Cayenne snakeweed (Stachytarpheta cayennensis)

Cayenne snakeweed is a small-to-medium sized shrub with long curved flower spikes. It can quickly invade bare ground and then outcompete other desirable vegetation.

Profile

How does this weed affect you?

Cayenne snakeweed thrives when groundcover is reduced. It can:

• rapidly invade pastures, reducing productivity
• invade crops including sugarcane
• outcompete and suppress native plants.

What does it look like?

Cayenne snakeweed is an evergreen, clumping perennial shrub with protruding flower spikes. It usually grows to 1.5 m tall but can grow up to 2.5 m tall.

Leaves are:

• 2–10 cm long, 1–5 cm wide
• oval-shaped with toothed edges and a pointed tip
• wrinkled with prominent veins
• mostly hairless, but sometimes with hairs on the underside along the veins
• in opposite pairs along the stem.

Flowers are:

•  blue, purplish, or sometimes white
• 6 – 7 mm long, tubular at the base spreading to 5 petals
• clustered along a curved spike, 10–45 cm long and 0.5 cm wide, that grows above the rest of the plant
• present throughout the year, mostly present during summer and autumn.

Flowers bloom in sequence from the base of the spike to the tip. As the flowers dry and fall, the flower stem develops a snakeskin like appearance.

Fruit are:

• dark brown to blackish.
• 3–4 mm long and 1.5–2 mm wide
• oblong and slightly flattened
• embedded in pits along the flower spike.

Stems are:

• branched and slender
• 50–150 cm [BV1] long and up to 6 mm thick
• greenish or purple-tinged and four-angled in cross section when young
• light brown, woody and round in cross section when mature
• mostly hairless, except for a few hairs near the nodes.

Roots are:

• a single woody taproot with many woody lateral roots
• white.

Where is it found?

A plant was found in a nature reserve on the North Coast region of NSW. This plant was treated, and no more plants have been found.

Snakeweeds are native to the tropical Americas, from the Caribbean through to Argentina. Cayenne snakeweed has naturalised in many tropical parts of the world. It was introduced to Australia as an ornamental plant. Four species of Stachytarpheta have naturalised in coastal areas of Queensland, the Northern Territory and Western Australia.

What type of environment does it grow in?

Cayenne snakeweed thrives in tropical and subtropical climates with annual rainfall above 1000 mm. In Australia it has been found from sea level to 1200 m. Cayenne snakeweed quickly spreads when ground cover is reduced. It has been found:

• along waterways
• along the edge of forests
• in pastures (especially degraded)
• in rangelands
• in sugar cane crops.

How does it spread?

How does it spread?

By seed

Seeds germinate all year provided there is enough soil moisture. Seeds are spread:

• by sticking to machinery, vehicles, animals and clothing
• in contaminated hay and pasture seed
• in soil
• by people dumping garden waste.

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.

References

Identic Pty. Ltd. & Lucid (2016). Weeds of Australia Fact Sheet: Stachytarpheta cayennensis (Rich.) Vahl retrieved 12 June 2025 from: https://keyserver.lucidcentral.org/weeds/data/media/Html/stachytarpheta_cayennensis.htm

Parsons, W.T. and Cuthbertson, E.G. (2001). Noxious weeds of Australia, CSIRO Publishing, Collingwood.

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved 12 June 2025 from: https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Stachytarpheta~cayennensis

Queensland DAFF. (2020). Invasive plant fact sheet: Snakeweed Stachytarpheta spp Retrieved 12 June 2025 from: https://www.daf.qld.gov.au/__data/assets/pdf_file/0005/54392/snakeweed.pdf

More information

back to top

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.

Pasture management

For heavy infestations, destock the paddock, treat snakeweed and re-establish dense pasture.

Physical removal

By hand

Pull or dig out individual plants, ensure all the larger roots are removed.

Slashing

Large infestations can be slashed before flowering. Follow up treatments will be required as slashing does not kill the plants.

Chemical control

Spraying

Spray actively growing plants, ensure all of the foliage is covered. Best results are achieved during summer.

Herbicide options

See Using herbicides for more information.

---

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: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

---

back to top

Biosecurity duty

The content provided here is for information purposes only and is taken from the NSW 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.

back to top

Reviewed 2025