Black willow (Salix nigra)

Black willow is an invasive tree of rivers, streams and wetlands. It is considered one of the most serious weeds of riparian and wetland ecosystems in Australia. It is a Weed of National Significance.

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How does this weed affect you?

Black willows form dense stands along waterways. They:

• outcompete native plants
• reduce food and habitat for land and aquatic animals
• reduce water quality (especially when they drop their leaves in autumn)
• take up large amounts of water
• change water flows
• divert flood waters and cause erosion
• restrict access to waterways.

What does it look like?

Black willow is an upright deciduous tree that grows up to 20 m high. It usually has one trunk but can have up to 4 main stems. Young trees have a conical shape which broadens with age.

Leaves are:

• bright green on both sides of the leaf
• slender and tapering to a pointed tip
• 4–10 cm long and 0.7–1.7 cm wide
• very finely toothed on the edges
• present from spring to autumn.

Flowers:

Male and female flowers are on separate plants and present in spring.

Male flowers are:

• yellow with no petals
• fragrant
• widely spaced in cylindrical clusters (catkins) 6–12 cm long.

 Female flowers are:

• green with no petals
• grouped densely in cylindrical clusters 4–6 cm long.

Fruit are:

• straw coloured capsules that split open to release seeds
• oval shaped with a long pointed tip
• 4.5–5.5 mm long

Seeds are:

• less than 1 mm long
• covered in long, silky hairs that can look like cotton wool.

Stems:

• Small branches are purplish-brown shiny and hairless.
• Trunks are covered with rough brown to grey bark with deep cracks when mature.

Similar looking plants

Black willow looks similar to Grey sallow (Salix cinerea). Grey sallow has many stems growing from the base and is usually a dome-shaped shrub - compared to the conical shape of young black willows. The leaves of grey sallow are shorter, wider, hairy underneath and without the toothed edges.

Where is it found?

The largest infestations of black willow grow in the Hawkesbury-Nepean and Murrumbidgee catchments. They could invade rivers, streams and wetland areas throughout NSW.

Black willow is native to eastern Canada and the USA.

Black willows were planted during the 1960s to stablise soil along streams and river banks in Victoria and southern NSW. However, they only provided short term erosion control in areas with no vegetation. In the long term they caused more erosion and have often changed the course of rivers.

What type of environment does it grow in?

Black willows prefer open sunny positions in temperate areas. They are tolerant of frosts, waterlogging and moderate salinity.  Black willows grow along the banks of streams, rivers, lagoons and swamps, spreading their roots out into the water. They can also grow in shallow waterways and sand bars in rivers. 

Maps and records

• Recorded presence of Black willow during property inspections (Map: Biosecurity Information System - Weeds, 2017-2023)
  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?

By seed

Black willows start producing seeds when they are 10 years old. Female plants produce thousands of seeds each year. The seeds are only viable for 2 - 6 weeks. In ideal conditions, with bare moist ground and sunlight, masses of seedlings can quickly germinate

Wind and water spread the light, fluffy seeds. The seeds can spread up to 100 km in waterways.

Black willow can cross-pollinate and breed with other willow species producing hybrids that can potentially be more invasive. 

By plant parts

Black willow can grow from small pieces of stems, twigs and even wood chips. Plant parts can float many kilometres downstream. New roots will grow in water from these pieces of black willow.

References

Holland Clift, S and Davies, J (2007). Willows National Management Guide: current management and control options for willows (Salix spp.) in Australia. Victorian Department of Primary Industries, Geelong.

Muyt, A. (2001). Bush invaders of South-East Australia: a guide to the identification and control of environmental weeds found in South-East Australia. RG and FJ Richardson.

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved 28 June 2021 from: https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Salix~nigra

Row, J.M. & Geyer, W.A. (2010). Plant Guide Black Willow Salix nigra Marsh. United States Department of Agriculture.

Stokes, K. E. (2008). Exotic invasive black willow (Salix nigra) in Australia: influence of hydrological regimes on population dynamics. Plant Ecology, 197(1), 91-105.

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

More information

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Control

Successful weed control requires follow up after the initial efforts. This means looking for and killing regrowth or new seedlings. Using a combination of control methods is usually more successful.

Clearing any vegetation along waterways may cause erosion and may require consent before any work starts. It is the landholder’s responsibility to obtain any approvals that may be required prior to undertaking clearing. NSW Local Land Services staff can assist you in understanding what other approvals may be required for your proposed activities under the Land Management Framework. Contact NSW Local Land Services for advice on the required approvals for your site.

 To manage black willow:

• work on upstream infestations first
• remove trees on inside bends of rivers first as these have more stable banks
• control plants before they produce fruit
• use appropriate methods of control for the size of the plants
• dispose of all plant parts away from flood zones
• look for and control new seedlings
• look for and control regrowth for 3–5 years.

Physical removal

By hand

Dig out or hand pull small seedlings up to 0.5 m tall. Small roots left in the ground do not usually regrow.

By machine

Only use excavators or bulldozers to remove larger trees and root systems in dry areas. In wet areas machinery pushes broken branches into the ground which produces many new plants.

Disposal

Trees killed by stem injection should be left for 12 months before they are cut and removed. Plants should not be chipped unless they have been treated with herbicide. Removed plant parts should be stored away from flood prone areas. Small plant parts such as twigs can be bagged and disposed of at some waste centres. Contact your local council for advice on how to dispose of willows.

Chemical control

Spraying

Only spray willows less than 2 m tall. Do not spray in autumn if the leaves have started to fall. Spray in areas where herbicides will not affect native plants or water bodies. Cover all of the foliage with herbicide. Only spray plants with clean foliage because silt on the leaves can reduce herbicide uptake.    

Stem injection

When: Year round but best results are in summer and early autumn.

Follow up: Wait at least 12 months after treatment.

Stem injection is suited to large trees. Make cuts or drill holes low all around the trunk to penetrate the sapwood below the bark. Inject herbicide within 15 seconds into each cut or hole.

Leave the tree undisturbed for at least 12 months after treatment. After 12 months the trunks can be cut and stacked away from water flows. The tree may regrow if the timber is cut too soon.

Cut stump method

When: Year round.

Follow up: Check and control regrowth or new seedlings in spring-summer.

Cut the trunk off below the first branches and immediately apply herbicide to both the trunk and the cut stump. Only use this method when all the cut material can be safely disposed of.

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: M, Inhibitors of EPSP synthase
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: M, Inhibitors of EPSP synthase
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: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Triclopyr 240 g/L + Picloram 120 g/L (Access™ )
Rate: 1.0 L per 15 L of diesel (or biodiesel such as Biosafe).
Comments: Cut stump application method for plants with stems more than 10 cm diameter at the base. Complete control may not occur due to the multi-stem growth of plant and difficulty treating all stems. See label for information about biodiesel.
Withholding period: Nil
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
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.
Greater Sydney Hornsby Shire Council, Hawkesbury City Council, Sutherland Shire Council, City of Canterbury Bankstown and Central Coast Council areas are classified as the core infestation area. An exclusion zone is established for the rest of the region.	Regional Recommended Measure* (for Regional Priority - Containment) Whole of region: Land managers should mitigate the risk of the plant being introduced to their land. Within exclusion zone: Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found. Within core infestation area: Land managers should mitigate spread of the plant from their land. A person should not buy, sell, move, carry or release the plant into the environment.
Murray	Regional Recommended Measure* (for Regional Priority - Eradication) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
North Coast Exclusion (eradication) zone: Ballina Shire LGA, Bellingen Shire LGA, Byron Shire LGA, Lord Howe Island, Port Macquarie-Hastings LGA, Richmond Valley LGA, Tweed Shire LGA. Core infestation (containment) zone: Clarence Valley LGA, Coffs Harbour City LGA, Kempsey Shire LGA, Kyogle Shire LGA, Lismore City LGA, Nambucca Valley LGA.	Regional Recommended Measure* (for Regional Priority - Containment) Whole of region: Land managers should mitigate the risk of the plant being introduced to their land. A person should not buy, sell, move, carry or release the plant into the environment. Exclusion zone: Notify local control authority if found. Land managers should eradicate the plant from the land and keep the land free of the plant. Core infestation: Land managers should mitigate spread of the plant from their land. Land managers should reduce the impact of the plant on assets of high economic, environmental and/or social value.
Riverina Core infestation areas: Snowy Valleys Council, Cootamundra - Gundagai Regional Council, Wagga Wagga City Council. Exclusion zone: All of Riverina except identified core infestation areas.	Regional Recommended Measure* (for Regional Priority - Containment) Within exclusion zone: Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found. Within core infestation area: Land managers should mitigate the risk of the plant being introduced to their land. Land managers should mitigate spread of the plant from their land. A person should not buy, sell, move, carry or release the plant into the environment. Land managers should reduce the impact of the plant on assets of high economic, environmental and/or social value.
	*To see the Regional Strategic Weeds Management Plans containing demonstrated outcomes that fulfil the general biosecurity duty for this weed click here

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Reviewed 2023