Giant knotweed
Botanical name
Fallopia 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?
Contact your regional council to determine the status of this species and responsibility for control.
1. Physical removal - Pull or dig out then burn or dispose of at refuse transfer station.
2. Cut and paste (all year round) - Cut the stem/trunk as close to the ground as possible and cover the entire stump with herbicide as soon as possible after cutting. Apply either glyphosate gel (120g/L strength) or metsulfuron gel (10g/l strength) or picloram gel (43g/l strength) to the entire cut stem.
3. Foliar spray - Apply herbicide using a hand held sprayer/knapsack to plants <1m tall or gun and hose for larger infestations. Apply either glyphosate (360 g/L active ingredient at 20ml/L knapsack, 1L/100L gun and hose) plus organosilicone penetrant (2ml/L), to cover entire plant. Note: Glyphosate overspray will kill other (desirable) plants OR Apply Metsulfuron herbicide (600g/kg active ingredient at 0.5g/L knapsack or 20g/100L gun and hose) plus organosilicone penetrant 3ml/L OR picloram/triclopyr herbicide (picloram 100g/L and triclopyr 300g/L active ingredient at 6ml/L) plus penetrant to thoroughly wet all parts of plant. Note metsulfuron and picloram have residual activity in the soil which may leach and kill other plants. Do not use under and around other (desirable) broadleaf plants and are not for use over or near water bodies or wetlands
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.
What can I do to stop it coming back?
Difficult to control as stem fragments and rhizomes resprout. Follow up 3-monthly for at least two years until eliminated.
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