Invasive Weed of the Month: Canada Thistle

Canada thistle (Cirsium arvense) is a wide-spread invasive weed found in all parts of Clackamas County. Unlike many native thistles found in Oregon, Canada thistle can quickly overrun an area and displace native plants. It can also reduce agricultural yield, and create problems for grazing animals when it infests a field or pasture. This can take a lot of money and time to control.

Canada thistle is not from Canada, so we can’t blame our northern neighbors for the problems this weed brings. Instead, this perennial noxious weed originated in southeast Europe and Asia. It is believed to have been imported into the United States in the 1700’s as a contaminant in crop seed. It is now currently found in every county in Oregon and is listed as an Oregon Class B noxious weed. This means it is a weed of economic importance but may have limited distribution in some Oregon counties. It also means that the propagation, transport, and sale of this plant are prohibited by law.

You may also know this weed as Canadian thistle, corn thistle, or creeping thistle. But not matter what you call it, this invasive weed can be found in disturbed areas that have abundant sun and moist soils. This includes crop fields, forest openings, gardens, hillsides, logged areas, pastures, range land, roadsides, stream banks, and vacant lots.

How Can I Identify Canada Thistle?

Mature Canada thistle plant
Flowers on mature plants can range from pink to purple.

Canada thistle is a tall, prickly, creeping perennial that grows to 3-5 feet tall. The plant starts as a rosette, but quickly develops rigid, hairy, branched stems and leaves with very sharp spikes. The plants usually die back during winter, but don’t be deceived. Their extensive underground system of roots and rhizomes have quietly spent the season storing up energy. Soon they will be ready for regrowth in the spring.

When Canada thistle blooms, it creates clusters of pink to purple, bristly flowers. These blooms are approximately 1/2 inch long and wide. Seeds from these blooms are scattered by the wind and can survive in the soil for up to 20 years. Most new plants, however, sprout up from the lateral root systems of former and existing plants. These root systems can extend as far as six feet deep. In a single season, this root system can spread horizontally up to 20 feet. Additionally, this noxious weed can quickly and easily reproduce from root fragments as small as one quarter inch long. Tillage and other cultivation methods can quickly spread a small infestation throughout an entire field.

Why Should I Care About Canada Thistle?

Canada thistle has thorny leaves.

Canada thistle is a problem for anyone who manages land. This aggressive, noxious plant forms dense patches on disturbed land. It also produces a chemical that inhibits the growth of other plants in its vicinity. It is very difficult to control by hand pulling, cutting, or burning. This is because the extensive root system allows it to quickly recover. Mechanical control such as discing or tilling can actually make the infestation worse! Equipment like this can quickly spread root fragments along a roadside or throughout a field.

Like many noxious weeds in Clackamas County, Canada thistle invades a variety of habitats including natural areas and forest clearings. It can displace the native vegetation needed by wildlife for food and shelter. It reduces pasture forage for grazing animals and complicates reforestation and landscape restoration efforts. Management of this weed costs cities, counties, and state agencies hundreds of thousands of dollars each year.

How Can I Control Canada Thistle?

Canada thistle reduces pasture forage.
Canada thistle reduces pasture forage

As noted above, traditional cultural and mechanical controls are not effective in treating this invasive weed. There are two biological controls, the Canada thistle gall-fly and a stem-mining weevil. These, however, have had very limited success in field settings and are ineffective overall.

The key to control is to stress the plant and force it to use its stored root nutrients. You can stress plants by mowing and hand-cutting small infestations before flowering. This will prevent the plant from producing seeds. Repeat mowing and hand-cutting regularly throughout the growing season to suppress regrowth.

Foliar herbicides have been shown to be the most effective management for killing large stands of Canada thistle. They do this by mobilizing their active ingredients into the root system of the plant.

Regardless of the control method employed, returning infested land to a productive state occurs only over time. This requires a sound management plan implemented over several years.

Do You Have Canada Thistle on Your Property?

Because Canada thistle is so widespread, property owners in Clackamas County are not required to report infestation. The District’s WeedWise program can provide advice on how to control Canada thistle. Take a look at their best management practices and herbicide guidance.

Learn More!

For more information on Canada thistle control, check out the Oregon Department of Agriculture Weed Profile for Canada thistle

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