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Salix species
Willow
All species of Willow except Salix babylonica (Weeping Willow), S. x calodendron and S. x reichardtii are Weeds of National Significance.
The images here are of the most invasive Salix species in Australia, from top: S. fragilis (Crack Willow), S. nigra (Black Willow), and S. cinerea (Grey Willow). Willows enjoy moist conditions and are often seen growing along rivers, streams, dams and lakes, where they can quickly take over.
Not only does severe infestation squeeze out all native vegetation but willows can also alter the shape of streambeds, channels and banks because they capture a huge amount of sediment.
Over summer months their dense shade reduces light levels and temperatures, which can change habitat for aquatic animals.
Willows also provide little food or shelter to most land and aquatic animals.
Spread
Salix spp. have separate male and female trees and reach sexual maturity between three and eight years of age. Hybridisation (cross-fertilising) between species is very common.
Some willows reproduce predominantly by seed and some by vegetative means (willows are notorious for being able to take root and regrow very readily from even the smallest piece of stem or root). Seed is dispersed by wind or water. Vegetative pieces are often dispersed by water, but also by machinery during removal.
Different species are problems in different States and Territories, but they all pose the same threat to waterways.
Control
Seedlings and small plants are easily dug up, but care should be taken to bag up the entire plant - anything left behind is likely to regrow.
Plants less than 10cm in diameter can be cut down and the remaining stump painted with a herbicide suitable for woody weeds. Material can be left to dry above floodlines.
Larger trees can be killed using the drill and fill method of herbicide control (where herbicide is applied to holes drilled into the tree but care must be taken that the herbicide is introduced to the cambium layer that transports water and nutrients, as this is just below the bark). The trees take a long time to die but there is minimal disruption to the environment and replanting can be undertaken as the trees deteriorate.
Care of waterways is best done on a catchment wide basis, starting with the upstream end of an infestation. Using machinery to remove willows along waterways is almost guaranteed to aid in their spread. And the wholesale removal of willows will also cause erosion problems and further environmental damage unless properly managed.
The common Weeping Willow (S. babylonica), shown here, is not considered a problem. This is because it has been planted for many years now and its spread is minimal. There are also no records of it spreading by seed.
However, it does hybridise with many invasive species overseas. It's thought that hybrid material is probably in Australia but the consequences are as yet unknown.
Information sources:
Blood, K., 2001, Environmental Weeds, CRC Weed Management Systems, published by C H Jerram Science Publishers.
Muyt, A., 2001, Bush Invaders of South-East Australia, published by R.G. & F.J. Richardson
http://www.environment.gov.au/biodiversity/invasive/publications/salix.html
S. cinerea image courtesy of Keele Arboretum, UK.
Click here to return to the invasive plants list.
The information contained on this page is Copyright © SGA and intended for personal use only.
You may not use the information for any other purpose without the express written permission of SGA.