Weed Alert: Hawkweed

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Hawkweeds (Hieracium spp.)

World Status | Identification | Growth and Spread | Control | Legislation

Hawkweeds (Hieracium species) have potential to be serious weeds in the temperate areas of south-eastern Australia, including the Australian Alps and Tasmanian grasslands. Prompt treatment of known populations of Hawkweeds has limited their spread so far.

Hawkweeds are highly invasive plants forming dense stands of up to 3800 plants per square metre. This is a major threat to biodiversity in conservation areas and native grasslands. Hawkweeds can also be a problem in pastures, on roadsides and in gardens. They are frost-tolerant and competitive across a wide range of soil types, preferring cool climates with annual rainfall above 500 mm.

World status

The genus Hieracium includes several hundred species known as hawkweeds. Hawkweeds belong to the Asteraceae or daisy family and were promoted as cottage garden plants.

Hawkweeds are native to the northern hemisphere, South Africa and South America. Several European species have become major weeds of pastures, gardens and natural areas in eastern North America, Japan, Patagonia and New Zealand.

Four Hieracium species are naturalised in Australia. These include Orange hawkweed (H. aurantiacum) in Victoria and New South Wales (NSW), King devil hawkweed (H. praealtum) in Victoria and Wall hawkweed (H. murorum) in NSW.

Small infestations have been found around ski fields as seed was introduced on equipment from New Zealand. Mouse ear hawkweed (H. pilosella) is widely distributed in New Zealand and a small population was found in Tasmania in 2001.

An eradication program has prevented any further spread. Hawkweed plants were previously sold by nurseries and these are likely to be sources of further infestations.

Identification

The general appearance of the hawkweeds is similar to dandelion (Taraxacum officinale) or flatweed (Hypochaeris sp.). They are perennial herbs of variable height (15–40 cm), and have a milky sap which is seen when their stems or leaves are broken.

Leaves

Plants form a rosette of stalkless leaves, hairy on both surfaces, with smooth or slightly toothed margins and are sometimes ‘sticky’ to touch. Occasionally 2–4 alternate leaves appear near the base of the upright stem. The flower stems grow up to 40 cm high and are covered in short, stiff hairs.

Flowers

Hawkweeds have yellow, orange or red ‘daisylike’ flowers that may be solitary or form a cluster of 5 to 30 flower heads. The flowers are 10–20 mm in diameter and have square-ended petals.

Seeds

The seeds are purplish-black and ribbed with a bristly tuft up to 6 mm long.

Growth and Spread

Hawkweed can reproduce and spread both by seed and vegetatively. Vegetative spread is the most common form of local spread and occurs when daughter plants form at the ends of the stolons. Stolons arise from buds at the base of the leaves.

Hawkweed seed usually germinates in spring after rain. Seedlings establish readily on bare soil and disturbed areas. Up to 40,000 seeds per square metre are produced in summer. Seeds have tufts that enable them to attach to hair, fur and vehicles. Seed can also be spread by wind, water, in contaminated fodder, garden waste and even ski or hiking equipment. The seeds can survive in the soil for many years.

Control

For small, scattered patches, dig out the plants, ensuring that below-ground growth is removed. Care must be taken to remove flower and seedheads which should then be burned. Follow up monitoring and control will be required for many years.

In NSW, a permit is available for the use of Grazon DS© herbicide for the control of hawkweeds in pastures and non-crop situations. Refer to the APVMA permit 11637 (expires September 2014) for full details and use patterns.

Competitive, well managed pastures are successful in reducing the size and impact of infestations.

Legislation

Hieracium sp. are declared noxious throughout NSW as Class 1 weeds under the NSW Noxious Weeds Act 1993. The plant must be eradicated from the land and the land must be kept free of the plant. The plant must not be sold, propagated or knowingly distributed. As a notifiable weed all outbreaks of hawkweed must be reported to the local council within three days.

Acknowledgements

Prepared by Annie Johnson. Comments provided by Scott Charlton, Andrew Storrie, and Birgitte Verbeek, NSW DPI.

References

• Burton, J. and Dellow, J. (2005) Hawkweeds. Agfact P7.6.58. NSW DPI, Orange.
• CRC for Weed Management (2003). Orange Hawkweed Weed Management Guide.
• Williams N.S.G and Holland, K.D. (2007) The ecology and invasion history of hawkweeds (Hieracium species) in Australia. Plant Protection Quarterly. 22(2): 76-80.
• National Herbarium of NSW. PlantNET- FloraOnline. www.plantnet.rbgsyd.nsw.gov.au. Accessed 24 January 2008.