Giant knotweed

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Botanical Name

Reynoutria sachalinensis

Family

Polygonaceae (dock)

Also known as

Falopia sachalinensis, Polygonum sachalinense

Where is it originally from?

Asia

What does it look like?

Giant, many-stemmed, thicket-forming perennial shrub (<2-4 m high) with roots with rhizomes and numerous hairless green stems (<20 mm diameter) that are woody at the base. Ovalish and pointed leaves (10-30 x 5-22 cm) with more than 14 pairs of lateral veins that are bluish below and usually on reddish stalks. White or greenish flowers (<2mm long) in densely-hairy, clusters (<6 cm long) appear from November to April, but no seed is produced in New Zealand.

Are there any similar species?

Fallopia japonica

Why is it weedy?

Grows extensively from rhizomes and multiple stems, tolerates wet to moderately dry conditions and warm to cold temperatures, but is intolerant of shade.

How does it spread?

Rhizomes are spread through soil movement and dumping.

What damage does it do?

Forms dense, long-lived thickets, excludes other species and prevents native seedlings establishing.

Which habitats is it likely to invade?

Disturbed shrubland and bare land.

What can I do to get rid of it?

1. Dig out small patches (all year round). Dispose of at refuse transfer station or burn.
2. Weed mat: leave for 6 months minimum. Dig or spray surviving shoots.
3. Stump swab (all year round): glyphosate (250ml/L) or metsulfuron-methyl 600g/kg (5g/L) or triclopyr 600 EC (200ml/L).
4. Stem injection (all year round): 5ml metsulfuron-methyl 600g/kg (50g/L) per stem.
5. Spray (spring-autumn): glyphosate (20ml/L) or metsulfuron-methyl 600g/kg (5g/L) or triclopyr 600 EC (6ml/L). Add penetrant.

CAUTION: When using any herbicide or pesticide, PLEASE READ THE LABEL THOROUGHLY to ensure that all instructions and directions for the purchase, use and storage of the product, are followed and adhered to.