Quick Facts

Quick facts

• This refers to Striga spp. apart from all those native species.
• All Witchweeds (Striga spp.) are parasitic plants.
• One of the worst parasites, Striga asiatica, has been found in Mackay, Queensland. This species is parasitic on grasses, including cereal crops.
• Native species of Striga in northern Australia do not attack cereal plants and are parasitic on native Australian species only.
• Plants produce millions of tiny seeds which in the case of S. asiatica easily contaminate the cereal crops in which they are growing.

What Does It Look Like?

What is it?

Witchweeds (Striga spp.) are upright short lived (annual) herbs growing to about 40 cm high although some species reach about 1 m. All species are parasitic, with roots that attach to the roots of host plants, and once established in this way gain all their nutrients from the host plants. The stems of Witchweeds are square in cross-section, covered with rough hairs or bristles, and are generally green in colour. The leaves are also generally green. The lower leaves tend to be arranged in opposite pairs along the stem, while the upper ones are alternate. The leaves are small, 5–75 mm long and 1–20 mm wide, narrow (lance-shaped; or linear, long and narrow), and rough textured (scabrous) (Navie 2004). They have entire leaf margins, i.e., without teeth or lobes and lack an obvious leaf stalk (petiole).

The flowers are on short stalks, or are stalkless (sessile), and are borne near the tips of the stems in clusters 10–15 cm long. The flower is a tube divided into an upper and lower lip; the upper lip with two lobes and the lower lip with three lobes. Flower colour varies among species and can be yellow, white, orange, purple, pink or red. The fruit are 2–4 mm long, oval capsules that turn black at maturity and contain numerous tiny seeds (0.2–0.3 mm long), that are brown, amber or blackish in colour, usually with a ribbed surface (Navie 2004).

The most problematic Striga species, S. asiatica, mostly attaches to grasses and is a danger to cereal crops in Australia. Native Striga species grow in native vegetation and are parasitic on native species (Barker 2008 pers. comm.).

For further information and assistance with identification of Witchweeds, contact the herbarium in your state or territory.

Flower colour

Yellow, White, Orange, Purple, Pink, Red

Growth form (weed type/habit)

Herb

Where it currently grows? Preferred habitat

Witchweeds are potential weeds of crops, pastures, open woodlands, and grasslands. They occur mainly in tropical, subtropical and semi-arid areas (Navie 2004).

Are there similar species?

Several native Witchweeds that are parasites of native species only can be easily confused with the potentially more problematic exotic Striga asiatica (Asiatic Witchweed), S. hermonthica (Purple Witchweed) and S. gesnerioides (Cowpea Witchweed). The native species are difficult to distinguish from the exotic ones, and their identity should be determined by an expert (Barker 2008 pers. comm.; Navie 2004).

Asiatic Witchweed is a small plant (15–30 cm high) with relatively small, narrow leaves (10–40 mm long and 1–4 mm wide) and small, often reddish flowers, 5–10 mm across. The flowers are loosely arranged in an elongated spike (Navie 2004).

Purple Witchweed is a larger plant (30–100 cm high) with larger leaves (25–75 mm long and up to 20 mm wide), with relatively large, often pink to purplish flowers 10–20 mm across. The flowers are densely arranged on the spike (Navie 2004).

Cowpea Witchweed is a small plant (15–30 cm) with reduced scale like leaves (5–7 mm long), and small mauve to purple flowers 5–10 mm across. The flowers are densely arranged on a short spike (Navie 2004).

Why Is It A Weed?

What are its impacts?

Agriculture: Witchweeds, particularly Striga asiatica, are particularly problematic in South Africa and the United States of America. They can infest a wide range of grass crops including maize, millet, rice, sorghum, and sugarcane, as well as some broadleaf plant crops, e.g., sunflower and tomatoes (Smith 2002).

Witchweed infestation can be a consequence of monocropping (growing the same crop year after year) with cereals causing declining soil fertility which can weaken the host plants resistance to parasitism. As a result of these cropping practices, Witchweed species have developed high numbers of long-lived seeds in the soil bank, with only some breaking dormancy each season (CIMMYT 2004).

The symptoms of parasitism by Witchweed species can be difficult to distinguish from those caused by drought, such as wilting, curling of leaves, stunted growth and a pronounced 'burnt' appearance to leaf edges. Witchweed infestations can significantly reduce yields (OISAT 2004).

How does it spread?

Witchweeds produce numerous tiny seeds that can be dispersed by wind, water, farm machinery and contaminate agricultural produce (Navie 2004). Autogamy (where self fertilisation and seeding occurs within the flower) has been detected in introduced plants of Striga asiatica in North America, although autogamy has not been observed in native African populations (Nickrent & Musselman 1979).

What is its history in Australia?

In 2013, S. asiatica was found on a small number of properties near Mackay, Queensland. A surveillance programme and control measures are in place to reduce the risk of further spread (Biosecurity Queensland 2013).

How To Manage It?

Best practice management

Witchweeds can be particularly difficult to control given that much of their life cycle occurs underground. Mechanical or chemical control can only be conducted after emergence (Johnson 2003). Hand removal or hoeing before the Witchweed species starts to flower can reduce the seed production of the weed (OISAT 2004).

Non-chemical control: Land management: An important preventative measure in agricultural situations is to avoiding using seed from harvests that have been contaminated (OISAT 2004). Crop rotation with 'trap crops' such as cotton, sunflower, linseed and Striga resistant maize varieties stimulate the germination of Witchweed seeds in the soil bank, which reduced the seed bank, without crop losses due to parasitism (Johnson 2003).

Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au 

Does it have a biological control agent?

NO

When does it grow? (lifecycle/growth calendar)

Witchweeds are parasitic and dependent on a host plant. For Striga asiatica, the most suitable germination temperatures are between 30-35 °C, with no development below 20 °C, and germination will only occur in the presence of chemicals given off by the host. After germination, it can spend several weeks underground obtaining nutrients from its host, after which it can emerge, flower, and produce seed very quickly (Johnson 2003).

Where Is It Found?

Which states and territories is it found?

Not naturalised in any Australian state or territory.

What areas within states and territories is it found?

In 2013, S. asiatica was found on a small number of properties near Mackay, Queensland. The native species occur across northern tropical Australia (Barker 2008 pers. comm.).

Where does it originate?

Many Witchweeds are native to tropical and subtropical regions in Asia and Africa. Native non-weedy witchweeds are relatively common in Australia, particularly in the northern regions of Western Australia (Navie 2004).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

NSW, NT, QLD, TAS, WA

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Striga spp. (other than native species)

Other scientific names (synonyms)?