Giant knotweed

What does it look like?

Giant knotweed is a large, thicket-forming herb that can grow up to 4m tall. It has large (up to 30cm long), heart-shaped leaves that are smooth on top and sometimes covered in fine hairs underneath. The stems are thick, hollow, often green to reddish-brown and die back to the root base in winter. It has a thick root mass with creeping underground stems. The flowers are greenish-white in dense drooping clusters near the end of stems.
Giant knotweed can grow in gardens, river and stream edges, forest margins and any waste areas. It tolerates wet to moderately dry conditions and warm to cold temperatures, but is intolerant of shade.

Why is it a problem?

Giant knotweed forms dense long-lived thickets, which shade and crowd out other species. Fortunately, Giant knotweed does not produce seed in New Zealand, however care must be taken with its removal, as even a small piece of the plant’s root or stem can grow into a new plant.

Control methods

Physical control

• Dig out small patches (all year round). Dispose of at refuse transfer station or burn.
• Weed mat: leave for 6 months minimum. Dig or spray surviving shoots.

Herbicide control

• Stump swab (all year round) using glyphosate (250ml/L) or metsulfuron-methyl 600g/kg (5g/L) or triclopyr 600 EC (200ml/L). Or,
• Stem injection (all year round) using 5ml metsulfuron-methyl 600g/kg (50g/L) per stem. Or,
• Spray (during spring and summer) using glyphosate (20ml/L) or metsulfuron-methyl 600g/kg (5g/L) or triclopyr 600 EC (6ml/L). Add penetrant.
• The plant can be difficult to control as stem fragments and rhizomes resprout. Follow up 3-monthly for at least two years until eliminated.
• CAUTION: When using any herbicide or pesticide, PLEASE READ THE LABEL THOROUGHLY to ensure that all instructions and safety requirements are followed

Related links

• Ministry for Primary Industries
• NZ Flora
• Unwanted Organisms Register
• Weedbusters
• NPPA and the control of weeds | NZ Government