Willow (Salix species)
Family: Salicaceae
Status: Willows are listed as a Weed of National Significance (that is, one of the top 20 weeds at a national level). All but weeping willow and two types of sterile pussy willow are now listed as Noxious throughout NSW. They are in category W4g, which means they cannot be sold or propagated, but there is no legal requirement to remove existing plants. However, many Landcare groups and landholders are embarking on programs to replace willows with native vegetation along rivers and creeks.
Description:
Several species of willows are potentially weedy on the south coast. All are deciduous trees or large shrubs, but they vary markedly in appearance.
Weeping willow (Salix babylonica) is the most familiar. It is usually single-trunked, with very long pendulous branches. It and crack willow (Salix fragilis), which is usually multi-trunked, are the most common species occurring along rivers.
Other species commonly planted as windbreaks or in gardens are:
With the exception of the pussy willows, the leaves of all species are long and narrow, with finely toothed edges, and usually a paler underside. Numerous tiny flowers are carried in upright catkins which are produced before the leaves in spring. Tiny seeds with a fluffy parachute are released quite early in the season, around November. At this time clouds of fluff can be seen drifting away from stands of willows which are seeding.
Not all willows produce seed. For a long time most willow species in Australia were represented by only one sex of plant, so that they spread only from broken off pieces taking root. However introductions of additional species and hybrids has allowed many willows which were formerly sterile to produce seed. Seed production requires a nearby partner tree of the opposite sex and a compatible variety, with the same flowering period. This condition is now very frequently met, as willows have been hybridising on the south coast for many years.
Preferred habitat and impacts:
River beds and banks. A moist seed bed such as wet sand is needed for seeds to germinate and become established. In some cases roadside ditches or swamps may also be colonised.
Their impact on rivers is substantial. They trap sediment, building up the river bed and filling in the waterholes needed by aquatic animals. Their large root masses, or tangles of trapped debris, can push water flow into the banks, causing erosion. The sudden drop of leaves in autumn can deplete the water of oxygen while the leaves are decomposing, making things difficult for fish and other animals. The dense shade they cast in summer can affect water temperatures.
Dispersal:
The very fine seed can drift for many kilometres on the wind. It is only viable for a few days, and needs a moist site such as a sandy river bed, so the odds of an individual seed landing in the right place and developing into a tree are low. Despite this, millions of seedlings manage to germinate in river beds in favourable years.
Look-alikes:
No natives look similar. There are many different willows, and all, including weeping willow, are potentially or actually weedy. The closely related poplars (Populus species) look similar, but have broad, glossy leaves and longer, dangling catkins. They are more often found planted away from rivers, but when they do occur in rivers can also cause problems by blocking flow and trapping sediment. They have not yet been shown to reproduce by seed on the south coast, but they do grow readily from broken branches or twigs.
Control:
Cut and paint or stem inject mature plants with glyphosate. This can be a difficult task, since willows are often multi-stemmed, and every stem will need to be treated. Their bases are also often buried in flood debris, blocking access. In this case burning the flood debris can kill the willow. This would be best done in the warmer months to avoid killing hibernating frogs and reptiles which might be sheltering in the piles.
Small numbers of seedlings can be easily hand-pulled in loose sand.
Spraying with glyphosate will be effective on smaller plants, but may produce too much spray drift to be acceptable with large plants.
Remember that a permit is required to use herbicides within a watercourse, and a permit is required from the Department of Land and Water Conservation to remove vegetation within a watercourse. Willow removal needs to be carefully staged to avoid causing more erosion. The willows may need to be replaced with suitable local native vegetation which can take on the job of protecting the river banks during floods. Get advice from your local Landcare Coordinator or DLWC office.