Generally, willow seed is very small, very short-lived (2-4 days), produced in massive quantities over several weeks, highly fertile, capable of dispersal over very large distances by wind, and very specific about the characteristics of a suitable seed bed. This means that usually most seeds do not survive but, when conditions are suitable, hundreds of thousands of seedlings can establish.
As willow seed is very short-lived, all of the early introductions of willows were made as cuttings or pot plants. This meant that most of the plants of any species, or at least the plantings at any one site, were derived from one or very few individuals. As willows are mostly either male or female this has meant that most of the clones were unisexual and plantings rarely produced seed or seedlings. Seed was only possible from hybridisation of two different species where the correct sexes were present; the flowering times overlapped; and the trees were closer than about 3-500 metres, (about the maximum effective distance for the pollinating insects.) The sparcity of plantings, other than massed single-clone plantings, and the lack of overlap of flowering times for the different species has meant that seeds were hardly ever formed.
Over the years more species and cultivars have been introduced. Some of these cultivars were hybrids between other species, often very rare. If such hybrids are planted, and either or both parent species are around, they can overcome the problems of non-overlapping flowering times, and also reduce the inter-species fertility barriers. There is now an increased opportunity for fertile and viable seed to be formed. As trees aged, the longer they tended to flower and the more likely that overlaps occurred. All such situations have occurred and there are now areas where there has been mass colonisation by seedling willows. Although these are, so far, usually serious to the 1ocal area only Salix cinerea, (a Pussy Willow), has proven to be a serious problem over large areas to date. It is a serious problem in Victoria, becoming a problem in southern N.S.W., but we now also have a population producing seeds around Sydney.
- Weeping Willow, Salix babylonica. Almost part of our heritage .The combination of rust and beetles means that the Weeping Willow is slowly becoming less common.
- Crack Willow, Salix fragilis; one of the commonest willows. Until very recently we thought that we only had male trees in Australia. Weeping Willow and Crack Willow can produce hybrids. One of these, sometimes called Salix x pendulina has been planted quite extensively. These new hybrids can produce fertile seeds by crossing among themselves or backcrossing to either parent. The progeny look like either Crack Willow, Weeping Willow or anything in between. This situation has developed at the junction of the Hawkesbury and Grose Rivers.
- Black Willow (Salix nigra) is native to America. It is now the willow species causing greatest concern. Most recent plantings of Black Willow have been derived from seedling stock. This means that both sexes are present in most plantings. Seedlings have been found in every area where the now-mature trees have been planted. In some areas there has been an incredible spread and establishment in those 20 years. It is now spreading along some of the tributaries of the Murray.
- What are thought to be hybrids of S. nigra x fragilis and S. nigra x babylonica have been identified. These are the two commonest species naturalised. There is a potential for these hybrids to backcross even more readily with either parent or to cross with other hybrids.
- S. alba has been reasonably common in Victoria for some time but since about 1950 it has become fashionable to plant one of the varieties, S. alba var. vitellina in N.S.W. This is a female clone and crosses readily with S. fragilis to produce the fertile hybrid, S. x rubens, which can backcross with either parent as well as producing a fertile F2. It is this complex that has caused problems on the Southern Tablelands (e.g. Numeralla River) and contributed to the problems in the Bega River.
- Salix matusdana x alba is a hybrid developed in New Zealand and then imported into Australia. Both sexes were imported and planted indiscriminately, and seedlings can now be found at several localities. One clone imported produces flowers of both sexes on the one catkin. S. matsudana is closely related to the Weeping Willow and male clones may produce fertile crosses with that species. The Tortured Willow, Salix matsudana cv. ‘Tortuosa’ (female) will also produce viable seed with the hybrid. The S. matsudana x alba hybrid is widely planted and the seedlings produced have caused serious problems in the Bega River.
- The Chilean Willow, Salix humboldtiana, is now quite commonly cultivated. It is quite closely related to Black Willow and could theoretically interbreed.
- Another species of concern is Salix x chrysocoma. This hybrid (S. babylonica x S. alba var. vitellina) was developed to produce a weeping tree that will grow in conditions too cold for S. babylonica. It is reasonably common around the Bathurst-Orange district. The main reason for concern is that it produces bisexual catkins, i.e. both sexes on the same catkin.
- Another group of species is the so-called Pussy WiIlows, Salix cinerea and hybrids. Most trees or shrubs grown as this are a sterile hybrid, S. x reichardtii, and are no cause for alarm. There is one sterile female hybrid (S. x calodendron) that is planted for stabilisation projects and is of little concern. The main give-away is that seedlings are readily produced if both sexes are present, and these seedlings are far less fussy about their seedbed than most willow seedlings. We have now found a seeding population in Sydney so it is yet another species to watch out for. If you only have male plants of any Pussy Willow there is no need for concern.
- The final group of concern is the Osiers or true Basket Willows. In Australia these are mainly S. viminalis, S. purpurea, or hybrids between them. They have mainly been planted in areas around the Snowy Mountains and Victoria. When females are planted near old plantings of males, or mixed sexes are planted together, seedling swarms have developed. The S. purpurea in Khancoban creek is a good example.
Dealing with the problem
Many people, once they hear that willows can be a problem, are convinced that the answer is simple, just kill all willows. Unfortunately life is rarely that simple. The first step is to identify any problems, identify if the species cause or exacerbate any of the problems, and then attempt to fix the problems. Controlling the species may be the logical option, but there are plenty of other examples where the species are often an indicator of massive changes and trying to control the species without trying to affect the major cause of the problem if not futile, is at least a job in perpetuity. Good examples of the latter are Water Hyacinth and Alligator Weed in the Hawkesbury, and many of the Thistles common as pasture weeds.
We have altered much of the habitat around us such that the original native species no longer have their competitive advantages. ‘New’ weedy species in many cases prove to be much better adapted to this modified landscape. If we take an irrational dislike to some of these species and attempt to eradicate them then we open the possibility of causing more or different problems; and replacing the weed with a species resistant to the control methods being used.
The next step is to identify the problems caused by willows.
- They can produce massive amounts of wind-dispersed seed and enormous numbers of seedlings in seedling swarms. These seedlings spread between catchments and become established in rivers and wetlands and dominate banks and islands.
- Willows produce stem pieces that spread within catchments.
- Willows do not provide suitable habitat for native animals or birds that rely on tree hollows.
- The massive leaf drop over a short period changes the invertebrate populations, and hence the vertebrate populations, usually reducing species richness.
- When they do fall into the stream, willow wood breaks down very quickly and does not form suitable habitat for native algae, invertebrates and vertebrates.
- The seasonally dense shade does not allow other species to establish underneath willows.
- Although willows are often planted in attempts to stabilise steam banks, they have been implemented in scenarios where rivers have changed bed (course), causing problems with our land ownership and management systems.
The benefits claimed for willows include:
- They look ‘nice’. This is a value judgement and is hard to comment on.
- They stabilise river banks, survive the techniques that engineers use and are cheap to use.
This is true but because of the misplaced criteria that have been used for selecting planting material, it is usually the most weedy and least effective species that have been used. There has been very little experimentation to see what native species are appropriate and how techniques could be changed to allow their use. Willows are forced to become tall and pot bound and then planted at the bottom of a deep hole with just the leafy tops showing. Everyone knows that Australian native plant species will not survive such treatment. Well think again, some of them do!!
What we need now is some studies to show which species, where and how.
- They are good stock feed.
We also need to look at potential problems if willows are removed:
- Increase in erosion through lack of stabilisation.
- Increase in flow velocities, potential for more erosion, higher flood peaks but shorter duration.
- Potential for them to be replaced by other weedy species.
- Potential for them to damage structures if large amounts of material are released at once.
We need to consider all of these points before we develop a management plan for Willows While it is possible to overcome most problem situations, it is not possible at this stage to overcome them all at every site.
The major problem to consider at any site, because it can affect distant catchments, is the production of viable seed. To control seed production effectively and economically it ‘s only important to remove one sex, not both. An understanding of the breeding systems allows you to make an informed decision to control the major problem first, and to address subsequent problems later. In one case 1 have even recommended continued grazing of a river bed because doing so reduced the size of the young willows and thereby reduced the potential for seed production. For sensible management, it is sensible to remove the fewer number of trees but other factors come into play, for example the ease with which pieces break off and are able to establish. To arrive at a sensible plan really does depend on knowing and being able to identify your willows.
The increased risk of erosion is a real one. If only a few trees are to be killed then the problem may not be great. If a large number are to be killed, then the risk to structures such as bridges also needs to be assessed. It mostly pays to kill the trees in situ and not try to remove them. There are exceptions, for example where there is a risk of dead branches or trees falling on a crossing, but generally as little interference as possible minimises the risk of erosion and further vegetative spread.
If there is already a good population of suitable native plant species then planting may not be necessary. Other actions may be, such as protecting young growth from grazing stock and /or rabbits. Willow control includes predicting and managing the post-willow scenario.
Control methods are important. There are few herbicides registered for use on willows yet but that is not a serious problem. RoundUp Biactive is, and is very effective. The biggest problem is that it is most effective in autumn yet sexing the trees and looking for seed has to be done in Spring. In control programs where this is necessary, the trees are often marked with tape or paint in spring, and treated with herbicide in autumn.
Nearly any application method works, but stem injection (battery drill,) on large stems, and spraying foliage of seedling swarms or multi-stem regrowth, seems to be the best combination. Spraying large trees often leads to drift and it can be difficult to get good coverage. Frilling works well with experienced operators but too often you see trees nearly ring-barked. This sets the trees right back but they can recover and often results in suckering because of poor translocation of the chemical. Cut and paint is also effective; experience shows that it is the outer 5 cm of the stump that is the most critical, and it is even better to paint the top 5 cm of bark than the inner part of the stump, but do both. The main trouble with the cut stump method is the mass of potential vegetative material that must be managed.
Most well-managed programs result in 95+% kill. It is always necessary to follow up and check for seedlings. Seedlings may not be obvious for 2-3 years, and there will be some plants survive the best program.
Steps for a willow control program:
- Identify the problem(s).
- Identify the species.
- Re-examine the problem.
- Identify problem species/trees; remember they also lurk in private gardens away from stream banks. If permission is not given to remove some of these trees it will determine those trees/species that you have to manage.
- Develop a draft management plan, paying particular attention to possible consequences of any actions and including monitoring and follow-up.
- If relevant, do a herbicide budget.
- Implement the plan, monitoring quality of work and progress.
- Modify plan if necessary.
- Initiate monitoring, evaluation and follow-up.