WILLOWS IN AUSTRALIA
Significance and Distribution
Willows are now widespread in south-eastern Australia. They are valued for river bank protection, landscaping, shade and windbreaks; but their ability to propagate vegetatively and grow vigorously in rivers has caused serious concern and expenditure because it enables them to displace native vegetation and to cause erosion where they divert floods to the banks. Their possible extensive spread by seed is now a further cause of concern.
Worldwide, there are over 300 willow species: mainly in temperate and cold regions of Europe, America and Asia. Willows range from dwarf shrubs to tall treas and nearly all are decidious. Each plant is usually entirely male or female with dozens of tiny flowers borne on a catkin. Pollination is by insects, and sometimes by wind. Planting is virtually by cuttings. Natural regeneration in Australia was thought to occur only by the rooting of accidentally detached cuttings.
Grown from cuttings and given reasonable water supply , willows can thrive over a very wide range of conditions: from Tasmania to southern Queensland, from sea level to the treeline, and from wet to semi-arid areas. Some do best in warm climates; most prefer the cooler climates. In Australia the regeneration of willows from seed is usually restricted to the wet beaches of rivers.
Willows are notorious for hybridisation between species and most of the hybrids are fertile. Plants propagated as cuttings taken from a single donor constitute a 'clone', all members of which are genetically identical to the donor.
All willows in Australia have been introduced. It was thought that most 'species' are represented here by only one sex and that they possibly originated from the introduction of only one clone, a similar situation to that in New Zealand. Only four or five 'species' were reported as spreading naturally in Australia and probably only vegetatively. In Australia willow regeneration by seed has only been recognised since 1993, with observations from Victoria and Tasmania.
Soil conservation and river management authorities have relied on a variety of willow species for bank stabilisation for many years, without realising the problems that can arise from combining different genders and species. Australia has continued to introduce numerious varieties of both sexes. Some of the emerging problem of willows spreading by seed is also due to tree planting by enthusiasts.
CRACK WILLOWS
Crack willow is a member of the Salicaceae family and is classified as Salix fragilis, or, more strictly, as one of its hybrid clones. Fragilis describes the brittle/'fragile' nature of the branches.
The clone affecting Tasmanian rivers is male and to date does not appear to be readily hybridising with other willows on stream banks. This means that the crack willow along Tasmanian rivers has established due to vegetative reproduction, that is, pieces of tree have broken off due to winds, floods, falling over or fishing poles and established into mature specimens. So strong is this characteristic that crack willow is being considered as an environmental weed and will require a long term committment to achieve successful removal from river systems.
Recently, some events have caused vegetation experts to believe that hybridising between crack willow and other willows is possible.
Meander Catchment
In the Meander Catchment crack willows are a significant problem within the riparian areas of the Meander River and its tributaries. A recent assessment of the exent of riparian vegetation along both sides of the river and streams in the Meander Catchment from aerial photographs estimated that 19% of the river banks in the catchment are covered in willows, 58% have native or other vegetation and 23% of the banks are bare. The current distribution of crack willows within the Meander Catchment is generally wherever there has been settlement or significant disturbance of native vegetation.
Effects of Crack Willow on Rivers
Rivers more than 20 metres wide, carry a strong flow in a clear, well-defined channel, are generally less effected by the presence of crack willow in regard to channel movement and flooding. Those rivers and tributaries less than 20 metres wide can be severely affected, with willows blocking the channel causing excessive flooding, erosion and channel movement.
In-stream stands and branches are often responsible for debris accumulation (log-jams) and the autumn leaf drop suddenly puts large amounts of material into a system that has evolved to accept the leaf fall from native evergreens.
Even if the presence of crack willow is having no present effect on the river system in terms of channel movement or obstruction, the invasive nature of this specimen can lead to a monoculture (single species) along the river system, which does little to support the native flora and fauna (biodiversity) and will, as a general rule, continue to grow and spread until it does begin to effect the system.
As the stands of crack willow become thicker, it creates its own "environment" in which nothing but crack willow grows.
Benefits
A considered approach to willow removal (asthey provide bank protection via a large root mat) and a documented plan of activity is critical, as unconsidered removal has the potential to leave the river banks exposed to erosion.
This root mass is the reason why willows growing on the banks should not be dug or pulled out - leave the root mat in place to protect the soil while the replacement vegetation is taking hold. The willow roots will rot down over a few years and usually do not cause many problems, provided they have been treated (dead) and/or regrowth is controlled.
However, willows growing in the river bed may be removed root-mat and all, as the river will generally fill the hole with gravel sands etc. Alternatively, the hole may remain and add to the habitat variety of the stream. Seek advise on this as too many holes in the bed may start an erosion problem so be careful, seek advise.
CONTROL MEASURES
FOLIAR SPRAYS - Suitable for small trees and regrowth up to 2 metres in height.
- With spray unit, apply chemical/herbicide to tree in full leaf but before leaves change colour in Autumn.
- Trees should have reasonable leaf area (1-2 metres in height) but at such height that spraying upwards is avoided.
- Glyphosate at a dilution of 1:100 has proved effective in treating regrowth.
- Once pronounced dead, trees may be cut down or pulled out.
Application
- Young willow trees.
- Recently infested areas.
- Managing Wiilow regrowth after initial treatment in previous year(s).
Limitations:
- Suited to small plants less than 2m, yet must be large enough to provide reasonable leaf cover.
- There is the possibility of affecting non-target species.
- Care must be taken when spraying around waterways. If spraying near waterways it is absolutely necessary, then spray from the stream side outwards onto land, NOT from land towards the stream.
- Knowledge of safe chemical handling application and use of protective clothing is essential.
Comments:
- A usefull means of tackling young willow treas and regrowth as part of ongoing management of willows.
CUT STUMP METHOD
Decription:
- Treecut off at or just above ground level.
- After the stem or trunk has been cut through with a chainsaw, immediatly all the sapwood out to the bark is targeted, squirting, brushing or swabbing a herbicide onto the cut surface. Undiluted Roundup (Glyphosate) and Garlon 600 (Tryclopyr) diluted 1:3 with distillate have proved successful.
- Once removed stem branches and leaves may be removed from site or cut into manageable pieces, packed tightly onto a small pre-lit fire and burnt on the spot. - i.e. burnt green.
- Best carried out whilst trees are in full leaf, from Summer to early Autumn, before leaves change colour.
Variations:
- Where trees or limbs are overhanging streams then such limbs could be secured by chain to a tractor before severing. Similar large such limbs could be secured by an excavator with crab-grab attachment prior to severing.
Applications:
- Ideal for willows on stream banks where it is necessary to leave root systems intact.
- Best suited to trees or limbs not overhanging streams.
- Treatment, removal and clean up are immediate.
- There is minimal herbicide use and wastage as well as minimal possible environmental contamination with herbicide.
Limitations:
- Best suited to trees or limbs not overhanging streams.
- Falling live willows provides many green twigs on the ground or in the stream which may be a source of propagation of new willows - thorough clean up is necessary.
- Not practical for removing in-stream willows.
- Netting of stream may be needed and/or people located down stream to collect dislodged branches.
Information source:
"Willow Management Guidelines" Parker & Bower, Landcare Tasmania, 1996.
"Willows spreading by seed - Implcations for Australian River Management" Cremer, Van Kraayenoord, Parker & Streatfield, Australian Journal of Soil & Water Conservation Vol.8, No4, November 1995.
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