Willow trees, in particular Salix cinera, the grey sallow or “wild pussy willow“, are serious problem weeds in southeastern Australia. They infest riverbanks, harming both indigenous flora and fauna: they drive out native plants and clog waterways for fish and other freshwater animals.
Because you have to know your enemy if you want to defeat them (thank you, Sun Tzu), CSIRO scientists Tara Hopley and Andrew Young have been researching grey sallows at sites in Victoria’a Ovens River catchment to find out how they reproduce and spread their seeds.
They found that a single tree can produce 330,000 seeds in a single season. They are pollinated by both wind and insects, and genetic paternity tests have shown they can spread their seed over 15 km.
This makes them very effective propagators, and so if you want to control them you need to work over a wide area with the cooperation of different landowners.
But on the plus side, the seeds were also found to be short-lived: they do not germinate 8 weeks after release. Also, a lot of the seed came from only a small number of sites: they calculated that it’d be possible to cut seed production by 50% from clearing only 20% of sites.
To read more about this effort to understand pests in order to beat them, see the CSIRO fact sheet – War on willows (PDF 2 MB).