectotype for the hybrid.[8] The lectotype was shown by molecular evidence to be the hybrid between S. alba and S. euxina,[9] the name of which is written as Salix × fragilis to show its hybrid status.[1][9]
Varieties
S. × fragilis is very variable, with forms linking both parents. Some formally named varieties include the following.[1][5] As of August 2021[update], none were accepted at this rank by Plants of the World Online, being treated as synonyms of the species.[1]Clive A. Stace suggested some would be better treated as cultivars.[5]
S. × fragilis var. decipiens W.D.J.Koch (S. × decipiens Hoffm.) – assigned to S. × fragilis by Plants of the World Online[1] and by Belyaeva,[8] but to S.euxina by Stace[5]
S. × fragilis var. furcata Ser. ex Gaudin – male tree with relatively wide leaves; of cultivated origin[5]
S. × fragilis var. fragilis – male and female trees known; leaves with even, well spaced teeth[5]
S. × fragilis var. glauca Spenn.
S. × fragilis var. polyandra Wimm.
S. × fragilis var. rubens (Schrank) P.D.Sell
S. × fragilis var. russelliana (Sm.) W.D.J.Koch – Bedford willow; female tree with long narrow leaves having somewhat uneven teeth; of cultivated origin[5]
Ecology
The plant is commonly called crack willow or brittle willow because it is highly susceptible to wind, ice and snow damage.[4] The name also derives from the twigs which break off very easily and cleanly at the base with an audible crack. Broken twigs and branches can take root readily, enabling the species to colonise new areas as broken twigs fall into waterways and can be carried some distance downstream. It is particularly adept at colonising new riverside sandbanks formed after floods. It also spreads by root suckers, expanding into pure 'groves'.[4]
Cultivation
Salix × fragilis is cultivated as a fast-growing ornamental tree. The cultivar 'Russelliana' (syn. S. × fragilis var. russelliana) is by far the most common clone of crack willow in Great Britain and Ireland, very easily propagated by cuttings. It is a vigorous tree commonly reaching 20–25 m (66–82 ft) tall, with leaves up to 15 cm long. It is a female clone.[2]
^ abcdefgStace, Clive A. (2019). "S. × fragilis". New Flora of the British Isles (4th ed.). Middlewood Green, Suffolk: C & M Floristics. p. 348. ISBN978-1-5272-2630-2.
^Bean, W. J. (1980). Trees and Shrubs Hardy in the British Isles 8th ed., vol. 4. John Murray ISBN0-7195-2428-8.
^ abcdBelyaeva, Irina (2009). "Nomenclature of Salix fragilis L. and a new species, S. euxina (Salicaceae)". Taxon. 58 (4): 1344–1348. doi:10.1002/tax.584021.
^ abcdKuzovkina, Yulia A. & Belyaeva, Irina V. (July 2018). Name change alert: Salix × fragilis L.(PDF). Fact Sheet No 2. University of Connecticut. Retrieved 2021-08-07.
^ abChristensen, Knud I. & Jonsell, Bengt (2005). "(1698) Proposal to conserve the name Salix fragilis with a conserved type (Salicaceae)". Taxon. 54 (2): 555–556. doi:10.2307/25065400. JSTOR25065400.
^Brummitt, R.K. (2009). "Report of the Nomenclature Committee for Vascular Plants: 60". Taxon. 58 (1): 280–292. doi:10.1002/tax.581028. JSTOR27756843.
^Zenni, Rafael D.; Ziller, Sílvia R.; da Rosa, Clarissa A.; Sühs, Rafael B.; Puechagut, Patricia B.; Marterer, Beloni T. P.; Heringer, Gustavo; de Matos, Ana Carolina L.; Bueno, Marina L.; Almeida, Jonathan W.; Silva, Carlos H. T.; Legracie-Júnior, José Renato; Chapla, Tatiani E. (2024). "Invasive non-native species in Brazil: an updated overview". Biological Invasions. 26 (8): 2397–2405. doi:10.1007/s10530-024-03302-9. ISSN1387-3547.
^ abGriffiths, J; Armstrong, H; Innes, R; Terry, J (January 2018). "Can aerial herbicide application control Grey Willow (Salix cinerea L.) and stimulate native plant recovery in New Zealand wetlands?". Ecological Management & Restoration. 19 (1): 49–57. doi:10.1111/emr.12299. ISSN1442-7001.
