... following biological features help to explain the spread and provide a basis for control. 1. The original cause of spread has been importation and planting usually as cuttings of just one or two clones at a time. 2. .The bases of the small branches of most of the tree willows in Australia are easily broken. These willows thus tend to spread by broken branches taking root in wet areas downstream. The shrub willow species are far less fragile and thus unlikely to spread by broken branches. 3. Male and female flowers are usually on separate trees. Female trees will usually produce viable seed with pollen from a male of the same species or of any other species of its botanical group (i.e. either tree willows or shrub willows), provided that the trees occur within pollinating distance of each other (up to at least 1000 m) and flower at the same ...

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