Salix fragilis (crack willow)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Biology and Ecology
- Latitude/Altitude Ranges
- Air Temperature
- Rainfall Regime
- Soil Tolerances
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Impact Summary
- Environmental Impact
- Impact: Biodiversity
- Social Impact
- Risk and Impact Factors
- Uses List
- Wood Products
- Prevention and Control
- Links to Websites
- Distribution Maps
Don't need the entire report?
Generate a print friendly version containing only the sections you need.Generate report
PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Salix fragilis L.
Preferred Common Name
- crack willow
- Salix fragilis f. bullata (Rehder) Rehder
- Salix fragilis var. decipiens (Hoffm.) W. D. J. Koch
International Common Names
- English: white Welsh willow
- Spanish: bardaguera blanca; sauce frágil
- French: saule fragile
- Portuguese: salgeiro-fragil
Local Common Names
- Germany: Bruchweide; Knackweide
- Italy: salice fragile
- Netherlands: kraakwilg
- Slovakia: vrba krenka
- Slovenia: krhka vrba
- Sweden: knäckerpil; skörpil
- SAXFR (Salix fragilis)
Summary of InvasivenessTop of page S. fragilis is an adaptable tree with a capacity to disperse over wide areas along watercourses by vegetative reproduction. The consequences of invasion include severe economic, environmental, biodiversity and social impacts. It has a history of invasiveness in a number of countries including South Africa, New Zealand and Australia.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Salicales
- Family: Salicaceae
- Genus: Salix
- Species: Salix fragilis
Notes on Taxonomy and NomenclatureTop of page S. fragilis is placed in subgenus Salix, section Salix, along with S. alba and S. babylonica (Newsholme, 1992). The specific epithet 'fragilis' means fragile, relating to the brittle, easily broken branches, as does the English common name, 'crack willow'.
According to Jalas and Suominen (1976), the area of original distribution of S. fragilis is much smaller than that reported in distribution maps, because a large number of plants identified as S. fragilis are hybrids, for example S. rubens (a hybrid with S. alba). Other hybrids exist, cultivated as ornamentals, such as S. x tinctoria (S. fragilis x S. pentandra), S. x alopecuroides (S. fragilis x S. triandra) and others that are not as common (Meikle, 1978).
Skvortsov (1999) mentions S. fragilis var. sphaerica, interesting as a ornamental species, with a dense spherical crown and a short trunk, common since the 18th century in the Baltic countries and still popular in Lithuania. Newsholme (1992) notes several varieties and hybrids; apart from S. fragilis var. fragilis, the crack willow, he lists var. furcata (S. 'Latungensis') with brighter brown, glossy branchlets and coarsely serrate leaves, sporadic throughout Scotland, England, also recorded in Belgium and Switzerland; var. russelliana (Bedford willow), larger overall than S. fragilis var. fragilis, and the most frequently planted variety in the UK and Ireland; S. fragilis var. decipiens (white Welsh willow) - only male trees occur in the UK, both sexes have been found in Germany and Austria; and S. fragilis var. bullata, a compact many-branched shrub rarely exceeding 4 m tall. Weber (2003) notes a hybrid formed with S. alba in the USA.
DescriptionTop of page S. fragilis is a robust tree, usually 10-15 m high, with a short, thick trunk, bark greyish, coarsely and deeply fissured when adult; branches spread to form a broad, rounded crown; twigs soon glabrous and rather lustrous, olive-brown, easily brittle at the place of attachment to the branch. Leaves lanceolate, long acuminate (9-15 cm long, 1.5-3 cm wide) dark shining green above and glaucous below. Catkins appear with the leaves in April and May, terminals on short, lateral shoots. Capsule 4-5 mm long, 2.5 mm wide rather ovoid. Male flower with two stamens (Meikle, 1984; Newsholme, 1992). The distinctive catkins appear with new leaves in the spring.
Plant TypeTop of page Broadleaved
DistributionTop of page S. fragilis is a tree with a Central European range, extending from western Europe (Spain and France) to the Balkans and Greece, relatively rare in the Mediterranean area (Sardinia and Corsica), to the northern areas of Asia Minor and Caucasus (Georgia); it is present in southern Scandinavia only (Jalas and Suominen, 1976; Skvortsov, 1999). However, according to Jalas and Suominen (1976) the area of original distribution of the species is much smaller than that reported in distribution maps, because a large number of plants identified as S. fragilis are hybrids, for example, S. rubens (a hybrid with S. alba).
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Planted||Reference||Notes|
|Georgia (Republic of)||Present||Native||Natural|
|South Africa||Present||Introduced||Invasive||Planted||Henderson , 2001; Henderson, 2001|
|South Africa||Present||Introduced||Invasive||Planted||Henderson , 2001; Henderson, 2001|
|Canada||Present||Present based on regional distribution.|
|-Ontario||Present||Introduced||Invasive||Planted||Urban Forest Associates Inc., 2002|
|-Ohio||Present||Introduced||Invasive||Cleveland Museum of Natural History, 2002|
|Russian Federation||Present||Present based on regional distribution.|
|-New South Wales||Present||Introduced||Invasive||Planted||Anon., 2000|
|-South Australia||Present||Introduced||Invasive||Planted||Anon., 2000|
|New Zealand||Present||Introduced||1880||Invasive||Holm and et al. , 1979; Cronk and Fuller , 1995; Owen , 1996|
History of Introduction and SpreadTop of page Where introduced, S. fragilis is an invader of river corridors, lakesides and wetlands (Weber, 2003). In Australia, S. fragilis is only associated with streamside habitats because the vegetative reproduction method requires twigs to be dispersed downstream (Anon., 2000). In was introduced into eastern states of the USA in colonial times (Newsholme, 1992).
Binggeli (1999) classifies S. fragilis as a moderately invasive woody species. In South Africa, it is declared a category 2 invader, capable of transforming habitat according to the Conservation of Agricultural Resources Act, 1983 (Henderson, 2001). S. fragilis is also demonstrating invasive behaviour in Botswana (Buss, 2002). The tree was introduced to New Zealand in 1880 and is now one of many weeds of concern on conservation lands (Owen, 1996). In Australia the planting of Salix species began soon after Europeans arrived but occurred on a large-scale from the 1950s to the 1970s and has only been discontinued relatively recently. S. fragilis is one of the most serious and widespread naturalized and invasive exotic willows in Australia, where it occurs in the states of Victoria, New South Wales, South Australia and Tasmania (Anon., 2000). Many hybrids of S. fragilis have also become naturalized in Australia, and S. fragilis is one of a number of Salix species designated Weeds of National Significance in Australia (Anon., 2000). The sale or cultivation of most willows is prohibited in New South Wales and South Australia (Anon., 2000). In Canada, S. fragilis is reported to be moderately invasive in wetlands where it can displace native Salix species in southern Ontario (Urban Forest Associates, 2002) and in the USA it is considered invasive in Cleveland, Ohio (Cleveland Museum of Natural History, 2002).
Risk of IntroductionTop of page S. fragilis is one of a number of willow species that has caused particular problems in Australia, exacerbated by a previous history of widespread planting. It has the capacity to disperse along river systems and the consequences of potential invasion should be evaluated prior to introduction in climate zones where it could naturalize.
HabitatTop of page S. fragilis is well adapted to a montane-submediterranean climate, colonizing riparian habitats, as does S. alba and hybrids between this and S. fragilis (Beismann et al., 1997). It is found along river valleys and mountain creeks, lakes, and wet soils between cultivated fields. It is found along ditches, channels and roads, where it can easily spread by means of wind-broken twigs and branches and root suckers (Martini and Paiero, 1988). It is widespread in cultivated landscapes, and is usually missing from undisturbed habitats (Skvortsov, 1999).
Habitat ListTop of page
|Terrestrial ‑ Natural / Semi-natural||Riverbanks||Present, no further details||Harmful (pest or invasive)|
|Wetlands||Present, no further details||Harmful (pest or invasive)|
Biology and EcologyTop of page Genetics
There is a wide variation in this species, evident from the large numbers of intraspecific and interspecific hybrids known (see Notes on Taxonomy and Nomenclature).
Physiology and Phenology
Catkins appear with the leaves in spring (Hillier Nurseries Ltd., 1994). S. fragilis establishes easily via branch cuttings struck into moist ground. Harvesting for fuelwood involves coppicing or lopping when the wood is required, often during the summer when the ground is not too wet to restrict access.
S. fragilis has a dioecious flowering system (Weber, 2003). Almost all reproduction is asexual and twigs and small branches, which are very prone to damage in the wind, are able to float to new locations where they form new plants (Agricultural & Resource Management Council of Australia & New Zealand, Australian & New Zealand Environment & Conservation Council and Forestry Ministers, 2000; Weber, 2003). Weber (2003) reports that the majority of individuals in Australia are male clones.
S. fragilis is a temperate species preferring moist habitats. It is found where annual rainfall is 500-2000 mm, but with no more than a 2-month dry season. It can tolerate very low temperatures, down to -25°C. S. fragilis is found almost exclusively on riversides, stream banks, pond sides, boggy ground and dry river beds with a permanent water table not far from the soil surface. It prefers lighter soils to heavy clays and acid soils to alkaline, and will tolerate infertile soils.
Latitude/Altitude RangesTop of page
|Latitude North (°N)||Latitude South (°S)||Altitude Lower (m)||Altitude Upper (m)|
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Absolute minimum temperature (ºC)||-25|
|Mean annual temperature (ºC)||5||16|
|Mean maximum temperature of hottest month (ºC)||15||24|
|Mean minimum temperature of coldest month (ºC)||0||10|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Dry season duration||0||2||number of consecutive months with <40 mm rainfall|
|Mean annual rainfall||500||2000||mm; lower/upper limits|
Rainfall RegimeTop of page Bimodal
Soil TolerancesTop of page
- seasonally waterlogged
Special soil tolerances
Notes on Natural EnemiesTop of page In the UK, willow species, including S. fragilis, are host to a very large number of herbivorous insects, and provide a winter host for an aphid (Cavariella aegopodii), which is a serious pest of carrot crops (Savill, 1991). Rust diseases (Melampsora spp.) are also problematic in the UK (Heyer, 1992). Willow beetles (Phratora spp.) can damage foliage (Kendall and Wiltshire, 1996).
Means of Movement and DispersalTop of page S. fragilis twigs and branches that have broken from the parent tree are carried by water downstream where they establish as new trees.
Impact SummaryTop of page
|Fisheries / aquaculture||None|
ImpactTop of page Willow debris can block streams during floods and damage bridges and roads (Anon., 2000). Where large infestations have established the costs of removal and restoration may be high.
Environmental ImpactTop of page Environmental damage from S. fragilis invasion includes changes to stream hydrology, higher erosion and sedimentation rates and flooding patterns, and they may also use more water than indigenous plant species and cause changes to nutrient cycling, water temperature, energy fluxes and general water quality may also result (Anon., 2000).
Impact: BiodiversityTop of page There are negative consequences for biodiversity where S. fragilis becomes invasive since the thick canopy created when it is dominant is sufficient to shade out other plants and reduce invertebrate abundance (Weber, 2003). These changes may also affect fish (Anon, 2000). Flora and fauna associated with both the banks and aquatic environment may be affected.
Social ImpactTop of page Willows such as S. fragilis may impede recreational activities such as canoeing and fishing (Anon., 2000).
Risk and Impact FactorsTop of page Invasiveness
- Proved invasive outside its native range
- Highly adaptable to different environments
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Highly mobile locally
- Has high reproductive potential
- Damaged ecosystem services
- Ecosystem change/ habitat alteration
- Negatively impacts tourism
- Reduced amenity values
- Reduced native biodiversity
- Pest and disease transmission
- Highly likely to be transported internationally deliberately
- Difficult/costly to control
UsesTop of page S. fragilis is considered an important species in hedgerows, shelterbelts and windbreaks, along fields and field channels. It was one of a number of species planted in Australia to counteract erosion caused by land clearance and cattle grazing (Anon., 2000). S. fragilis is not a timber species (wood density is about 450 kg/m³ at 15% moisture content) and has been mainly used for fuelwood. S. fragilis was once an important species for basket-making industries. Although pollen and nectar are provided relatively early in the year, its use as bee forage is limited by its poor nutritional quality (Anon., 2000).
Uses ListTop of page
- Erosion control or dune stabilization
- Shade and shelter
- Soil improvement
- Miscellaneous fuels
Human food and beverage
- Honey/honey flora
- Source of medicine/pharmaceutical
Wood ProductsTop of page
Wood gas (and other hydrocarbons
Prevention and ControlTop of page Grazing is thought to check the spread of willow and in Australia cattle, sheep, goats and possums feed on the trees. It is possible to burn willow in a hot fire and this kills trees but it is not a generally recommended method because fire may also affect native vegetation (Anon., 2000).
Weber (2003) suggests following similar mechanical control methods to those adopted for Salix cinerea, i.e. small individuals may be pulled or dug up, ensuring that the entire root system is extracted. In Australia mechanical removal of root mats and stems is practised where willows interfere with stream flow (Anon., 2000). Weber (2003) also suggests following similar chemical treatments as used on S. cinerea, i.e. herbicides may be applied according to the 'drill and fill' approach with herbicide applied to cut stumps to prevent resprouting with repeat applications if necessary. If removed for biological or other reasons in situtations where there is little risk to structures such as bridges, plants treated with herbicide may be left to decay over time (Anon., 2000).
Although not established at present, biological control techniques are of interest in Australia because there are no native willows in the country and because there are many potential insects and pathogens including the willow sawfly (Nematus oligospilus) that damages plants in New Zealand. The Keith Turnbull Research Institute, Frankston, Australia, have conducted some preliminary investigations into biological control (Anon., 2000).
ReferencesTop of page
Anon., 2000. Weeds of National Significance Willow (Salix taxa, excluding S. babylonica, S. x calodendron and S. x reichardtii) Strategic Plan. National Weeds Strategy Executive Committee, Launceston, New Zealand. Agricultural and Resource Management Council of Australia and New Zealand, Australian and New Zealand Environment and Conservation Council and Forestry Ministers.
Binggeli P, 1999. Invasive woody plants. http://members.lycos.co.uk/WoodyPlantEcology/invasive/index.html.
Buss CM, 2002. The potential threat of invasive tree species in Botswana. Department of Crop Production and Forestry, Ministry of Agriculture, Government of Botswana, 40 pp.
Cleveland Museum of Natural History, 2002. Invasive, non-native vs. native plants of the Cleveland region. http://www.cmnh.org/collections/botany/documents/Invasive_Plants_of_Northeast_Ohio.html.
Cronk QCB; Fuller JL, 1995. Plant invaders: the threat to natural ecosystems. London, UK: Chapman & Hall.
Henderson L, 2001. Alien Weeds and Invasive Plants. Plant Protection Research Institute Handbook No. 12. Cape Town, South Africa: Paarl Printers.
Heyer S, 1992. The rust diseases of willows in Britain. Proceedings of the Royal Society of Edinburgh, 98B, 119-134.
Holm L; Pancho JV; Herberger JP; Plucknett DL, 1979. A Geographical Atlas of World Weeds. Toronto, Canada: John Wiley and Sons Inc.
Jalas J; Suominen J, 1976. Atlas Florae Europeae. Distribution of vascular plants in Europe. Vol. 3. Salicaceae to Balanophoraceae. Helsinki, Finland.
Kendall DA; Wiltshire CW, 1996. An applied study of clonal resistance to willow beetle attack in SRC willows. ETSU [Energy Technology Support Unit (UK)] Biomass Study B/M4/00532/27. 38 pp.
Martini F; Paiero P, 1988. I salici d'Italia. Trieste, Italy: LINT.
Meikle RD, 1978. Salix. In: Stace CE, ed Hybridization and the flora of the British Isles. London, UK: Academic Press.
Meikle RD, 1984. Willows and poplars of Great Britain and Ireland. London, UK: BSDI.
Owen SJ, 1996. Ecological weeds on conservation land in New Zealand: A database. Department of Conservation, Wellington, New Zealand: DOC Science Publications. http://www.hear.org/weedlists/other_areas/nz/nzecoweeds.htm.
Rechinger KH, 1964. Salix L. In: Tutin TG, Heywood VH, Burges NA, Valentine DH, Walters SM, Webb DA, eds Flora Europaea, Volume 1. Cambridge, UK: Cambridge University Press.
Rechinger KH, 1964. Salix L. In: Tutin TG, Heywood VH, Burges NA, Valentine DH, Walters SM, Webb DA, eds. Flora Europaea, Volume 1. Cambridge, UK: Cambridge University Press.
Rice PM, 1997. INVADERS Database System (http://invader.dbs.umt.edu). Missoula, Montana, USA: Division of Biological Sciences, University of Montana.
Skvortsov AK, 1999. Willows of Russia and adjacent countries. Taxonomical and geographical revision. Report Series No. 39. 307 pp. Joensuu, Finland: Faculty of Mathematics and Natural Sciences, University of Joensuu.
Skvortsov AK, 1999. Willows of Russia and adjacent countries. Taxonomical and geographical revision. Report Series No. 39. Joensuu, Finland: Faculty of Mathematics and Natural Sciences, University of Joensuu. 307pp.
Tutin TG; Burges NA; Chater AO; Edmonson JR; Heywood VH; Moore DM; Valentine DH; Walters SM; Webb DA, 1993. Flora Europaea. Volume 1. 2nd edition. Cambridge, UK: Cambridge University Press. World Wide Web page at http://www.rbge.org.uk/forms/fe.html.
Urban Forest Associates, 2002. Invasive exotic species ranking for Southern Ontario. Urban Forest Associates Inc., Canada. http://www.serontario.org/pdfs/exotics.pdf.
Distribution MapsTop of page
Unsupported Web Browser:
One or more of the features that are needed to show you the maps functionality are not available in the web browser that you are using.
Please consider upgrading your browser to the latest version or installing a new browser.
More information about modern web browsers can be found at http://browsehappy.com/