Witchweed

Striga spp.
Herb
Alternate
Simple
Multi-coloured
Green

Native to semi-arid and tropical Africa, and recorded in more than 40 countries, 'witchweed' is a common name loosely applied to up to 30 different species in the genus Striga. All witchweed species are small, parasitic herbs. 

Non-native witchweeds are currently not found in Australia but have the potential to become costly pests in Queensland. An opportunity exists to prevent their naturalisation here.

Witchweed is a prohibited invasive plant under the Biosecurity Act 2014.

Common names 
Also known as: Witchweed,
Family 
Orobanchaceae (also placed in Scrophulariaceae)
Deciduous 
No
Flowering time 
Flowering time varies with species and environmental conditions
Native/Exotic 
Exotic
Origin 
Native to semi-arid and tropical Africa
Notifiable 
Yes
State declaration 
Category 1
Council declaration 
As per State Declaration
Known distribution 

Recorded in more than 40 countries, non-native witchweeds are currently not found in Australia.

Habitat 

Prefers semi-arid savannahs and grasslands, generally in tropical areas but sometimes subtropical and warm temperate areas. Grows on roots of other plants.

Habit 

Witchweeds are parasitic herbs that grow on the roots of host plants. Some species only emerge above the ground to flower. 

Heavy infestations can reduce grain crop yields by up to 70%. Causes export grain to be rejected. Affects maize, millet, legume crops, rice, sugar cane and sorghum. Has the potential to become serious pest if introduced or cultivated as ornamentals.
USA has spent more than $250 million on eradication from the Carolinas. Striga are obligate parasites that draw nutrients, water and carbohydrates from the roots of their hosts. 

Stem and leaves 

Herb up to about 30cm tall, leaves are 6-40mm long, 4mm wide, with tapered pointed tip. Morphology varies across the genus.  In general, Striga have bright green stems that are square in cross-section and 1–2.5 mm in diameter. The stems are sparsely covered with coarse, short, white, bulbous-based hairs. Leaves are nearly opposite, narrowly lanceolate, with successive leaf pairs perpendicular to one another. Underground stems are round with scale-like leaves and white, but turn blue when exposed to air.  The roots are succulent, round, without root hairs, and attached to the root system of hosts.

Flowers and fruits 

Flowers are 5-8mm long and can be red, pink, white, yellow, orange.or purple. The fruit is a five-sided capsule with narrow wings. Capsules contain 250–500 tiny dust-like seeds, 0.15–0.2 mm long, can persist for 15 years.

Reproduction and dispersal 

A single Striga plant can produce thousands of seeds.Seeds only germinate in close proximity to roots of suitable host. When a host is

present, seeds require 1–2 weeks of moisture and temperatures of at least 20 °C (with 25–35 °C being optimal) before they will erminate.

Some species only emerge from below ground to flower (hence name ´witchweed´). Seeds spread by wind, water, soil movement, feet, fur or feathers. Seeds could be contaminant of imported grain/pasture seeds or in soil adhering to imported machinery. Seeds remain viable in soil for up to 15 years.