Alias: Creeping thistle
- Stems: 1 to 4 feet tall, erect, rigid, branching.
- Leaves: Sessile, deeply lobed, edged with stiff yellow
- Flowers: Many 1/2-inch flowering heads; male and female
flowers borne on different plants. Flowers purple, lavender, or
white, with overlapping, weakly spine-tipped bracts. Flowers in
July and August.
- Seeds: 1/8-inch long, somewhat flattened, tan, with an
apical circle of long hairs.
Canada thistle is a perennial that grows from a deep, complex root
system. The roots spread horizontally and send up new stems, forming
dense colonies. It grows in a wide range of soils and environmental
conditions. It is aggressive, spreading by root and seed. The deep
root system makes it difficult to control.
Control: An integrated system must be used for many years
to control this pest.
- Biological: Competition with grasses and alfalfa is effective,
as are seedhead weevils, and the painted lady butterfly. A 3-year
intensive cattle grazing program has proved effective in southwestern
- Chemical: Fall treatments, combined with mowing, are
effective. Get in touch with you local weed control office for
the latest information on controlling this pest.
- Mechanical: Mowing can be effective if combined with
herbicide application. Plowing increases plant numbers and should
Report all sightings to your local