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. Crop losses can be as high as 100%.
Worldwide at least 11 species of witchweed are known to attack crops. The most significant species are Striga hermonthica, Striga asiatica (both on cereals) and Striga gesnerioides (on legumes). All Striga species except for the native Striga parviflora are Prohibited Weeds in New South Wales (NSW).
Parasitic weeds like witchweed are among the most destructive and difficult-to-control weeds in agriculture.
Witchweeds are native to tropical Africa, India, the Middle East and China. They infest an estimated two-thirds of all cropping in Africa.
Striga asiatica has the widest distribution and is listed as a weed in 35 countries, including a large infestation in the United States.
In July 2013 Striga asiatica was found on a small number of properties near Mackay in Queensland. Witchweeds are not known to occur in NSW. Specimens of the native witchweed (Striga parviflora) have been collected from woodlands on the north coast and central western slopes of NSW.
Witchweeds are dependent on a host plant, only germinating when exposed to certain chemicals that host plants give off. A number of witchweed plants can attach to a single host plant.
For Striga asiatica, Striga hermonthica and Striga gesnerioides the most favourable temperatures for germination are 30°– 35°C, and germination does not occur when temperatures are below 20°C. After germinating, plants spend the first four to seven weeks underground obtaining all their nutrients from their host.
After emergence, witchweeds can flower and produce seed rapidly. Each plant is capable of producing at least 50 000 tiny seeds. These may remain viable in the soil for over 10 years.
Seeds are spread short distances by wind, and further by water and soil attached to animals, machinery, tools, footwear and clothing. Contaminated crop seed is the most likely way for witchweeds to be introduced into an area.
The presence of witchweed may be indicated by symptoms in the host plant, which are similar to severe drought stress, nutrient deficiency and vascular disease.
Key identification features
Witchweed prefers intensive agriculture where frequent crops, monocultures and fertilisers encourage growth and seed production.
2006 edition prepared by Annie Johnson; 2013 edition prepared by Elissa van Oosterhout; Reviewed by Phil Blackmore, Birgitte Verbeek
Holm LG, Plucknett DL, Pancho JV and Herberger JP (1977) The world’s worst weeds: Distribution and Biology. The University Press of Hawaii, Honolulu
Control is difficult as witchweeds are inaccessible until they emerge, by which time it is usually too late to prevent yield losses. Rotations with trap crops that stimulate witchweed germination can be beneficial. Contact your local council weeds officer for assistance if you suspect you have found witchweed.
See Using herbicides for more information.
PERMIT 9907 Expires 31/03/2020
Fluroxypyr 333 g/L (Starane™ 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
All species in the Striga genus are declared except the native Striga parivflora
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State Prohibited Weed
The plant must be eradicated from the land and that land must be kept free of the plant