About the Catchment Area : Issues & Problems

Water Quality - Deciduous Leaves

Many people have grown attached to the seasonal colour changes of the willow, ash, elm and poplar, but for the managers of waterways across Australia, autumn represents a time for dramatic negative effects on the ecological functioning of our watercourses.

The following facts relate specifically to willows, but they also apply to all exotic deciduous trees when found on a watercourse.

• The dense shade from the canopy of a willow reduces the growing opportunity for native riparian (watercourse) vegetation, resulting in banks devoid of groundcover and prone to erosion.
• While native plants have a fairly constant rate of leaf fall throughout the year, willows drop large quantities over a short period (autumn). This results in high levels of nutrients which strip the water of oxygen, resulting in the death of native aquatic animals and increase in potentially toxic algal blooms
• Willows provide poor habitat and few food resources for native animals.
• Willows can cause dramatic changes to the channel shape of a watercourse. The shallow, dense root mat of a willow growing out into the watercourse can cause bank widening and erosion.
• Willows spread mostly from broken branches and twigs taking root. This means that willows can spread rapidly and dominate entire reaches in short periods, if not controlled.

Read more about the problem caused by exotic deciduous trees....

While the Board is able to assist with exotic tree control in some cases, demand for assistance far outweighs available resources. The mammoth task of protecting and rehabilitating our waterways can only be achieved if landholders are adequately informed and prepared to tackle the problems in concert with the Board.

Removing exotic trees

Before you sharpen up the chainsaw ...
If your site is in the urban area or near a hills township, recent Significant Tree legislation may require that approval is obtained from your local council prior to the removal of any tree with a circumference of more than 2.5 m.

Bear in mind that tree removal can be a dangerous job! Make sure you are adequately equipped and skilled. If in doubt, consult a professional tree removal contractor.

Willow control is only one aspect of the planned approach to guide all good watercourse rehabilitation. A rehabilitation strategy should include fencing the watercourse to exclude livestock; revegetation as soon as possible after willow removal, and follow-up weed control (both for willow regrowth and herbaceous weed growth).

Landholders can access technical advice from the Board when planning watercourse rehabilitation, and should do so before undertaking extensive exotic tree removal.

Control methods
All outlined control methods should occur while the willows are green and leafy and the environment is reasonably dry (November to April). Appropriate herbicides to use in these applications include glyphosate (such as Roundup Biactive TM). This can be used undiluted, but some operators report better results when diluted 1:1 with distilled water. These procedures may need to be repeated if spring regrowth is evident.

• 1) Cut and poison: In this approach, the willow is cut down and herbicide immediately applied to the cut stump. The stumps will rot away after 3-5 years. This can be expensive, as it requires heavy machinery to pile the debris for burning.
• 2) In-situ poisoning: The tree is killed by frilling the living tissue (cambium layer or sapwood, figure 1) at the base of the trunk and filling the wounds with herbicide. Alternatively, holes can be drilled at a 45 angle into the cambium layer every 5-10 cm around the base of the trunk, then filled with herbicide. The tree is left to die and decompose over a number of years. Revegetation commences in the autumn/winter after the tree has lost its leaves. This is by far the cheapest option as it’s easily performed by the landholder for the cost of the herbicide. The main drawback lies in aesthetics, and where fallen timber may clog nearby infrastructure, such as culverts.
• 3) Poison and cut: Willows are killed in the same way as in-situ poisoning, but once dead, the tree is cut down and removed. This reduces the risk of willows spreading via living branches and twigs breaking off during the removal stage and re-establishing. The additional safety risks associated with lopping dead, brittle timber must be considered. This method is often as expensive as the cut and poison approach.
  Any use of poison next to a watercourse requires careful consideration and effort. If you have any concerns about the use of herbicides, contact your Catchment Water Management Board or Animal and Plant Control Board in your region.

It is essential that you are well prepared before undertaking any rehabilitation works along your watercourse. This not only includes assessing your equipment and skill, but also ensuring you have an action plan that considers at least the first three years of ongoing work required to achieve good watercourse management objectives.