Prickly acacia was first introduced to Queensland in 1890 as an ornamental and shade tree. In 1926 members of the Department of Agriculture and Stock deemed the plant to be of significant benefit as a shade tree for Mitchell grass downs. From here landholders starting planting the tree and it spread rapidly throughout the downs. Due to it’s invasive potential and extensive spread, the plant was declared as noxious in 1957. In the mid-seventies there was a population explosion, but since then the population has been reduced.

Description of Weed
Prickly acacia is one of North West Queensland’s worst woody weeds. It grows as a thorny shrub or small tree up to 4-5m, and occasionally up to 10m. Young trees are sometimes multi-stemmed with an orange or green bark and the mature trees are mostly single stemmed with a dark, rough bark. The leaves of the plant are fern-like, it has small yellow ball-like flowers and the pods are 10-15cm long.

Distribution
Prickly acacia is currently distributed over most of the Mitchell grass downs in varying levels of densities. Prickly acacia covers approximately 6.6 M ha of Mitchell grass downs costing the rural industry around $5 million annually in reduced production and $3-4 million in controlling the infestations. It has the potential to spread into other states of Australia, which has led to the development of a National Containment line for prickly acacia.

Research
The research being conducted involves using the steps from the process of adaptive management to combat the prickly acacia weed problem on the Mitchell grass downs.

1. Develop the plan, which will include steps for removal, follow up and restoration
2. Implement the plan at a large scale field trial level
3. Gather results and analyse
4. Distribute findings to all those who participated in the field trials and other key parties
5. Review the original plan – make changes if necessary
6. Adopt changes into new plan and implement

At each trial site, permanent transects will be established to record the affects of each control technique and the success of restoration techniques. The transects record dead or alive adult plants, seedling recruitment, pasture composition and paddock yields. All findings from the trials conducted will be passed back to the group and other interested parties.

Results
Currently the project has established two groups, one in Hughenden and one in Winton with large scale field trials under way. At these sites, Velpar L has been trialed on broad acre densities for prickly acacia adult tree and has also been trialed to evaluate effectiveness of the chemical on regrowth/seedlings.

The Velpar trials have been very successful and both groups are pleased with the results and have expressed strong interest in pursuing techniques for Mitchell grass restoration. It has become a growing concern of landholders that techniques for eradication and management are demonstrated.

There is a long way to go to achieve sustainable land practices in the Mitchell grass downs, but the use of adaptive management has certainly provided a new, innovative and effective angle for woody weed management.

Publication
The final output of this project will be a booklet containing case studies of the five regions of prickly acacia. The booklet will include the steps taken to achieve restoration of the Mitchell grass downs. It will also include personal comment from landholders relating to their experiences throughout the project.