Annual Ragweed

Ambrosia artemisiifolia

Annual Ragweed is listed as a priority invasive plant for the Sunshine Coast Region. It is a category 3 restricted invasive plant under the Biosecurity Act 2014. It must not be given away, sold, or released into the environment. Penalties may apply. We must take all reasonable and practical measures to minimise the biosecurity risks associated with dealing with annual ragweed under our control. This is called a general biosecurity obligation (GBO). At Crystal Waters Sunshine Coast Council has required us to control this weed since it was reported 3-4 years ago. Further information on Annual Ragweed can be found here: Annual Ragweed.

Information and Identification

Annual Ragweed originally comes from North America and is a fast-growing but short-lived herbaceous weed which normally dies at the beginning of winter, although here some plants survive the winter and start growing again in spring.

It is mostly a weed of degraded pastures and can seed prolifically, with the seed lasting for many years in the soil. The seeds are easily spread by animals or by machinery if the plants are left to seed then slashed. Its pollen can cause hay fever and aggravate asthma.

Identification:

• Up to 1–2m tall (or even 3-4 metres tall here!).
• Fern-like leaves which 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 slightly rough, deeply divided with hairy undersides, 1–16cm long and 1–7cm wide.
• Closely-packed plants tend to be single-stemmed, but isolated plants can develop a multitude of stems from a single base.
• 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.

Impacts

Economic

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

Social

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

Distribution at Crystal Waters

Annual Ragweed is widely distributed at Crystal Waters, with new infestations appearing each year. Since it was reported here about four years ago, I have mapped the distribution of most infestations each year.

In the above maps you can see the changes in distribution each year (brown in 2022, lighter brown in 2023, orange in 2024), as earlier patches shrink in size and new patches appear. In general our control efforts seem to be relatively successful at reducing the size of existing infestations, but we are obviously missing some plants each year which seed new areas.

Control methods and efforts

Control at Crystal Waters has focussed on trying to prevent all plants from flowering so that there is no seed production. Larger infestations are slashed, while smaller ones are brush-cut and individual plants are removed by hand. If plants are cut too early, they just regrow and produce seeds. However, if they’re left too late we miss the critical time for seed production. So most control work is done in February-March, when the plants are just starting to produce buds. The main flowering period is generally towards the end of March.

The table below shows the distribution and control efforts in all the main infestations over the last 3 years: