Willows (Salix species)

Willows are deciduous trees or shrubs that form large, dense root-mats on the surface of the soil or in shallow water and slow-moving streams. They invade thousands of kilometres of riverbanks and numerous wetlands in temperate Australia.

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How does this weed affect you?

Willows are among the worst weeds in Australia due to 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. Unlike most other vegetation, 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. This, together with the amount of water willows use, damages stream health. The replacement of native vegetation by willows reduces habitat for both land and aquatic animals.

What does it look like?

Willows are deciduous trees or shrubs. They have small seeds with long, silky hairs attached to one end like a parachute, which help them spread. The seeds are usually short-lived, from days to a few weeks. 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. Upright catkins (flower stalks) carry numerous tiny flowers. The trees form large, dense root-mats on the surface of the soil or in shallow water and slow-moving streams.

Where is it found?

 Willows have only invaded about 5% of their potential geographic range in temperate Australia. The most seriously invasive willow, grey sallow (Salix cinerea), is expanding its range rapidly in Victoria and New South Wales, and possibly in Tasmania.

Grey sallow or pussy willow (Salix cinerea) is the most seriously invasive willow in Australia. It is a large spreading shrub or small tree with twigs or branches that are hard to break. It reproduces mainly by seed. Pussy willow is highly invasive in swamps, drainage lines and other moist sites including lowland and mountain streams. Large and rapidly expanding populations occur in Victoria, and this species will probably become a major wetland and riverside weed (as it is in New Zealand). It forms hybrids with other shrub willows.

Crack willow (Salix fragilis var. fragilis) and basket willow (Salix x rubens) are by far the most widespread and abundant willows in Australia, and are the most serious problem willow in Tasmania. They are found along thousands of kilometres of streams in southeastern Australia where they were widely planted for stream stabilisation. Crack willow spreads almost exclusively by plant parts so it is only associated with streams.

Black willow (Salix nigra) has been widely planted in northeastern Victoria and at several sites in New South Wales. It is now very abundant in some streams. Black willow has the potential to behave in the same invasive manner as grey sallow in wetlands.

Maps and records

• Recorded presence of Willows during property inspections (Map: Biosecurity Information System - Weeds, 2017-2024)
  These records are made by authorised officers during property inspections under the Biosecurity Act 2015. Officers record the presence of priority weeds in their council area and provide this to the NSW Department of Primary Industries. Records reflect the presence of the weed on the date of inspection.
  
  

How does it spread?

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. Seed carried by wind or water easily travels more than 1 km, with small amounts potentially spreading up to 100 km. Seed production is becoming more common as more willows are introduced into Australia. However, the conditions required for germination (ie continuously wet, bare sediment) do not commonly occur and the seed only remains viable for between two and six weeks, depending on the species.

Willows are either male or female and most groups in Australia are single-sex clones. However, they readily hybridise when opposite sexes come together. They flower in spring, the flowers only lasting for 2–3 weeks. The tiny seeds ripen about 3–4 weeks later in late spring or early summer. Germination is very fast, occurring within 24 hours, and seedlings grow rapidly under favourable conditions. The hybrid species are vigorous and can breed just two or three years after germination.

There are 32 different groups (species, varieties, subspecies and hybrids) of willows in Australia. Nearly all the different species have become naturalised here and can cross-breed with other willow species that flower at the same time. Most naturalised willow populations are hybrids and can be practically impossible to identify precisely.

The introduction of New Zealand willows (Salix matsudana hybrids) throughout the Murray–Darling Basin in the 1980s and their widespread sale since then is now causing problems, as the females produce abundant seed and the males fertilise the weeping willow (Salix babylonica), a widespread species that in the past usually did not seed because it had no male partner flowering at the same time.

What type of environment does it grow in?

Willows occur naturally in permanently or seasonally wet, inundated or waterlogged sites. The largest infestations in Australia are in Victoria, Tasmania, New South Wales and the Australian Capital Territory. Several species (weeping, basket and crack willows) have been widely planted along the rural waterways of southeastern Australia for erosion control.

Acknowledgements

Information and guide revision: Bob Trounce (NSW Agriculture), Lynton Auld (Blue Mountains City Council), Richard Carter (NSW Agriculture/Weeds CRC), Vanessa Richardson (NSW NPWS) and John Thorp (National Weeds Management Facilitator). Maps: Australian Weeds Committee.

This information was generated by the CRC for Australian Weed Management. 

References

Weeds of National Significance (2003) Weed Management Guide Willow - Salix spp. 

More information

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Control

Willows are relatively easy to kill and mechanical and chemical control techniques are well understood. However, it should be noted that indiscriminate removal of willows is not recommended as it may lead to stream instability. Control should be conducted in consultation with you local council weeds officer. 

A willow control strategy

Pull seedlings by hand

The simplest strategy is to pull all seedlings (and rooted branches) while they are still small. This works best if it is done regularly, especially if there are limited sources of seed and few suitable regrowth sites.

Kill mature tree where they stand unless this is not possible for safety, practical or aesthetic reasons. Use stem injection of a registered herbicide to avoid chemical runoff. Best results will be achieved from summer to early autumn. Leave trees undisturbed for 12 months after herbicide application to ensure a successful kill.

Start control in the uppermost part of the catchment, preferably on the insides of bends

A long-term planned approach to control is needed. Staged removal should start in the upper reaches of the catchment. In the case of seeding species (eg pussy willow and black willow) which can recolonise treated areas, a coordinated catchment-scale intensive attack is the best option.

First remove trees on the inside of bends because these banks are more stable. Where willows have been planted to stabilise soil or creek banks, alternative vegetation should be established before all willows are removed.

Be aware of stream flow dynamics

The flow of the river will change once the willows are removed, and this may place greater pressure on restricted points downstream. In these cases it may be advisable to start working on the lower end of the section, progressing upstream.

Follow-up will be required

Monitor treated areas and use followup control on any regrowth for 3–5 years after the initial control.

Prevent spread

Early detection and control are essential to prevent the spread of new infestations. The deliberate planting of willows along waterways has virtually ceased and extensive removal operations are common. It is fairly easy, given enough resources, to prevent the spread of willows that propagate by plant parts, as they are confined to streams and are spread downstream. For seeding willows, prevention of spread is difficult because seed can be dispersed over large areas. Willows are still widely planted, eg for windbreaks on farms, and many groups (including weedy ones) are sold by the nursery trade in Australia. There is potential for additional willow taxa to become naturalised if importation is not closely regulated.

Surveys and staged removal

A long-term plan should be devised before any attempt is made to eliminate problem willows. Removal of trees can actually increase erosion problems, so a plan to replace willows with more desirable species is needed. Start by carrying out an extensive survey to identify potential seed sources. The willow species that set seed flower between September and November, so this is the best time to search for catkins on or under trees.

CSIRO recommends identifying seed trees by attaching conspicuous plastic ribbons to them which will endure floods and grazing animals and last for 2–3 years. Trees growing more than 2 km away from a river may still be a significant seed source.

Staged removal should be undertaken over a number of years, starting in the upper reaches of each catchment and working downstream. Where willows have been planted to stabilise soils or banks, alternative vegetation should be established before the willows are removed.

Remove trees first which will not destabilise banks (eg on the inside of bends). Anticipate stream flow changes and be aware that removal of constrictions will allow greater pressure at restricted points further downstream. In these cases it may be advisable to start working on the lower end of the section, progressing upstream.

Mechanical removal

Mechanical removal of seedlings, or of larger trees in dry areas. Elimination of young seedlings is a cost effective way of keeping waterways free of potential blockages, erosion and streambed change. Hand pulling of seedlings less than 0.5 m tall is the most practical and environmentally safe way of removing young plants. Leaving small roots in the ground does not lead to suckering or regrowth. Using large machinery such as excavators or bulldozers to remove larger trees and root systems is not recommended except in dry areas. In wet areas bulldozers push broken branches into the ground and thus generate numerous new plants. Disposal recommendations Trees killed while they are standing (ie by stem injection) should be left for 12 months before they are removed. They can then be cut at a suitable height and stacked away from watercourses. If it is necessary to remove live trunks and limbs from the site, stack them to dry above flood level, taking care to minimise the spread of small pieces. Smaller twigs should be bagged and disposed of at tip facilities so that they do not sprout and cause further problems.

Control with herbicides

Stem injection  

Herbicides available for woody weeds are effective in controlling willow. Trees can be killed by stem injection, application to leaves and stems, bark (chemical girdling) and cut and paint methods (check with state/territory agencies for current recommendations). In dry conditions herbicide can also be applied by basal bark spraying and treatment of seedlings. Although stem injection may be a slower, more laborious method, it is an important option for avoiding chemical runoff and protecting native vegetation. In general, herbicide should be applied from summer to early autumn, although stem injection or cut and paint application is effective year round.

Stem injection is suited to large trees. Make cuts or drill holes below the branches, around the trunk, 20–30 mm into sapwood. The injection points should be single cuts spaced at less than 130 mm intervals, or holes drilled at 50–100 mm intervals, around the circumference. Angle holes and cuts downwards to minimise herbicide leakage. Herbicide should be immediately injected into each cut or hole at the recommended rate. Leave the tree undisturbed for at least 12 months after herbicide application to ensure a successful kill.

Cut stump application

Cut stump application should only be used to kill willows that can be easily and safely disposed of (ie smaller specimens). Cut the aerial trunk off completely at a level below the first branches and immediately apply a recommended herbicide to the cut stump. Remove all material to prevent regeneration from pieces. The cut surface of the removed stem should also be painted with herbicide for safe disposal. Minimal transport of branches and stems will help avoid broken fragments being spread. Willow wood chips can take root and grow so trees for chipping should be killed prior to removal.

New infestations can occur when trees are cut and moved away from waterways with heavy equipment. Small pieces of branch embedded in the attached soil may take root or enter the water to float away to new sites.

Foliar spraying

The entire plant can be foliar sprayed if it is less than 2 m tall before the start of leaf fall and where herbicides will not affect native plants or make contact with water bodies.

Follow-up

Regrowth from stumps, pieces of stems or seeds will need to be followed up with monitoring and further control for 3–5 years after the initial effort. Check that treated trees have died, and remove trees that could cause problems if they become snared elsewhere by floods. Look for the spread of any new willows and follow up with substantial re-assessments at least every five years.

Herbicide options

See Using herbicides for more information.

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Glyphosate 360 g/L (Various products)
Rate: 1.0–1.3 L in 100 L of water
Comments: Spray to wet all foliage. Use the higher rate for trees 1–2 m high.
Withholding period: Nil.
Herbicide group: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

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Glyphosate 360 g/L (Various products)
Rate: Undiluted
Comments: Stem injection. For trees with a basal diameter of 0 - 25 cm use 1 mL/cut. For trees with a basal diameter of 25 - 60 cm use 2 mL /cut.
Withholding period: Nil.
Herbicide group: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

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Picloram 44.7 g/L + Aminopyralid 4.47 g/L (Vigilant II ®)
Rate: Undiluted
Comments: Cut stump application: Apply a 3–5 mm layer of gel for stems less than 20 mm. Apply 5 mm layer on stems above 20 mm. Stem inject application for trees: Make a series of cuts 15-20 mm deep around the trunk using an axe or saw. Space cuts evenly with no more than a 20-40 mm gap between them. Apply a 5 mm layer of gel over the lower surface of the cut.
Withholding period: Nil.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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Triclopyr 240 g/L + Picloram 120 g/L (Access™ )
Rate: 1.0 L in 15 L of diesel
Comments: Cut stump application for plants with a basal stem greater than 10 cm in diameter. Need to treat all stems.
Withholding period: Nil
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area	Duty
All of NSW	General Biosecurity Duty All pest plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
All of NSW	Prohibition on certain dealings Must not be imported into the state, sold, bartered, exchanged or offered for sale. All species in the Salix genus have this requirement, except Salix babylonica (weeping willows ), Salix x calodendron (pussy willow) and Salix x reichardtii (sterile pussy willow)

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Reviewed 2021