Witchweeds (Striga species)

PROHIBITED MATTER: If you see this plant report it. Call the NSW DPI Biosecurity Helpline 1800 680 244

Witchweeds are parasitic herbs that grow on the roots of host plants. They are serious weeds of maize, millet, rice, sugarcane, sorghum and legume crops.

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How does this weed affect you?

 Witchweeds are parasitic plants that:

  • take nutrients from host plants stunting or killing them
  • can completely destroy maize, millet, rice, sugarcane, sorghum and legume crops
  • are very difficult to control
  • usually can’t be found in time to save the crop.

Parasitic weeds like witchweed are among the most destructive and difficult-to-control weeds in agriculture. Worldwide, at least 11 species of witchweed are known to harm crops. The most damaging witchweed species are:

  • Purple witchweed (Striga hermonthica) and red witchweed (Striga asiatica) on cereals and grasses
  • Cowpea witchweed (Striga gesnerioides) on legumes.

All Striga species except for the native Striga parviflora are Prohibited Matter in New South Wales (NSW). This native species can damage sugar cane and maize crops.

What does it look like?

Witchweeds grow near the base of their host plants and can’t be seen until they emerge from the soil. This may take four to seven weeks for some witchweeds. They can flower and produce seed rapidly once they emerge. Most witchweeds are only 15-20 cm tall when fully grown, but some are up to 60 cm.

Host plants are often stunted, with symptoms resembling severe drought stress, nutrient deficiency or disease. The symptoms can appear before and after witchweeds emerge. A key sign of witchweed is host plant leaves shrivelling and wilting, despite moist soil.

Leaves are:

  • green
  • 6–40 mm long and 1–4 mm wide (or reduced to scales for cowpea witchweed)
  • tapering to a pointed tip.

Flowers are:

  • attached near the top of the stem, next to a leaf
  • 5–8 mm wide
  • red, pink, white, yellow, orange or purple
  • with four or five petals.  

Fruit are:

  • capsules
  • about 4 mm long and 2 mm wide
  • green to brown as they mature
  • with over 500 seeds.

Seeds are:

  • prolific, each plant can produce at least 50,000 seeds
  • so small that they look like dust
  • 0.2–0.3 mm long
  • brown.

Stems are:

  • round and white when underground
  • four-sided and covered with short, hard hairs above the ground
  • usually singular (i.e. not branched)
  • above ground stems can be red-purple for cowpea witchweed.

Where is it found?

Exotic witchweeds are not known to occur in NSW. Red witchweed was found growing on several sugarcane properties near Mackay, Queensland, in 2013 and is the target of a national eradication program. It is a weed in 35 countries, including the United States. 

Most witchweeds are native to tropical Africa, India, the Middle East and China. Even in their native range, witchweeds are troublesome and cause significant crop losses.

One native witchweed Striga parviflora has occasionally been found in woodlands on the north coast and central western slopes of NSW. This native witchweed has been found on sugar cane and maize crops in Queensland. 

What type of environment does it grow in?

Witchweeds usually grow in intensive agricultural monocultures. However, they can also grow in grasslands, pastures and bushlands if their host plants are present.

Witchweeds grow best in well-drained soils, although some will tolerate wet areas.

How does it spread?

By seed

Each plant can produce at least 50,000 tiny sticky seeds. The seeds can live in the soil for over 10 years. The tiny seeds are hard to detect during inspections and can easily be spread to new places by:

  • contaminated soil on machinery, tools, footwear and clothing
  • contaminated crop seed
  • livestock eating plants and spreading seeds in their manure
  • sticking to the coats of animals
  • wind and water. 

Witchweed seeds only germinate when exposed to certain chemicals produced by host plants. Witchweed seeds do not germinate when temperatures are below 20°C. The ideal temperature for germination of purple witchweed, red witchweed and cowpea witchweed is 30–35°C.

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Control

Please do not attempt to treat or dispose of this weed yourself. Report this plant if you see it anywhere in NSW by calling the hotline listed at the top of this page immediately. NSW DPI will lead an initial response for the treatment and disposal of the plant to stop it from spreading.  

Do not touch the plants as they can be covered in small sticky seeds that are easily spread to new locations.

Herbicide options

WARNING - ALWAYS READ THE LABEL
Users of agricultural or veterinary chemical products must always read the label and any permit, before using the product, and strictly comply with the directions on the label and the conditions of any permit. Users are not absolved from compliance with the directions on the label or the conditions of the permit by reason of any statement made or not made in this information. To view permits or product labels go to the Australian Pesticides and Veterinary Medicines Authority website www.apvma.gov.au

See Using herbicides for more information.


PERMIT 9907 Expires 31/03/2025
Fluroxypyr 333 g/L (Staraneā„¢ Advanced)
Rate: 300 to 600 mL in 100 L of water
Comments: Spot spray application
Withholding period: 7 days.
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 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 Prohibited Matter
A person who deals with prohibited matter or a carrier of prohibited matter is guilty of an offence. A person who becomes aware of or suspects the presence of prohibited matter must immediately notify the Department of Primary Industries
All species in the Striga genus are Prohibited Matter in NSW, except the native Striga parviflora

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For technical advice and assistance with identification please contact your local council weeds officer.
For further information call the NSW DPI Biosecurity Helpline on 1800 680 244 or send an email to weeds@dpi.nsw.gov.au

Reviewed 2020