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Willows Management |
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Willows and their management Worldwide there are over 300 species of willow (genus: Salix). Most of these originated from Europe, America and Asia (Cremer et al 1995). In Australia around 100 species of willows have been introduced, often through nurseries. Canberra has been a major centre for introductions. Some of the more common and widely introduced willows include the Weeping Willow (Salix babylonica), Crack Willow (S. fragilis), Black Willow (S. nigra), Golden Upright Willow (S. alba var vitellina) and Pussy Willow (S. cinerea). There are also many types of willow resulting from the crossing of different willow species. These are known as willow ‘hybrids’, and they are generally fertile. Many different willow hybrids have been identified but precise identification can be difficult in many cases (Cremer et al 1995). It is thought that all fertile willows can hybridise as long as their flowering times overlap, and that some of these hybrids are superior to their parents in terms of survival (Cremer 1999). Willows have different methods of spreading. One method is by seed where separate male and female willows cross pollinate to produce seed. Willows can also spread vegetatively where bits of the plant break off and take root. Many willows are also intentionally planted or unintentionally moved around by machinery. Willows are valued by land managers for many reasons including river bank protection, landscaping, shade, and windbreaks. Some species such as the Weeping Willow are also attractive to many people. Many willows are well adapted to the riparian zone and Australian conditions, and some are very capable colonisers. This can result in significant environmental and economic impacts (CRC Weed Management 2003).
It is important to make a distinction between problem species and non problematic willows because this has implications for riparian management. Most species of willows in Australia are now on the ‘Weeds of National Significance’ list (CRC Weed Management 2003). The significance of this is discussed in the Legislation section. The most problematic species include those that spread by seed - for example Black Willow, Tortured Willow, Purple Osier and Pussy Willow. Aggressive willows that spread vegetatively such as Crack Willow also pose a major problem. Some of the issues connected with these species are outlined in Cremer et al, 1995; Cremer 1999; and CRC Weed Management 2003. These include:
Strategies employed nationally for management of problem willows include restrictions on importation, sale and planting of problem species, and the total eradication of the most aggressive species (Black Willow and Pussy Willow). Millions of dollars are spent every year in south-eastern Australia controlling willows (CRC Weed Management 2003).
Weeping Willows and two hybrid species of Pussy Willow (S. x calodendron and S. reichardtii) are not listed as Weeds of National Significance as they don’t readily reproduce (CRC Weed Management 2003). Only female Weeping Willows were introduced into Australia. However it has been discovered that they are now crossing with males of some problem species because flowering times are aligning as willows adapt to local conditions.
Any willow control program should ideally be undertaken in a strategic manner based on surveys of willow distribution. This is not always possible since some land managers may not want their willows controlled. A pragmatic course of action should be undertaken. Some basic principles include:
Control methods include hand pulling, cut stump, stem injection and foliar spraying with Glyphosate (registered for use on waterways). The technique(s) chosen should minimize impacts on waterways. Large willows should ideally be poisoned prior to, or at the time of removal by, experienced contractors to minimise the risk of spread. Wholesale removal can result in erosion, and every branch that breaks off can potentially reshoot elsewhere. It is also important to replace willows with other (preferably native) vegetation. In dense infestations a phased approach may be required that integrates revegetation over many years. This is especially important where no other canopy species exist and waterways are left in danger of a lack of shading and high water temperatures (which is detrimental to aquatic life).
Another very important consideration is the management of dead willow debris. In some waterways it is acceptable to leave a certain level of debris where there is no threat to infrastructure downstream. Willows can start to break down within around a few years depending on type, location and climate (pers. comm. Miners 2006). In other areas all debris needs to be removed to protect infrastructure such as bridges and fences and to preserve visual amenity and recreational access. The physical removal of debris adds significantly to cost so the benefits need to be carefully assessed. Physical removal needs to be carried out carefully to protect banks from erosion. Excavators are usually the most appropriate machinery for this task, particularly for mid stream willows (preferably with a log grab attachment). For willows located on banks the root ball is commonly left in situ to prevent erosion. This work requires experienced chainsaw operators working alongside excavators who can ensure safe and effective removal. There is a recently discovered sawfly larvae (Nematus oligospilus) affecting willows in the southeast region of NSW and South Australia. Previously unknown in Australia, it is now well established on willows around Lake Burley Griffin and anecdotal evidence suggests it has been around the area since the summer of 2003-04 (CSIRO 2005).
Originally from the northern hemisphere, the sawfly was found for the first time in southern Africa in 1993/94. A sawfly previously identified as N. desantisi (but probably the same species) was found in Argentina in about 1980. In both places, the sawfly has been associated with large areas of willow defoliation.
By 1997 the sawfly had spread to Rotorua in New Zealand, and by 2004 had spread throughout both islands (Charles et al 1999). In their area of origin, outbreaks of sawfly larvae tend to be short lived although repeated defoliation can slow tree growth and even result in mortality (especially during drought). In New Zealand, it has caused severe damage to willow plantings. In Australia the extent of outbreaks are unknown. However, in Canberra the sawfly larvae have resulted in the repeated and severe defoliation of willows at some sites (CSIRO 2005). The main species of willow affected in New Zealand include Crack Willow, Weeping Willow, Tortured Willow, Golden Willow, Pencil Willow and Peking Willow (Charles et al 1999). Implications of the sawfly larvae on the success of willow control are unclear. It is unknown at this stage whether willow control will be made easier or more difficult as their presence may impact on the capacity of the plants to take up herbicide (in that defoliated trees may be less vulnerable to the application of herbicides). Research on this is currently being undertaken (pers. comm. Holland-Clift 2006). Further information can be obtained from: http://www.ento.csiro.au/about_insects/willow_sawfly.html Willow Management in a National Context The ‘Willows
National Priority Action Framework’ ( There are a number of strategies and documents in the ACT relevant to control of willows. These include the Upper Murrumbidgee Willow Management Strategy and the ACT Weeds Strategy. While willows are not specifically mentioned in this latter document, it outlines a strategy that is relevant to willow control. There is also a Weeds Working Group made up of representatives of the community, various government agencies and commercial industry. This group:
Willow management forms a part of the focus of these activities. |
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