Salix cinerea (grey sallow)

S. cinerea (grey sallow) is included in the section Capreae of subgenus Caprisalix (Rechinger, 1964). It is a broadleaved willow, typically a large shrub generally much branched from the base with numerous intercrossing branches. Bark is dark grey, becoming fissured with age. The species forms a broad, rounded crown. Twigs are densely pubescent, remaining so at least until the end of the first year; wood of peeled twigs with long, scattered striae. Leaves very variable, usually ovate or oblanceolate, 2-9 cm long, 1-3 cm wide, dull grey-green and pubescent above or dark green and slightly lustrous. Catkins appearing in advance of the leaves in March and April. Male flowers with 2 free stamens. Capsule up to 10 mm (Meikle, 1984; Newsholme, 1992).S. cinerea is a Eurasian species, common throughout Europe, from the Mediterranean to Scandinavia (including Spain and Portugal, Sicily, Sardinia, Corsica, the Aegean Islands and Turkey). Its range extends eastward to Asia, from the Crimea to the Caucasus, from northern Iran to Siberia, and north of the Caspian and Aral seas to the Chinese border (Jalas and Suominen, 1976; Skvortsov, 1999).It is a useful species in erosion control projects, and to protect marshlands and riverbanks. Because of its early flowering, it is also important for honeybees (Schiechtl, 1996). Hybrids with S. viminalis (S. x calodendron) are grown in the UK, Germany and Denmark to produce biomass as a energy source (McElroy et al., 1983).

Willows reproduce either by seed or vegetatively, typically by broken branches taking root. Both sexes of S. cinerea occur in New Zealand and reproduction is almost exclusively by seed that is capable of very wide dispersal, and the prolific production of light wind dispersed seed is probably an important factor in the invasiveness of this species (Harman, 2004). S. cinerea seed will float on water while it remains attached to its cotton parachute, though that soon falls off unless the water is very still, but it is possible for seedlings to be transported by flooding up to 100 km (Cremer, 1999, 2003). The very light seeds can travel some distance in the wind and it can thus invade areas such as cleared woodland where the soil has been disturbed. However, in Australia, spread of S. cinerea is considered to be mostly by stem sections spreading downstream (Cremer, 2003).  

Long distance dispersal has been due to intentional introduction and planting, largely for riverbank erosion control, and it continues to be promoted as such.

S. cinerea is able to hybridize with other members of subgenus Vetrix.

S. cinerea is a dioecious species, and both male and female flowers are highly attractive to bees, and as such considered to be commonly pollinated by insects such as the introduced European bee (Apis spp.) or native bees (Cremer, 1999) though maybe partly pollinated by wind. Flowering and the production of viable seed may begin from 2-3 years old. Ripe fruits open when dry, and the movement of cottony hairs levers seed out, accelerated by wind. Seed will germinate on and under water and tiny seedlings can survive under water for up to a month but cannot grow until exposed to air (Cremer, 2003).  

  All willows in Australia generally flower in September and October, with fruit maturing a month later (CRC Weed Management, 2003), and S. cinerea as with most other willows will tolerates waterlogging and can sucker. It cannot grow in the shade. It can tolerate strong winds but not maritime exposure. Seedlings grow quickly even in exposed conditions and it provides good shelter for the establishment of woodland plants thus making a good pioneer species and, except in wetter soils it will eventually be out-competed by the other woodland trees.  

S. cinerea is a temperate species and can tolerate hard and persistent frost. Being a riparian species, it can also tolerate a wide variety of rainfall regimes as long as the soil in which is grows is permanently moist or wet. S. cinerea can grow on light (sandy), medium (loamy) and heavy (clay) soils and even heavy clay soil as long as they are regularly moist or wet soil. It can tolerate permanent water logging and poor aeration, and prefers acid and neutral soils with a pH down to 3.5, making it an extremely hardy species (Cremer, 2003).

The cost of willow management is about $2 million per year in Victoria, Australia alone, and control costs in New South Wales and New Zealand may also be expected to be equally high. However, Salix spp. play a vital role in flood prevention and erosion control, and there are no known suitable alternatives (Harman, 2004).  

In Australia, the spread of S. cinerea has negative effects on biodiversity and stream morphology via impacts on stream hydrology, stream ecology and riparian ecology, and Cremer (2003) and others have detailed the environmental imapcts. This includes impacts on: river geomorphology, river dynamics, the pool-riffle sequence, frequency of flooding, rates of bank erosion, physio-chemical properties of water, in-stream habitat quality, nutrient input, displacing native vegetation such as native Eucalyptus spp., and affecting faunal food and habitat quality. S. cinerea obstructs and diverts streamflow due to thicket establishment on banks and invasion of shallow water by branches falling into the water and rooting. S. cinerea also produce dense shade during the growing season eliminating many native terrestrial plants growing beneath and possibly decrease water temperature, and leaf-fall may reduce oxygen content.

However, in contrast to these negative impacts, willows do have a positive impact, for example, providing stream stabilisation as well as food and habitat for native animals where native vegetation is absent.