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Willow other than weeping, and sterile pussy willow

Crack willow on the Bega River

Willow catkins

Willow seed

Pussy willow, S caprea lvs

S. caprea catkins

Black willow & Lombardy poplar

Black willow leaves

White poplar

Lombardy poplar

     


Family:

Salicaceae

Status:

Willows (Salix species) are listed as a Weed of National Significance. All but weeping willow (Salix babylonica) and two types of sterile pussy willow (Salix x reichardtii and S. x calodendron) are now listed as noxious in class 5 throughout NSW. Black willow (Salix nigra) is listed as noxious in class 2 in most of the LCAs of the Murrumbidgee and Murray River catchments but not to date in the Southern Tablelands and South East Region LCAs. It was common in the Tumut and Tumbarumba areas, but eradication attempts have been made on infestations in these areas.

Description:

Deciduous trees 4-20m high, single or multi-trunked. Leaves vary but most have finely toothed edges and a paler underside. Tiny flowers in erect catkins may be followed by fluffy seeds. A single plant usually only carries flowers of one sex.  The first three photos are the very common crack willow (Salix fragilis) or unidentified hybrids, the two broad-leafed and short-catkined photos are pussy willow (Salix caprea).

Usually found along rivers where they were sparsely planted in the early days of European settlement, but they have subsequently become very dense in many rivers. Willows were also used to stabilise road batters in the Snowy Mountains but many have now been removed.

Several species of willows are weedy or potentially weedy. All are deciduous trees or large shrubs, but they vary markedly in appearance.

Weeping willow (Salix babylonica) is the most familiar. It is usually single-trunked, with very long pendulous branches. It and crack willow (Salix fragilis), which is usually multi-trunked, are the most common species occurring along rivers.  While weeping willow is not listed as noxious it is safer not to plant it, as it is capable of hybridising with other willows to produce viable seed.

Other species commonly planted as windbreaks or in gardens are:

* Golden upright willow (Salix alba var. vitellina), whose bare stems in winter are very yellow;
* New Zealand hybrid willows (Salix matsudana hybrids), which have a narrow upright habit;
* pencil or Chile willow (Salix humboldtiana ‘Pyramidalis’), which has a very narrow columnar form and may be evergreen in warm areas;
* tortured willow (Salix matsudana ‘Tortuosa’), which has markedly contorted branches and leaves;
* pussy willows (Salix cinerea, S. caprea), which are multi-stemmed and shrub-like (though up to 10m high) and have broad oval leaves and short very fluffy catkins.

With the exception of the pussy willows, the leaves of all species are long and narrow, with finely toothed edges, and usually a paler underside. Numerous tiny flowers are carried in upright catkins which are produced before the leaves in spring. Tiny seeds with a fluffy parachute are released quite early in the season, around November. At this time clouds of fluff can be seen drifting away from stands of willows which are seeding.

Black willow (Salix nigra) can be distinguished by its tall, single-trunked erect habit, rough, furrowed bark, shiny red-brown twigs and the fact that the leaves are nearly equally bright green on both surfaces, not markedly paler or whitish underneath.

Preferred habitat and impacts:

River beds and banks and other wet areas. A moist seed bed such as wet sand is needed for seeds to germinate and become established. In some cases roadside ditches or swamps may also be colonised. Burnt alpine bogs were massively colonised by willow seedlings in Victoria after the 2003 Alps wildfires, and an intensive effort was required to remove them all.

Their impact on rivers is substantial. They trap sediment, building up the river bed and filling in the waterholes needed by aquatic animals. Their large root masses, or tangles of trapped debris, can push water flow into the banks, causing erosion. The sudden drop of leaves in autumn can deplete the water of oxygen while the leaves are decomposing, making things difficult for fish and other animals. The dense shade they cast in summer can affect water temperatures.

Dispersal:

The very fine seed can drift for many kilometres on the wind. It is only viable for a few days, and needs a moist site such as a sandy river bed, so the odds of an individual seed landing in the right place and developing into a tree are low. Despite this, millions of seedlings manage to germinate in river beds in favourable years and small numbers of plants have penetrated deep into wilderness areas.

Not all willows produce seed. For a long time most willow species in Australia were represented by only one sex of plant, so that they spread only from broken off pieces taking root. However introductions of additional species and hybrids has allowed many willows which were formerly sterile in Australia to produce seed. Seed production requires a nearby partner tree of the opposite sex and a compatible variety, with the same flowering period. This condition is now very frequently met, as willows have been hybridising on the south coast for many years.
Black willow (Salix nigra) and NZ hybrids (S. matsudana X alba) are potentially the most invasive as both sexes were imported into Australia and they therefore do not need another willow species with a compatible flowering period to be present to produce seed.

Look-alikes:

No natives look similar. There are many different willows, and all, including weeping willow, are potentially or actually weedy. The closely related poplars (Populus species) look similar, but have broad, glossy leaves and longer, dangling catkins. They are more often found planted away from rivers, but when they do occur in rivers can also cause problems by blocking flow and trapping sediment. They have not yet been shown to reproduce by seed on the south coast, but they do grow readily from broken branches or twigs. White poplar (Populus alba) suckers very vigorously, producing dense thickets.  Lombardy poplar (Populus nigra var. Italica) is better behaved in amenity plantings, but in rivers it often produces thickets by falling over, with each branch then developing into an individual tree.  It is brittle, and snapped off branches frequently take root to become trees.  They can be shifted well downstream in floods.

Control:

Cut and paint or stem inject mature plants with glyphosate. This can be a difficult task, since willows are often multi-stemmed, and every stem will need to be treated. Their bases are also often buried in flood debris, blocking access. In this case burning the flood debris can kill the willow. This would be best done in the warmer months to avoid killing hibernating frogs and reptiles which might be sheltering in the piles.

Small numbers of seedlings can be easily hand-pulled in loose sand.

Spraying with glyphosate will be effective on smaller plants, but may produce too much spray drift to be acceptable with large plants.

Remember that a permit is required to use herbicides within a watercourse, and a permit is required from your local Catchment Management Authority to remove vegetation within a watercourse. Willow removal needs to be carefully staged to avoid causing more erosion. The willows may need to be replaced with suitable local native vegetation which can take on the job of protecting the river banks during floods. Get advice from your local Landcare Coordinator or CMA office.