Thistles are one of the most common weed problems that property owners deal with in Clackamas County. Many of the thistles we encounter are invasive and can grow to dominate a property. But did you know there are at least four thistles that are not only non-invasive, but native to western Oregon? These native thistles […]
Canada thistle, Canadian thistle, corn thistle, creeping thistle
Cirsium arvense (syns. Breea arvensis, Carduus arvensis, Cirsium incanum, Cirsium orchrolepideum, Cirsium setosum, Cnicus arvensis)
Canada thistle is a tall, prickly, creeping rhizomatous perennial with multiple purple flowers. The erect plant stands around 2 to 4 feet tall on rigid, hairy, branched stems that usually die back during winter. This aggressive noxious plant forms dense unisex patches on disturbed land, and produces allelopathic chemical that inhibit growth of other plants. Canada thistle is native to Europe, not Canada, and can be found throughout the United States.
Canada thistle leaves are oblong to lanceolate or lance-shaped, 2 to 7 inches long with deep and irregular lobes. Leaves are toothed with prickles that are 3 to 6 mm long. The green leaves are slightly hairy on their upper surface. Canada thistle leaves are alternate and are attached directly to the stem; do not have a leaf stem. Young leaves also alternate but are oval-shaped with no point, teethed with a softened prickle.
The purple disk flowers of the Canada thistle plant are surrounded by rows of soft spine-tipped phyllaries. Flowers appear from June to October with 1 to 5 heads per branch. Petals are often purple but can be pink or even white. Both sexes have umbrella-shaped flowers, with female flowers being slightly bigger with longer pappus (male 10-14 mm, female 14-20 mm).
Canada thistle has dry, small, single seed fruit called achenes. Oval, slightly curved, hairy and shiny; the achenes have feathery pappus attached at the base. Both the achenes (2-4 mm long) and the pappus bristles (12-20mm) are tan.
Canada thistle has a deep and extensive root system consisting of vertical and several horizontal roots extending as far as 15 feet. Roots are rhizomatous, with new stems sprouting in spring. Roots are stiff and fragile but long-lived. In a years time, one plant’s root system can take over an area up to 25 ft². Seedlings start with deep taproot with creeping rhizomes developing between 2 and 4 months.
Canada thistle reproduce both by seed and vegetatively through creeping roots. March through April, Canada thistle seeds germinate and grow to form rosettes. New plants can develop from underground shoots and roots longer than 1 cm. In May, new shoots bolt. June through August the plant flowers with seed production starting in August and continuing throughout September. Female flowers need to be pollinated to produce viable seeds which take 8-10 days to develop. Plants produce 1,000 seeds on average and may produce up to 5,300 seeds. Seeds are dispersed by the wind.
Seeds are transported mostly by wind. The Canada thistle plant returns to the rosette stage near October and November. Flower stems usually stay up for some time after seed reproduction. New root and shoot buds develop over winter.
New plants developed from vegetative reproduction are hardy, not sensitive to competition. New plants developed from seeds are slow-growing and sensitive to both competition and shading.
The seeds may remain viable in the soil for over 20 years.
Canada thistle can be found in disturbed areas with abundant sun and moist but not wet soils. Crop fields, forest openings, gardens, hillsides, logged areas, pastures, range land, roadsides, stream banks, vacant lots and waste places are the usual infestation locations for Canada thistle. It is found throughout most of the continental United States.
Canada thistle was introduced to the Willamette Valley between 1875 and 1899. Although it was reported to be not common when first collected, it is now very common throughout the area.
Clackamas County:
Canada thistle can be found throughout Clackamas County. It is widespread and directly impacts properties throughout the county. As an ubiquitous weed, this is not a species that is actively surveyed and the mapped distributions do not represent the full extent of the Canada thistle population in Clackamas County.
The management of invasive weeds is best served through a process know as Integrated Pest Management (IPM). IPM is a weed management methodology that utilizes:
An IPM based strategy ensures the maximum effectiveness of treatment measures. IPM strategies typically use more than one management method to target one or more susceptible life stages. It is adaptive to site conditions in the field and to the response of a plant to management. The utilization of multiple management tools also inherently reduces the use of herbicides in a management plan. The IPM process ultimately provides a framework for the establishment of Best Management Practices (BMP) which outlines the best approach for controlling a weed particular infestation.
Only young plants can be removed by digging up the roots. More developed infestations have deep rhizomatous roots that greatly complicate removal. Any remaining root fragments can produce new plants. Do not compost flower heads, seeds or rhizomes as most home compost piles do not reach a temperature sufficient to kill all seed.
Mowing can help te reduce the nutrients stored in the plants, but to be effective mowing must be repeated every few weeks for several growing seasons. Mowing can be an important tool, especially when combined with other management practice such as herbicide application.
Tillage increases Canada thistle infestations. Rhizomes are cut and spread, allowing new plants to grow. Tillage can be effective in combination with chemical treatments.
Burning and grazing are not effective controls for Canada thistle management. Burning may reduce seed production.
Solarization of patches using plastic sheeting can be successful in controlling seedlings and small plants.
Herbicide application is an effective means to control Canada thistle infestations. Canada thistle is susceptible to several systemic herbicides. Be cautious when using herbicides on pasture land with grazing animals.
Product Names: Transline
Rate:
0.67 to 1.33 pt product per acre (4 to 8 oz ae/acre)
Time: Apply in spring (March-May) before plants bud.
Comments: Re-treatment may be required to achieve effective control. Clopyralid is not harmful to grasses.
Product Names: Milestone
Rate:
5 to 7 oz product per acre (1.25 to 1.75 oz ae/acre)
Time: Apply in late spring (April-June) after plants are budding and in the flowering stage.
Comments: Re-treatment may be required to achieve effective control. Aminopyralid is a very effective herbicide for Canada thistle control. Aminopyralid is not harmful to grasses.
Product Names: Accord XRT II, Rodeo, Roundup, and various others
Rate:
Broadcast: 2 qt per acre (2.25 ae/acre)
Spot treat: use 2% v/v solutions.
Time: Apply post emergence in late spring to early summer, (March-June) when plants are growing fast and past budding stage. A fall application can be made before a killing frost (August-October).
Comments: Re-treatment may be required to achieve effective control. Do not mix. Glyphosate is not selective and will harm grasses. Use care when working around desirable plants to avoid damage. Leaves should be sprayed until wet but not dripping to achieve good control.
Product Names: Several names
Rate:
2 qt per acre (1.9 lb ae/acre)
Time: Apply in spring (March-May) after plants are in the rosette or bolting stage.
Comments: Re-treatment may be required to achieve effective control. 2,4-D is broad leaf selective herbicide and not will harm grasses. Use care when working around desirable plants to avoid damage. Leaves should be sprayed until wet but not dripping to achieve good control.
Product Names: Banvel
Rate:
4 pt product per acre (2 lb ae/acre)
Time: Apply in early spring (March-April) while plants are in the rosette stage.
Comments: Re-treatment may be required to achieve effective control. Dicamba is not a very effective herbicide for Canada thistle control without the addition of another chemical. Dicamba is a broad leaf selective herbicide and is not harmful to grasses.
Product Names: Telar
Rate:
1 – 1.33 oz per acre (0.75 – 1 oz ai/acre)
Time: Apply post emergence in spring (March-May) when plants are bolting but before blooming.
Comments: Re-treatment may be required to achieve effective control. Chlorsulfuron can be harmful to grasses. Leaves should be sprayed until wet but not dripping to achieve good control. Chlorsulfuron needs to be agitated constantly during application.
Product Names: Perspective
Rate:
4.75 to 8 oz per acre
Time: Apply in spring (March-May) after plants are in the rosette or bolting stage.
Comments: Re-treatment may be required to achieve effective control. Amincocyclopryachlor+chlorsulfuron is broad leaf selective herbicide and not will harm grasses. Use care when working around desirable trees and shrubs to avoid damage. Leaves should be sprayed until wet but not dripping to achieve good control.
Product Names: Arsenal, Habitat, Stalker
Rate:
4-6 pt product/acre
Comments: Labeled for use, but not typically recommended for control of Canada thistle
Product Names: Oust and others
Rate:
6 – 8 oz product per acre (4.5 – 6 oz ai/acre)
Time: Apply pre-emergence to post-emergence in early spring (February-April) when plants are actively growing and germinating.
Comments: Re-treatment may be required to achieve effective control. Sulfometuron can be harmful to grasses and natives. Use care when working around desirable plants to avoid damage. Leaves should be sprayed until wet but not dripping to achieve good control.
Product Names: Tordon 22K
Rate:
2 pt per acre (8 oz ae/acre)
Time: Apply post emergence in spring (March-May) when plants are growing rapidly before the bud stage.
Comments: Picloram can cause long term soil activity and should not be used around trees because of root uptake. Use care when working around desirable plants to avoid damage. Leaves should be sprayed until wet but not dripping to achieve good control. Restricted herbicide.
Several different insects are have been used to target Canada thistle populations in Oregon. While effective in reducing infestations, biological control methods will never result in the complete eradication of a weed. Populations of all biological controls will vary from year to year.
Canada thistle flowers and seeds should be collected and put in trash or yard waste. Do not compost flowers and seeds. Non-flowering plants can be chopped up and composted or put in trash or yard waste.
Follow-up treatments are often necessary to control large infestations.
