Declared Plant Policy willows (Salix spp.)

Declared Plant Policy
willows (Salix spp.)

Willows are fast-growing deciduous trees and shrubs from the northern hemisphere.
Their ability to grow very easily from cuttings of any size makes them popular as
quick-establishing farm trees; it also enables them to spread from branches swept
downstream by floods. Branches can also take root while still attached to the parent
tree, so thickets can 'walk' out into the stream. Consequently, many species are
weeds of waterways where they displace native vegetation, hinder access and slow
the movement of floodwater.

Willows are normally dioecious, having male and female flowers on separate
individuals. So far there has been little spread of willows by seed because the vast
majority grown were a female clone of S. babylonica var. babylonica. But now a
wider range of tree willows including both male and female plants has been imported
and is being distributed by nurseries as farm trees. These species and hybrids are all
tetraploids (Darlington & Wylie, 1961) and most are able to interbreed, meaning that
any female willow tree may become a source of tens of thousands of seeds each
spring. Pollen can be spread over many kilometres by wind and insects, and some
of the hybrids are monoecious, producing both male and female flowers on the same
tree. The seeds only live for a month or two, but if they find suitable conditions can
germinate in large numbers and establish rapidly. Most species need bare ground
with the water table at the surface from November to midsummer.

The existing problem willows in South Australia are the common weeping willow (a
mixture of Salix babylonica var. babylonica, S. pendulina and S. sepulcralis) along
the Murray; and S. X rubens on streams in the Mt Lofty Ranges, the south-east, and
also on the Murray.

Concern was triggered by the promotion of the hybrids of S. babylonica var.
pekinensis X S. alba from New Zealand (Cremer et al., 1995). Willow control was
highlighted as a problem needing consistent response across the States at a national
workshop last year (Outhet et al, 1997). The Australian Weeds Committee has now
formed a working group on willows, and NSW is also moving to prohibit the sale of
most willow species.

Section 177 of the Natural Resources Management Act empowers the crown to
prohibit the sale of proclaimed plants. This power has proved effective in keeping
demonstrably weedy plants such as prickly pear off the market, but only with the co-
operation of the nursery industry and the gardening public. To retain this co-
operation, it is desirable to invoke this section only for plants that the public generally
perceives as weeds. It would be futile, for example, to prohibit sale of the common
weeping willow - a familiar tree that anyone can grow from a twig picked up in a
neighbour’s garden or from public land.

Many Salix are either proven weeds, or have similar biology to the weedy species
and hybridise freely with them (Cremer et al., 1995).

Willows known to be weedy

S. alba var. alba - (white willow) Small tree. Male and female plants are present,
and it has become naturalised in New South Wales, Victoria, Tasmania and SA by
seed production, forming dense stands along streams.

Native range of Salix alba in Europe

S. babylonica var. babylonica (weeping willow) Large tree to 25 m tall, all female. Has
been planted in great numbers along rivers in New South Wales, Victoria and SA,
where it spreads from broken branches and even twigs stuck into riverbanks as rod-
rests by fishermen. Has not caused practical problems yet, but is an aesthetic affront
to conservationists who want to retain native vegetation along rivers. Its deep roots
enable it to tap underground water, allowing it to naturalise on the Swan coastal plain
in WA (Hussey et al., 1997) in an environment comparable to parts of SA.

S. babylonica var. pekinensis - (Matsudana willows) All females; have interbred
with S. alba to form wild seedlings in Victoria and New South Wales. Other
deliberately bred hybrids of this parentage have been imported from New Zealand
including the named cultivars ‘Cannock’, ‘Makra’, ‘Te Awa’ and ‘Tangoio’.

S. cinerea (common sallow) Large shrub that was widely planted in mistake for the
ornamental ‘pussy willow’. Males and females naturalised in Victoria, New South
Wales and Tasmania.

S. fragilis (crack willow) Medium tree, apparently all male in Australia. Hybridises
with S. alba and spreads downstream by broken-off branches.

S. nigra (black willow) Medium tree. Unlike most willows, it does not need wet
ground as a seedbed. Male and female forms are naturalised in Victoria and New
South Wales.

S. X mollissima is a hybrid of S. triandra and S. viminalis; female plants are
naturalised in NSW.

S. purpurea (purple osier) A shrub used for soil stabilisation on Snowy River scheme,
and now spreading from seed. Cultivars 'Holland', 'Irette' and 'Pohangina' are male;
'PMC' is a fertile female; 'Booth' is said to be sterile.

S. X rubens (basket willow or goldcrack willow) Medium tree, a natural hybrid of S.
alba and S. fragilis that is being produced in quantity along rivers in Victoria and NSW.
Infestations occur on the Murray upstream and downstream from Murray Bridge.

S. X rubra is a hybrid of S. purpurea and S. viminalis; male and female plants are
naturalise in NSW.

S. sepulcralis var. chrysocoma (Dode)Meikle (Golden weeping willow) is a garden
hybrid of S. alba and S. babylonica var. babylonica Most trees are female. Naturalised
in NSW, Victoria and Tasmania after planting for erosion control.

S. sepulcralis var. sepulcralis is a garden hybrid of S. alba and S. babylonica var.
babylonica Naturalised in Victoria, mostly as female plants.

S. viminalis (common osier) Large shrub. Male and female cultivars 'Continentalis',
‘Mealy Top' and 'Jacob Brown' were planted for soil stabilisation on Snowy River
scheme and are now naturalised there.

Some other willows

S. alba var. caerulea Cricket bat willow is cultivated for cricket bat wood, and has not
become naturalised anywhere in Australia. It is seen as a growth industry in Australia,
and several landholders have started plantations of this variety (Carson, 1996).

S. babylonica var. pekinensis ‘Tortuosa’ (Tortured willow) is a female clone
widespread in the nursery trade, and has not become naturalised in Australia.

S. chilensis 'Fastigiata' is also known as S. humboldtiana Willd. 'Pyramidalis', pencil
willow or Chilean willow. It is widespread in the nursery trade, a male clone that has
only become naturalised in warm summer-rainfall areas of NSW.

Native range of Salix myrtilloides in Europe

S. myrtilloides is a shrub native to northern Europe and within the arctic circle
(Myklestad & Birks, 1993; Suominen, 1976), but absent from southern Europe where
climates match some parts of SA. It has not become naturalised in Australia

The subgenus Chamaetia is a distinct group comprising all the ground-cover species
such as S. apoda and S. reticulata from arctic, subarctic and alpine regions. They have
not been found to hybridise with the tree willows and have never become naturalised.
As they are of some value in gardens, their sale should not be prevented. S. X boydii
is a similar shrub, used in alpine gardens and for bonsai.

Declaration

That the following sections of the Natural Resources Management Act, 2004 apply to
willows of all species throughout South Australia excluding the following:

Salix babylonica
Salix calodendron
Salix reichardii

177(1)(2) [Prohibiting sale of the plants].

References

Carson, J. (1996) Cricket bat willow industry revived. Aust. Farm Journal June 1996:
14-15.

Cremer, K.W. (1995) Willow identification for river management in Australia.
Technical Paper No.3 (CSIRO Division of Forestry: Canberra).

Cremer, K.W., Van Kraayenoord, C., Parker, N. & Streathfield, S. (1995) Willows
spreading by seed - Implications for Australian river management. Aust. J. Soil
and Water Conservation 8(4): 18-27.

Darlington, C.D. & Wylie, A.P. (1961) Chromosome Atlas of Flowering Plants. 2nd
edn (Allen & Unwin: London).

Harden, G.J. & Rodd, A.N. (1990) Salix In Harden, G.J. (ed) Flora of New South
Wales. 1: 448-451.

Hussey, B.M.J., Keighery, G.J., Cousens, R.D., Dodd, J. & Lloyd, S.G (1997) Western
Weeds. (Plant Protection Society: Victoria Park).

Ladson, A. et al. (1997) Willows along Victorian Waterways. (Department of Natural
Resources and the Environment: Victoria).

Myklestad, Å. & Birks, H.J.B. (1993) A numerical analysis of the distribution patterns
of Salix L. species in Europe. J. Biogeogr. 20: 1-32.

Outhet, D. et al. (1997) Outcomes of the Willow Workshop. NSW Department of Land
and Water Conservation.

Rechinger, K.H. (1964) Salix. Flora Europaea 1: 43-54.

Rodd, A.N. (1982) Salicaceae. Flora Australia 8: 202-206

Spencer, R.D. (1997) Horticultural Flora of South-eastern Australia. 2. (University of
NSW Press: Sydney).

Suominen, J. (1976) Salicaceae. Atlas Florae Europaeae Vol.3 (Committee for
Mapping the Flora of Europe: Helsinki).