How do weeds affect wetlands?

Without weed control, wetlands like Te Waihora will continue to be choked by invasive species like willow and reed canary grass (Phalaris arundinacea). This will affect water movement, water quality, siltation of the channels and will eventually displace the native vegetation that supports fish, bird and invertebrate species. The result is the gradual degradation of the catchment that will spoil the recreational activities people are used to enjoying in the area; as well as displacing the plant, fish and bird species that have become an integral part of the area.


What makes a weed?

There are many definitions for weeds but essentially they are plants growing in places where they are not wanted. Weeds have specific characteristics that often distinguish them from other plants in terms of their reproduction, growth and the resources they use to survive. When weeds are introduced to a new environment, they often grow rapidly and invade that environment more effectively than the native species that would normally be found growing there. Most weeds in New Zealand are introduced species that have readily adapted to our unique climate, landscape and ecosystems. Most are becoming a significant issue for conservation managers, farmers and urban householders.


What are the main weed species causing problems at Te Waihora/Lake Ellesmere?

Beggar’s tick (Bidens frondosa), purple loosestrife (Lythrum salicaria), gorse (Ulex europaeus), reed canary grass (Phalaris arundinacea) and yellow flag iris (Iris pseudacorus) are all recognised weed pests at Te Waihora, along with Grey willow (Salix cinerea) and Crack willow (Salix fragilis.

Grey willow is one of the top ten weed species in New Zealand recognised by the Department of Conservation. It threatens many wetland and riparian areas throughout the country. The goal of willow management is not to eradicate all willows but to decide why, where, when and how to manage them.

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