COVID-19 alert: South East Queensland in lockdown from 4pm, Saturday 31 July. Business restrictions apply.

Annual ragweed

Native to North America, annual ragweed is a fast-growing, fern-like plant.

Annual ragweed can invade and suppress weak and overgrazed pastures, reducing productivity. Its pollen can cause hay fever and aggravate asthma.

Annual ragweed is a restricted invasive plant under the Biosecurity Act 2014.

Scientific name

Ambrosia artemisiifolia

Other names

  • Ambrosia, asthma plant, bitterweed, common ragweed, hay-fever weed, hog-weed, horseweed

Similar species


  • Fern-like plant up to 1-2m tall.
  • Leaves grow in a rosette at the plant's base during early stages of growth.
  • Older leaves are short-stalked, 2-3cm long, opposite at base, alternate at top.
  • Leaf blades are fern-like, slightly rough, deeply divided with hairy undersides, 1-16cm long and 1-7cm wide.
  • Flowers are small, greenish, to 20cm long on upper part of plant.
  • Flower spikes appear yellow when mature because of pollen production.
  • Male flowers grow at top of spike and females at base of spike.
  • Seeds are black, small, top-shaped and rough.


  • Often colonises bare areas on roadsides and banks of watercourses, and can invade pastures from these areas.


  • Naturalised in South East Queensland.
  • Infestations are found near Stanthorpe, Inglewood, Gympie, Gin Gin, Atherton and around Brisbane.

Life cycle

  • Establishes each year, normally germinating from spring to summer.
  • Germination can occur at other times of year in suitable conditions.
  • Flowering is triggered by decreasing day length and occurs from mid-to late March, after which plants die.
  • Late-germinating plants may overwinter before flowering the following autumn.

Affected animals

  • Can cause hayfever and asthma in humans



  • Invades and suppresses weak and overgrazed pastures, reducing productivity.
  • Infestations can become particularly dense in overgrazed pastures.


  • Pollen contains potent allergens that can aggravate asthma and cause respiratory allergies such as hay fever.

How it is spread

  • Seeds spread by floodwater, stock, or in fodder or topsoil from infested areas.


A method that combines mechanical, herbicidal and biological controls with land management practices is most effective.

Physical control

  • Hand-pull young plants; however, anyone prone to allergies should avoid contact with flowering plants and pollen.
  • Minimise infestations by maintaining healthy, dense pastures that suppress annual ragweed germination and growth

Mechanical control

  • Slash or mow before plant sets seed.

Herbicide control

  • Spray plants when young, before flowering (i.e. before the end of December).
  • Herbicides are not selective against legumes and damage to legume species may result.

See the Annual ragweed fact sheet (PDF, 1.9MB) for herbicide control and application rates.

Biological control

  • A leaf-eating beetle and a stem-galling moth have been introduced into Queensland and have reduced the size and vigour of annual ragweed.
  • However, annual ragweed is still a significant problem and other control methods are necessary.

Legal requirements

  • Annual ragweed is a restricted invasive plant under the Biosecurity Act 2014.
  • It must not be given away, sold, or released into the environment without a permit.
  • The Act requires everyone to take all reasonable and practical steps to minimise the risks associated with invasive plants and animals under their control. This is called a general biosecurity obligation (GBO).
  • At a local level, each local government must have a biosecurity plan that covers invasive plants and animals in its area. This plan may include actions to be taken on certain species. Some of these actions may be required under local laws. Contact your local government for more information.

Further information