Canada thistle

Cirsium arvense · Aquatic plants

Whole plant · Photo: Matt Lavin / CC BY-SA

InvasiveShoreline

Also called Field thistle, Creeping thistle

State noxious weed listing

Idaho: Containment
Oregon: B list
Wyoming: State designated noxious weed

How to identify Canada thistle

Spiny, wavy, lobed leaves on stems 1 to 4 feet tall
Many small purple to pink flower heads, about 1/2 to 3/4 inch across
Grows in patches from creeping roots rather than as single plants
Fluffy white seed heads in late summer

Similar plants

Bull thistle is a biennial with larger flower heads, spiny winged stems and prickly hairs on top of the leaves, and does not form patches; native thistles usually grow as single plants, have larger flower heads and often have woolly, gray white leaves.

Growth profile

Growing season	Shoots come up from roots in April and May, flowers June to August, seeds July to September, and roots overwinter; often regrows in fall.
Starts growing	Early spring as soils warm
Light	Full sun to part shade
Depth	Moist banks above the waterline; not in standing water
How it spreads	Creeping roots that resprout from small pieces and wind blown seed
Habitat	Pond and reservoir banks, canal and ditch banks, wet meadows, drawdown zones and other disturbed ground near water; also common in fields and pastures.

Why it matters in a lake

Listed noxious in all three states. Its deep creeping roots resprout from small pieces, so mowing and pulling alone rarely control it, and spraying at the water edge needs an aquatic labeled product.