Knotweed: Japanese and giant
Japanese and giant knotweed are already here but we're working to get rid of them.
Why they are pest plants
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Japanese and giant knotweed are woody herbs which grow to around 2-4m high from rhizomes (fleshy underground roots). Their root system can be up to 3m deep and extend to 7m across.
- Produces masses of creamy white flowers.
- Flowers appear from December to March.
- Glossy brown fruit shaped like a bluntly-angled nut.
- Fruit are about 3mm long.
- Leaves are broadly ovate, pointed at the tip and flattened at the base.
- Leaf stalk is dark crimson.
- Stems form zigzags, reddish-purple when young, become green and unfurl and when mature, become hollow with purple speckles on them.
Japanese knotweed and giant knotweed grow rapidly from an extensive fleshy underground root system, forming dense, long-lived thickets. They exclude other desirable species and prevent native seedlings from establishing.
Japanese knotweed and giant knotweed are spread through the movement of their roots or when shoots and roots are transported by people. They are very hardy plants and are difficult to eradicate. Originally from Japan, knotweed is a very serious invasive pest plant in parts of Europe. It grows primarily in disturbed areas, roadsides and river banks. Japanese knotweed and giant knotweed occur at sites in Taupiri, Karangahake (south Coromandel), and in Hamilton and Tamahere.
Responsibility for control
Waikato Regional Council is responsible for controlling Japanese and giant knotweed – do not attempt to remove them yourself. However, landowners/occupiers are encouraged to report Japanese and giant knotweed on their properties and liaise with Waikato Regional Council in areas where control programmes are in place. Japanese and giant knotweed are also banned from being sold, propagated, distributed or included in commercial displays.
How to control japanese and giant knotweed
If you see these weeds on your property, do not cut or treat them. Call 0800 BIOSEC (0800 246 732) to report them to your local biosecurity pest plant officer.
See our tips overleaf for ideas on how you can help us prevent Japanese and giant knotweed from spreading.
Tips – stopping the spread
WARNING! Fragments as small as 0.7 grams can form new plants, so be extremely careful with any equipment used on site.
Japanese and giant knotweed can spread by soil movement, garden waste and equipment such as diggers and farm machinery. When disturbed, Japanese and giant knotweed breaks up easily into small fragments which can easily regrow. Take special care not to disturb it or transport it to new sites.
- For advice and additional information on control methods, call our pest plant staff on freephone 0800 BIOSEC (0800 246 732).
- Chemical company representatives, farm supply stores and garden centres can also be good sources for advice.
View, download or order the following publications or call our freephone 0800 800 401.
- National Pest Plant Accord (Manual of plants banned from sale, propagation and distribution) ($10.00 plus GST)
- Plant Me Instead! (Plants to use in place of common pest plants) (free)
- Waikato Regional Pest Management Plan (RPMP) (free) (Section 5.23, page 83)
- Waikato Regional Council pest guide (free)