Willows are a group of introduced tree species that were brought to New Zealand in the early 1800s and widely planted for bank stability. Since then, crack and grey willows have replaced native wetland and river-bank vegetation in many places.

If well managed, and the right species, willows can contribute to bank stability and offer positive benefits to some wildlife. 

Crack willow (Salix fragilis) is the most common willow in Otago and across New Zealand. It was introduced around 1860 and can live for around 100 years. Crack willows typically form dense canopies up to 25 m high along riparian margins and within wetlands. Although there are only male plants in NZ, it grows easily from small stem fragments that can be spread by water and machinery movement.

Grey willow (Salix cinerea, also called pussy willow) is a deciduous shrub or small tree that can grow to 7 m tall. Grey willows spread by wind-dispersed seed or by suckering from the roots, and plants are either male or female. Grey willows can replace native species in wetlands and forms dense stands.

Less invasive willow varieties have been used in recent decades for riverbank and hill country erosion control, including Moutere and Tangoio hybrid clones.  The Weeping willow (Salix babylonica) is a common amenity tree found in many public parks.

Details on a wide range of willow species can be found at www.poplarandwillow.org.nz and you can identify different species using the Manaaki Whenua Landcare Research guide.

Benefits:

• All willow species are useful for erosion control on hillsides and along rivers, due to their extensive root systems and rapid growth. The New Zealand Poplar and Willow Research Trust has detailed practical guidance for farmers on the effective use of willows to reduce erosion.
• Willow canopy shade reduces stream temperature and can also provide shade for stock
• Willows can be used for shelter belts and provide drought fodder for stock
• Willow canopy shade can reduce macrophyte growth (aquatic and riparian plants)
• Habitat for birds and insects
• Visual amenity and shade for people
• The willow grub hatch supports trout and related fishing activity

Problems:

• Crack, grey and some hybrid willows are highly invasive, outcompeting and displacing native plant species.
• Willows can choke stream channels and restrict flood flows, forcing streams to divert and cut a new course which can damage infrastructure and adjacent land. Fallen trees and large broken branches can also form debris dams or potentially block downstream culverts
• In-stream roots can form a dense fibrous and/or web-like network that traps fine sediment and reduces the stone and cobble habitat needed by many fish and invertebrate insect species
• The autumn leaf drop is a major biomass input that can deplete dissolved oxygen levels as they decompose , and can further restrict flow if caught up in debris dams of broken branches
• The water used by willows can reduce stream flow – more on this below
• Dense riparian willows limit public access to waterways