Name:
Willow (Salix spp.)
Description
There are many different species, varieties, cultivars and hybrids of Willow in Australia. They are all deciduous trees or shrubs, with four distinctive growth habits recognisable among the group: the pencil willows, the tree willows, the weeping willows and the shrub willows.
The pencil willows have a single trunk capable of growing into a large tree, with very short lateral branches forming a narrow crown, where the branches get progressively shorter from the base of the trunk to the apex. The tree willows have a single or several trunks, capable of growing into large spreading trees, with upright large branches. Within the tree willows is another group that have branches that hang down and this group is known as the Weeping Willows. The shrub willows are multi-stemmed and never grow into trees.
All willows form large dense shallow root masses. The leaves of most species are long and narrow with fine toothed margins. Salix cinerea (Grey Sallow or Pussy Willow) has broader, shorter leaves than the rest. The flowers of willows are unisexual and group together in what is called a catkin, with male catkins and female catkins usually formed on separate plants. The small seeds develop quickly inside a capsule. Each seed has long silky hairs at one end which aids in the wind dispersal of the seed once the capsule opens. The seeds are short lived and normally don’t live longer than 10 days. Most naturalised Willow populations are hyrbids and can be practically impossible to identify precisely.
For further information and assistance with identification of willows contact the herbarium in your state or territory.
Distribution:
Willows are confined to the south-east regions of Australia from south-east Queensland through the Northern, Central and Southern Tablelands and some coastal regions of New South Wales. They are widespread in eastern and central Victoria and found in south-eastern South Australia, particularly along the Murray River and some of its tributaries. Willows are also widespread in Tasmania.
Habit:
Tree, Shrub
Key points:
- Willows are invasive weeds of many waterways and wetlands in south-eastern Australia.
- Willows spread their roots into the bed of a watercourse, slowing the flow of water and reducing aeration.
- Willows are easily killed with herbicide but difficult to control because broken stems and twigs easily take root.
- Some varieties of Willow can also spread by seed, which can be carried up to 100 km by wind or water.
- Willows have started to invade natural habitats.
How it spreads:
Most willows spread by fragments of stems or twigs breaking off and growing new roots in water. Pieces can travel many kilometres before establishing at a new site. Fishermen often break off twigs and stick them in the riverbank to hold their lines, and these pieces will also grow.
Seed is the main method of spread for several species, especially Grey Sallow and Black Willow. These species can invade off-stream wetlands from sea level to alpine locations. Seed carried by wind or water easily travels more than 1 km, with small amounts potentially spreading up to 100 km.
Willows can also reproduce prolifically from seed, producing seedlings resulting from cross-pollination between different types. Hybrids, resulting from interbreeding between different hybrids or cultivars, are still being identified in the north-east of Victoria. Willows can germinate in massive numbers (half a million seedlings were recorded at one site), and form islands in watercourses.
However, often these seeds do not germinate or grow, possibly due to the lack of suitable seedbed, rising or rapidly falling water levels and floods that uproot or bury the seedlings.
Where it grows:
Willows occur naturally in permanently or seasonally wet, inundated or waterlogged sites.In Australia they are mainly found in similar situations along water courses or in seasonal or permanently wet areas.
Flower colour:
White
Distribution map:
Impacts:
Most species of Willow are Weeds of National Significance. They are among the worst weeds in Australia because of their invasiveness, potential for spread, and economic and environmental impacts. They have invaded riverbanks and wetlands in temperate Australia, occupying thousands of kilometres of streams and numerous wetland areas.
Willows spread their roots into the bed of a watercourse, slowing the flow of water and reducing aeration. They form thickets which divert water outside the main watercourse or channel, causing flooding and erosion where the creek banks are vulnerable. Willow leaves create a flush of organic matter when they drop in autumn, reducing water quality and available oxygen, and directly threatening aquatic plants and animals. This, together with the amount of water willows use, damages stream health.
The replacement of native vegetation (e.g. river red gums) by willows reduces habitat (e.g. nesting hollows, snags) for both land and aquatic animals.
Willows have only invaded about 5% of their potential geopgraphic range in temperate Australia.
Origin:
All willows in Australia have been introduced from other countries, with most probably coming from Europe. Most species presently growing in Australia are found naturally in Great Britain and Ireland.
History:
Willows have been used for soil stabilisation and river bank protection, basketry, landscaping, shade and shelter. Tortured Willows (Salix matsudana hybrids) from New Zealand were introduced into the Murray-Darling Basin during the 1980s and have only recently begun to cause problems.
The Cricket-bat Willow is cultivated in parts of Australia but has not naturalised.
