Cayenne snakeweed (Stachytarpheta cayennensis)

Cayenne snakeweed is a small-to-medium sized invasive shrub of subtropical climates. It dominates degraded pastures and the understoreys of open grasslands and bushlands.

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How does this weed affect you?

Cayenne snakeweed is an opportunistic weed that can rapidly invade pastures, sugar cane crops, neglected areas and roadsides. It will quickly spread and thrive when ground cover is reduced or excluded, such as newly sown pastures or crops, cleared areas and overgrazed pastures.

Disturbed native vegetation areas are at risk of invasion. Cayenne snakeweed can quickly out-compete and eliminate native species.

What does it look like?

Cayenne snakeweed is an evergreen, clumping perennial shrub. It can grow up to 2.5 m high but more commonly reaches 1.5 m. It is distinguished by long, curved and snake-like flower heads.

Leaves

• 4-10 cm long
• arranged in opposite pairs
• oval in shape with toothed edges and a pointed tip
• wrinkled texture
• prominent veins
• mostly hairless, sometimes hair occurs on the underside along the veins

Flowers

• many small flowers occur along a long, curved, thin spike
• flower spike extends past the plant foliage, is 10-45 cm long and 0.5 cm wide
• 5 dark blue to violet coloured petals fuse together at their base to form a short tube 6-7 cm long
• individual flowers bloom a few at a time in sequence from the base of the spike to the tip
• a small green pointed bract (modified leaf) occurs where the flower joins the spike, 3-6 mm long

Seeds

• 4-6 mm long
• embedded in pits along the spike
• dark brown to black in colour

Stems

• branched and slender
• young stems are square and greenish or purple-tinged in colour
• mature stems are round, light brown in colour and woody
• mostly hairless, except for a few hairs near the nodes (joints)
• 50-150 cm long and up to 6 mm thick.

Roots

• single woody taproot
• white
• many woody lateral roots extend from the taproot.

Where is it found?

Snakeweeds are native to the tropical Americas, extending from the Caribbean through to Argentina. Cayenne snakeweed has naturalised in many tropical parts of the world including Madagascar, tropical Africa, Oceania, south-eastern USA and tropical Asia.

Originally introduced as an ornamental plant, four species have now naturalised along the coastal areas of Queensland, the Northern Territory and Western Australia. All are considered weeds. Cayenne snakeweed is predominantly found along the eastern and northern Australian coastlines, particularly northern Queensland and the Northern Territory.

At present, cayenne snakeweed is not known to occur in NSW.

How does it spread?

Cayenne snakeweed only reproduces from seed. The seeds of cayenne snakeweed can be dispersed in contaminated fodder, hay and pasture seed. It can also be spread from the movement of contaminated soil and garden refuse.

Lifecycle

Seeds germinate all year provided there is enough soil moisture. This is usually following summer rain. Young plants grow quickly and flower in the late summer of their first growth season. Once established, new stems are produced continuously.

Flowering and seed set occurs throughout the year but is most abundant during spring, summer and autumn.

What type of environment does it grow in?

Cayenne snakeweed thrives in tropical or subtropical climates with annual rainfalls above 1000 mm. It is more often found in wet coastal regions, along waterways and adjacent pasture areas.

Acknowledgements

Written by Rachele Osmond.

References

Biosecurity Queensland (2014) Fact sheet PP52–Snakeweed: Stachytarpheta spp. Department of Agriculture, Fisheries and Forestry, Brisbane. Available at http://www.daff.qld.gov.au/__data/assets/pdf_file/0005/54392/IPA-Snakeweed-PP52.pdf

Department of Employment, Economic Development and Innovation (2011) Dark blue snakeweed: Stachytarpheta cayennensis. Queensland Government Available at http://keyserver.lucidcentral.org/weeds/data/03030800-0b07-490a-8d04-0605030c0f01/media/Html/Stachytarpheta_cayennensis.htm  

Hosking JR, Sainty GR, Jacobs SWL & Dellow JJ (in prep) The Australian WeedBOOK

Parsons, WT and Cuthbertson, EG (2001) Noxious weeds of Australia, CSIRO Publishing, Collingwood.

More information

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Control

Pasture management is an important component of cayenne snakeweed control and should be incorporated into the integrated weed management program.

Pasture management

Heavy infestations required destocking, physical and chemical treatment, followed by pasture re-establishment.

Physical control

Individual plants within small infestations can be physically pulled or grubbed out. Take care to remove larger roots.

In large infestations, physical control will need to be conducted together with chemical control to obtain the best results. Slash infestations prior to flowering. Follow with foliar herbicide treatment on actively growing plants.

Continued follow-up and re-treatment is essential to maintain control.

Herbicide control

Foliar spray larger infestations when plants are actively growing. Best results are achieved during summer months.

Small infestations can be spot sprayed using a foliar application.

Herbicide options

See Using herbicides for more information.

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Glyphosate 360 g/L (Various products)
Rate: 10 mL per 1 L water
Comments: Spot spray. For general weed control in Domestic areas (Home gardens), Commercial, Industrial and Public Service areas, Agricultural buildings and other farm situations.
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
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.

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Reviewed 2021