Willow Control Methodology and Debris Management
All willow debris in the Lake area and lower parts of the Molonglo River needs to be removed following or at the time of poisoning (except for very small shrub willows and seedlings). Leaving willow debris would potentially impact on recreation, aesthetics and/or public safety. Damage to off-site infrastructure by isolated debris is a remote possibility due to the lack of stream flow and the ‘soft’ nature of willow wood. However, a large quantity of debris may result in damming and blockage of spillways and overflows on Scrivener Dam and this could lead to potential damage of the dam and adjacent infrastructure.
There are two main options when managing debris. The most appropriate method depends on willow density, size and location.
The first is to cut and poison the stump as the tree is being removed. The tree then needs to be mulched and/or removed well away from wet areas and the area ‘raked’ for live branches and twigs, which could potentially take root. This requires extremely experienced (and preferably accredited) contractors, as every part of the plant that breaks off has the potential to reshoot, possibly creating a bigger problem than the one being addressed.
The second method is to stem inject willows and poison them in-situ followed by removal of debris 12-18 months later once they are completely dead. This is the preferred method as is minimizes the risk of further spread, however it may not be acceptable in highly visible areas (although, given the current willow sawfly infestation, this may be less of an issue). In the case of high use areas it will be important to remove debris before the willows start to break down. This usually doesn’t occur for several years after poisoning. This method may be an option in some less visible areas, but the issue of public amenity needs to be considered. Given that willows look dead for the colder months of the year and people are used to this, this method may be acceptable.
Glyphosate 360 is the only chemical registered for use around (though not in) aquatic areas. Special permission may be granted by the Environment Protection Authority (EPA) for the use of other chemicals or over-water spraying. This is perhaps more relevant for control of weeds, such as Blackberry (Rubus fruticosus), that infest areas after willows are controlled.
Control can be carried out at any time of the year if experienced contractors are used. Ideally, however, it should be carried out around late summer to early autumn.
In more remote areas of the Molonglo River it would be acceptable to leave willow debris, providing that the guiding principles were adhered to and an assessment was made of potential impacts downstream. Landholders consulted through this process expressed a concern about willow debris and the impact it has had in the past on their floodgates along tributary streams such as Jerrabomberra Creek. There are no floodgates on the Molonglo River within the Plan area.
The Molonglo River is a priority as a major source of infestation of Crack Willows, although infestations upstream of the NSW border would need to be given consideration as an ongoing source of willows. A coordinated approach needs to be taken to maximise funding investment.
The surveys elicited a range of information to guide on-ground management actions – for example densities, depth of infestation between water and the bank, height, dominant sex and so on. Maps presenting this information are attached as Appendices. An interactive version of the on-ground surveys is also available on GIS where a variety of layers can be displayed together. This information will also help to give an indication of potential cost.
A selection of maps have been attached to assist with on-ground operational management, however use of these layers on a GIS system will be more beneficial as layers can be switched on and off as needed and will be easier to overlay.
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