Asiatic knotweed

What does it look like?

Asiatic knotweed is a thicket-forming herb that can grow up to 2 metres tall. It has zig-zagging stems and red-purple shoots which appear early in spring. As the canes grow, the leaves unfurl and the plant turns green. The mature canes are hollow and have a characteristic pattern of purple speckles. The leaves are long, triangular (15 x 10cm) and pointed at the tip, with a flattened leaf base. In late summer it produces masses of creamy white flowers.
Asiatic knotweed grows primarily in disturbed areas, urban sites, roadsides and along riparian margins. It can tolerate a wide range of soil conditions, high temperatures, high salinity and drought but its growth is depressed by shade.

Why is it a problem?

Asiatic knotweed spreads rapidly, forming dense stands that compete with native vegetation. Its tough shoots and roots can break through gravel, tarmac and concrete; causing damage to foundations, walls, pavements, drainage works, and flood prevention structures.
Its dead stems and leaf litter decompose very slowly, forming a deep organic layer and prevents native seeds from germinating. Asiatic knotweed reproduces from seed and can regrow from detached or broken fragments of roots or stems.

Control methods

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

Herbicide control

• Stump swab (all year round): Glyphosate (250ml/L) or metsulfuron-methyl 600g/kg (5g/L) or triclopyr 600 EC (200ml/L). Or,
• Stem injection (all year round): 5ml metsulfuron-methyl 600g/kg (50g/L) per stem. Or,
• Spray (spring-autumn): Glyphosate (200ml/10L) or metsulfuron-methyl 600g/kg (5g/L) or a product containing 100g picloram+300g triclopyr/L (6ml/L) or triclopyr 600 EC (6ml/L). Add penetrant.
• Stem fragments and rhizomes resprout. Seeds germinate in bared areas and it’s difficult to control. 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.

More information

Also known as Japanese knotweed.

Related links

• Ministry for Primary Industries
• New Zealand Plant Conservation Network
• NZ Flora
• Unwanted Organisms Register
• Weedbusters