HomeMy WebLinkAboutCanada Thistle.pdf
Leaf Male Flower Female Flower
Maple Ridge Noxious Weeds Program
Canada thistle
(Cirsium arvense)
Designation: Provincially Noxious
Photo: L. Scott
History:
Introduced from Eurasia likely as a
contaminant of crop seed as early as
the late 18th century.
How to Identify:
Flowers: Flower heads are white to
purple, about 1 cm in diameter, borne
on clusters of 1 – 5 at branch tips, and
have a sweet vanilla scent. Flower
bracts are spineless.
Seeds: One‐seeded, pale yellow (straw)
or light brown in colour; straight or
slightly curved.
Leaves: 5 – 17 cm long, narrow, and
alternate on the stem with crinkled,
deeply lobed, and spiny edges. Base
leaves are stalkless and clasping, or
extended down the stem.
Stem: Mature plants range from 0.3 –
2.0 m in height.
Location: Canada thistle is found in
almost every plant community over a
wide range of elevations where there is
soil disturbance or bare ground
including roadsides, railway
embankments, lawns, gardens,
cultivated and non‐cultivated fields,
margins of forests, meadows, wetlands,
and native plant communities.
Mistaken Identity: Often confused with
other non‐native thistles such as Bull
thistle (Circium vulgare), and Scotch
thistle (Onopordum acanthium). Canada
thistle can be differentiated from all
similar species by the lack of spines on
the main stem, small flowers, and
height (less than 2 m tall).
Impacts:
Economic: Plants can crowd out forage
grasses in pastures and rangelands,
reducing yields and productivity.
Ecological: Single plants can spread
rapidly (up to 5.5 m per season) and
form dense patches, particularly in
riparian areas, thus out‐competing
native plants.
Management:
Prevention:
• Monitor for Canada thistle in both disturbed and
undisturbed areas.
• Ensure soil, gravel, and other fill material are not
contaminated.
• Avoid unloading, parking, or storing equipment and
vehicles in infested areas.
• Minimize soil disturbance during activities and re‐
vegetate exposed soil as soon as possible.
• Remove plants, plant parts, and seeds from personal
gear, clothing, pets, vehicles, and equipment. Wash
vehicles, including tires and undercarriage, and
equipment at designated cleaning sites before
leaving infested areas.
• Bag or tarp plants, plant parts, and seeds before
transporting to a designated disposal site (e.g.
landfill).
• Take special care when controlling Canada thistle
near streams, or ditch lines, to prevent the
movement of plant parts downstream.
• Maintain or establish healthy plant communities
that are resistant to invasion by invasive plants.
Mechanical Control:
• Mowing is most effective when completed at the
bud stage. Do not mow when in seed.
• Regular cutting or tillage can help wear down plant
reserves, reduce plant growth, and reduce
populations, but is not likely to kill the plant.
Chemical Control: Herbicides such as Milestone have
been highly effective in controlling Canada Thistle.
Always consult the product label and appropriate
regulations when choosing a herbicide.
Disposal: If plants are cut prior to flowering, the plant
material can be left on the site to decompose. If plants
are cut post flowering, all plant parts, including flower
heads, should be bagged and deeply buried at a
landfill. Care should be taken to ensure that plant
parts are not distributed during transport.
Restoration and Planting Alternatives:
Canada thistle is an uncommon garden species but is
found in many disturbed areas. The best way to
prevent the spread of noxious weeds is to eliminate
new seedlings before they form a well developed root
system. Plant newly disturbed areas with native
species before the weeds have an opportunity to
grow.
Photo:
http://www.nwcb.wa.gov/publications/WesternFieldGuide.pdf
Distribution in DMR:
Widespread but sporadic (low density).
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 from: T.I.P.S. datasheet for Canada Thistle; A Guide to Weeds in British Columbia – Canada Thistle
http://www.anpc.ab.ca/wiki/index.php/File:Cirsarve_3.jpg; http://www.nwcb.wa.gov/publications/WesternFieldGuide.pdf
http://iscmv.ca/target‐species/species‐profiles