HomeMy WebLinkAboutKnotweed.pdf
Young knotweed In flower Full‐growth
Maple Ridge Noxious Weeds Program
Knotweed species
Japanese, Giant, Bohemian (Fallopia sp.) and Himalayan (Polygonum polystachyum)
Designation: Provincially Noxious
Photo: e‐flora BC
History:
Japanese and Giant knotweed were introduced into North
America from eastern Asia in the 1800’s as garden ornamentals.
Those two plants hybridized to create Bohemian knotweed, the
predominant plant in the metro Vancouver area. Less is known
about the introduction of Himalayan knotweed to North
America except for the origin; the Himalayan mountain region
of southern Asia.
How to Identify:
Early growth: Newly emerging knotweed is likened to red
asparagus emerging from the ground.
Mature growth: Large, woody bamboo‐like plants that can
grow 1 – 3 m tall in dense thickets.
Flowers: Not all plants flower. Those that do have small,
white/green flowers grow in showy, plume‐like, branched
clusters along the stem and leaf axils (joints).
Leaves: Variable. Japanese: spade‐shaped; Giant: larger, heart‐
shaped; Bohemian: hybrid of Japanese and Giant; Himalayan:
lance‐shaped, pointy. Leaves appear in zigzag pattern along
stems.
Stem: Stems, or canes, are hollow with varying thicknesses,
upright, and bamboo like with red segments and thin, papery
sheaths. Stems die in winter turning brown and woody.
Location: Found just about anywhere. Thrive on freshly
disturbed soil in roadside ditches, low‐lying areas, irrigation
canals, and other water drainage systems. Also found in riparian
areas, along stream banks, and in other areas with high soil
moisture. Able to grow in partial shade or full sun.
Mistaken Identity: Elderberries (Sambucus spp.) have lance‐
shaped, pointed, and sharply toothed leaflets. Red elderberry
(Sambucus racemosa) is found in moist areas but the stems
have small nodules (bumps) along the stem.
Danger / Impact
Forms dense, impenetrable thickets which displace native
vegetation. Allelopathic nature of knotweed discourages the
growth of other native vegetation. Dominates stream banks,
which increasing erosion potential when plants die and expose
banks during winter. Exposed knotweed roots break off and
float downstream to form new infestations. Degrades wildlife
and fish habitat. Extensive root system can lead to knotweed
growing through weaknesses in cement, house foundations and
walls.
Management:
Knotweed is extremely difficult to control
once established. It spreads prolifically by
root and stem segments, and in some cases
seed. New research shows that seeds of
plants in the region are highly viable and
little is known about their long‐term
viability in the soil.
Do not treat manually. Manual treatment is
ineffective and may cause further spread.
Should be controlled by a professional
using herbicide application. Live knotweed
should not be cut as this method is
ineffective and disposal results in a high
likelihood of spread during transport.
Timing: Herbicide treatment should occur
two to three times per growing season for
maximum control. Spring treatment, mid‐
summer and fall treatments show best
efficacy. Plant is dormant during the winter.
Disposal: Herbicide killed material can be
left on site to decompose. Cut material can
be disposed of at the Maple Ridge Transfer
Station. Do not compost in home compost
bin.
Follow‐up: Monitor at least twice annually.
Continue monitoring for several years even
after no re‐growth appears.
Restoration and Planting Alternatives:
Restoration should not occur for a couple
of years after management as the
allelopathic nature of knotweed can have
inhibited newly planted native vegetation.
Red‐osier dogwood (Cornus
stolonifera)
Willow species (Salix spp.)
Native tree species:
Red alder (Alnus rubra),
Black cottonwood (Populus
balsamifera ssp. trichoparpa),
Douglas‐fir (Pseudotsuga menziesii),
Western redcedar (Thuja plicata),
Sitka spruce (Picea sitchensis)
Photo: J. Craig
Distribution in DMR:
Several locations, some small and some extensive.
Requires coordinated control.
Early detection required to prevent new infestations.
Who Do you Call?
1) REPORT PROHIBITED WEEDS Report‐A‐Weed: www.reportaweedbc.ca
1‐888‐WEEDSBC
2) DMR: 604‐467‐7363 AND SEE: http://www.mapleridge.ca/714/Invasive‐
Plant‐Species
Information and photos:
http://www.bcinvasives.ca/invasive‐species/invasive‐plants/knotweeds
http://www.iscmv.ca/species‐profiles/japanese‐knotweed
http://westvancouver.ca/sites/default/files/dwv/assets/home‐building‐property/docs/pets‐wildlife‐environment/Invasive‐Plants/APPENDIX_A_‐
_JANUARY_28__2014_DRAFT.pdf