Winter senna (Senna septemtrionalis)

Also known as: arsenic bush

Winter senna is a shrub with bright yellow flowers. It invades bushland and outcompetes native plants.

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How does this weed affect you?

Winter senna is an environmental weed in coastal forests and woodlands. It grows quickly and can:

• form dense thickets
• outcompete native plants
• reduce food and habitat for native animals.

What does it look like?

Winter senna is a perennial woody shrub that grows 1–3 m tall.

Leaves

The leaves are made up of 3 to 5 pairs of opposite leaflets that are larger at the end of the leaf. There is usually a small gland between each pair of leaflets.

Leaflets are:

• glossy green on top
• paler green underneath
• up to 10.5 cm long and 3.5 cm wide
• elongated with long pointed tips.

Flowers are:

• bright yellow
• pea-like with five petals up to 1.6 mm long, the top petal is larger than the others and has a notched tip
• in clusters of 4–10 flowers
• present from spring to autumn.

Fruit are:

• cylindrical pods
• 7–8 cm long and up to 1.2 cm in diameter
• green when young
• straw-coloured to brown when mature
• usually upright on the stalk.

Seeds are:

• olive or brown coloured
• 2.5–5 mm long
• flattened
• smooth or with a minutely pitted surface.

Similar looking plants

Winter senna looks like Cassia (Senna pendula var. glabrata) when in flower. Cassia’s leaflets are shorter and broadly oval with rounded tips.

Where is it found?

Winter senna grows all along the east coast of NSW from the Queensland to Victorian border.

It is native to Mexico. It has been planted as an ornamental shrub in gardens

What type of environment does it grow in?

Winter senna tolerates a wide variety of soil types and moisture levels. It grows in both forests and open woodlands and disturbed areas such as along roadsides.

How does it spread?

By seed

Winter senna produces seeds which can still be viable after 16 years in the soil. Seeds are spread in water, mud, and contaminated agricultural produce.

References

Datiles, M.J. & Acevedo-Rodríguez, P. (2014). Senna septemtrionalis (smooth senna) In: Invasive Species Compendium. Wallingford, UK: CAB International. Retrieved 17 February 2021 from: https://www.cabi.org/isc/datasheet/49597

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved 17 February 2021 from https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Senna~septemtrionalis

Richardson F.J.,Richardson R.G. and Shepherd R.C.H (2006).Weeds of the south-east an identification guide for Australia. (R.G. and F.J. Richardson, Melbourne).

More information

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Control

Successful weed control relies on 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.

Physical control

Small seedlings can be hand pulled or dug out.

Chemical control

Spot spraying

Spray actively growing plants. Apply herbicide to all of the foliage to the point of visible wetness.

Splatter gun

Splatter guns can be used for dense infestations that are difficult to reach. The specialised nozzle produces large droplets. This allows plants up to 10 m away to be sprayed with a limited chance of spray drift. Spray small amounts of concentrated herbicide on the weeds. It is not necessary to cover all of the foliage.

Wiping

Wipers or wands apply herbicide directly onto leaves. This method can minimise damage to other plants.

Basal barking

Apply herbicide mixed with diesel to cover the lower stem, all the way around.

Disposal

Dried seed pods can be burnt in a hot fire. Contact your local council for further advice on how to dispose of seed pods

Herbicide options

See Using herbicides for more information.

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PERMIT 9907 Expires 31/03/2025
Fluroxypyr 200 g/L (Comet® 200 herbicide)
Rate: 500 mL to 1 L per 100 L water
Comments: Spot spray
Withholding period: Do not graze failed crops and treated pastures or cut for stock feed for 7 days after application. See label for further information.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Fluroxypyr 200 g/L (Comet® 200 herbicide)
Rate: 35 mL per L diesel/kerosene
Comments: Basal bark
Withholding period: Do not graze failed crops and treated pastures or cut for stock feed for 7 days after application. See label for further information.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Fluroxypyr 333 g/L (Starane™ Advanced)
Rate: 300 to 600 mL per 100 L water
Comments: Spot spray
Withholding period: Do not graze failed crops and treated pastures or cut for stock food for 7 days after application. See label for more information.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Fluroxypyr 333 g/L (Starane™ Advanced)
Rate: 21 mL per L diesel/kerosene
Comments: Basal bark
Withholding period: Do not graze failed crops and treated pastures or cut for stock food for 7 days after application. See label for more information.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L (Various products)
Rate: One part product to 50 parts water
Comments: Spot spray
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L (Various products)
Rate: One part product to 9 parts water
Comments: Splatter gun
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L (Various products)
Rate: One part product to 20 parts water
Comments: Wipe onto leaves
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Metsulfuron-methyl 600 g/kg (Various products)
Rate: 10 - 20 g per 100 L water plus surfactant
Comments: Spot spray
Withholding period: Nil (recommended not to graze for 7 days before treatment and for 7 days after treatment to allow adequate chemical uptake in target weeds).
Herbicide group: B, Inhibitors of acetolactate synthase (ALS inhibitors)
Resistance risk: High

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PERMIT 9907 Expires 31/03/2025
Metsulfuron-methyl 600 g/kg (Various products)
Rate: 10 g per 1 L of water plus surfactant
Comments: Wipe onto leaves
Withholding period: Nil (recommended not to graze for 7 days before treatment and for 7 days after treatment to allow adequate chemical uptake in target weeds).
Herbicide group: B, Inhibitors of acetolactate synthase (ALS inhibitors)
Resistance risk: High

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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 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.

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Reviewed 2021