Also known as
Reynoutria japonica, Japanese knotweed, R. japonica cv ‘Compacta’, Polygonum reynoutria, P. cuspidatum
Where is it originally from?
What does it look like?
Many-stemmed, thicket-forming perennial shrub (<1-2 m high) forming roots with rhizomes and numerous, zigzagging, hairless, bluish to reddish stems (<15 mm diameter) that are woody at the base. Ovalish, pointed leaves (8-23 x 5-17 cm) with less than 14 pairs of lateral veins are bluish below, and on dark crimson stalks. White flowers (<2.5 mm long) in densely-hairy, branched hanging clusters (<6 cm long, Dec-Apr) are followed by glossy brown nuts (3 mm).
Are there any similar species?
Why is it weedy?
Grows rapidly and extensively from rhizomes and multiple stems. Produces relatively long-lived and well dispersed seed, and tolerates wet to moderately dry conditions, and warm to cold temperatures, but is intolerant of shade.
How does it spread?
Seed and rhizomes are spread by soil and water movement.
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?
Shrubland and areas around waterways.
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 (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.
What can I do to stop it coming back?
Stem fragments and rhizomes resprout. Seeds germinate in bared areas. Difficult to control. Follow up 3-monthly for at least two years until eliminated.