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March’s Weed of the Month: Canada thistle

Canada thistle (Cirsium arvense) is a widespread invasive weed found in all parts of Clackamas County.  Unlike the many native thistles found in Oregon, Canada thistle can quickly overrun an area and displace native plants, reduce agricultural yield, and create problems for grazing animals when they infest a field or pasture. They also cost a […]

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welted-thistle-slider

The new thistle you don’t want on your property!

We have plenty of thistles growing in our area and landowners across the region are working hard to combat these prickly invaders.  While most of the thistles growing in our area are invasive, some of them are actually native.  Unfortunately, a recent discovery from eastern Oregon marks the addition of a new invasive thistle to […]

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Blessed Milkthistle (Silybum marianum)

Gallery:

Common names:

blessed milkthistle, milk thistle, Marian thistle, Mary thistle, Saint Mary’s thistle, Mediterranean milk thistle, variegated thistle

Scientific Name:

Silybum marianum (Syn. Cardus marianus)

Description:

Blessed milkthistle is a sparsely branched thistle growing up to 6 feet tall and forming dense stands. It’s a tap-rooted biennial or annual that forms large rosettes followed by 2 inch purple blooms borne singly on unbranched, grooved and somewhat cottony stems. The leaves are oblong to lanceolate, hairless, shiny dark green with distinctive white patterns running along the veins, reaching up to 20 inches long and 10 inches wide. The white mottling gives the plant the appearance of having been drenched in milk, thus the common name of milkthistle. The leaf margins are tipped with spines up to 1/2 inch in length. Large rosettes can reach 3 feet in diameter. Its solitary, composite, red-purple flowers reach 2 inches in diameter and are surrounded by leathery, spiny, hairless bracts. The all-disk flowers are similar to other thistles, with large spines extending out in layers from under the pincushion flower head.

Life cycle:

Biennial

Height of mature plants

Up to 6 feet.

Flower color:

Purple

Bloom time:

April to October

Look-a-likes:

None

Habitat:

Blessed milkthistle can be found in full sun or part shade.  They typically grow in poorly managed pastures and on roadsides where nitrogen is high and disturbance regimes are frequent. This plant is also traded horticulturally and found in ornamental and medicinal gardens.

Impacts:

  • Serious threat to livestock. Ingestion by livestock can cause nitrate poisoning and death.
  • Forms dense stands that shade out forage species and exclude livestock.
  • Spines can cause injury to people and livestock.
  • Displaces native vegetation.

Noxious Weed Listing:

Origin:

Southern Europe

Present in Clackamas County:

Yes

Links:

Oregon Noxious Weed Pprofile
Washington Noxious Weed Profile
Invasive.org profile
CABI Invasive Species Compendium

 

BMP: CANADA THISTLE (Cirsium arvense)

Common name:

Canada thistle, Canadian thistle, corn thistle, creeping thistle

Scientific Name:

Cirsium arvense (syns. Breea arvensis, Carduus arvensis, Cirsium incanum, Cirsium orchrolepideum, Cirsium setosum, Cnicus arvensis)

Noxious Weed Listing:

Description:

General:

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.

Leaves:

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.

Flowers

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 bell or umbrella shaped flowers, with female flowers being slightly bigger with longer pappus (male 10-14 mm, female 14-20 mm).

Fruits

Canada thistle have 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.

Roots

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 28 ft². Seedlings start with deep taproot with creeping rhizomes developing between 2 and 4 months.

Reproduction:

Canada thistle reproduce both by seed and vegetatively through creeping roots. March through April Canada thistle germinates, grows and forms 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. Seeds are transported short distances 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.

Habitat:

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.

Impacts:

  • Canada thistle spreads quickly and forms dense infestations
  • Reduces crop yields and pasture productivity
  • Competes with and replaces native vegetation
  • Reduces animal diversity
  • Host plant for several crop-damaging insects

Introduction:

Canada thistle was introduced to the Willamette Valley between 1875 and 1899. Although it was reported to be not common when first collected, currently it is very common throughout the area.

Distribution:

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.

State of Oregon:

United States:

Management:

Strategy:

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:

  • Management thresholds to determine when and if to initiate control,
  • The ecology and life history characteristics of the targeted invasive weed,
  • Site specific conditions and land use considerations to inform management practices,
  • The effectiveness and efficiency of various control methods.

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.

Manual:

Only young plants can be removed by digging up the roots. Developed infestations have deep,  rhizomatous roots. Any remaining fragments can produce new plants.

Solarization with black plastic can be successful with small infestations (Invasive Plants)

Do not compost flower heads, seeds or rhizomes!

Mechanical:

Mowing can be effective, but only if repeated every few weeks for several growing periods or if combined with another management practice such as herbicide application. Do not let plants flower between mowing. Regular mowing may take up to 4 years to be effective.

Tillage increases Canada thistle infestations.  Rhizomes are cut and spread, allowing new plants to grow. Tillage can be effective in combination with chemical treatments (Native Plants)

Cultural:

Burning and grazing are not effective controls for Canada thistle management. Burning may reduce seed production.

 

Chemical:

Applying a directed spray of glyphosate or clopyralid at the flower bud stage before flowering, or in the fall before the first frost will control Canada thistle. Generally, fall treatments are more effective then spring treatments.  Taking advantage of natural periods of stress can be helpful when controlling Canada thistle. Clopyralid provides the best and most consistent control. Metsulfuron and aminopyralid or triclopyr products are also effective. You may need to be repeat herbicide applications for several years.

Before you Start:

  • Before purchasing any herbicide product it is important to read the label.  The label is the Law.  Carefully review all parts of the label even if you have used the product before.  Select a product that is most appropriate for your site.  If you have questions, ask your vendor before purchasing a product.
  • When selecting herbicides always use a product appropriately labeled for your site. Follow label recommendations and restrictions at all times.  If any information provided here contradicts the label, the label takes precedence.  Always follow the label!
  • Protect yourself.  Always wear the recommended protective clothing identified on your label and shower after use.
  • When applying herbicides use spot spray techniques whenever possible to avoid harming non-target plants.
  • Do not apply during windy or breezy conditions that may result in drift to non-target plants
  • Avoid spraying near water.  Hand-pull in these areas, to protect aquatic and riparian plants and wildlife.
  • Avoid exposure to pets, pollinators, and wildlife.  Remove animals from treatment areas to avoid exposure to herbicides. Follow the reentry instructions on your herbicide label and keep pets out of the area until the herbicides have dried.  Avoid spraying when insects and animals are active.  Avoid spraying blooming plants to minimize an effects to bees and pollinators.
  • Be sure to store any chemicals, out of the reach of children and pets to keep your family safe.
  • Product labels and formulations change regularly.  Check the Pacific Northwest Weed Management Handbook and the label for current control recommendations.

Herbicides:

The mention of any brand name product is not, and should not be construed as an endorsement for that product.  They are included here only for educational purposes.  Suggested rates are generalized by active ingredient.  Specific rates will vary between products.  Be sure to review the label before application and use the recommended label rate at all times.

Active Ingredients

Product Names: Several names

Rate:
2 qt per acre (1.9 lb ae/acre)

Time: Apply in spring (March-May) after plants are in rosette rosette or bolted 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: Perspective

Rate:
4.75 to 8 oz per  acre

Time: Apply in spring (March-May) after plants are in rosette or bolted 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: 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 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: 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: 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: 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 budding 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: not registered in California.

Product Names: Accord, 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: 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: Arsenal, Habitat, Stalker

Rate:
4-6 pt product/acre

Comments: Not a recommended control for 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.

Biological:

Canada thistle stem weevil larvae (Ceutorhynchus litura), bud weevil larvae (Larinus planus) and thistle stem gall fly larvae (Urophora cardui) were specifically released in the United States to control Canada thistle. However, these insects provide little to no control of infestations.

Disposal:

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:

Follow-up treatments are often necessary to control infestations.

Best Management Practices:

Small Infestations:

  • Consider the land use practices on site.  Identify, and site specific considerations that should be taken into account before initiating control.
  • Be sure you can properly identify Canada thistle.  If you are unsure about your weed bring a sample to the Conservation District, and we can help to identify your particular weed.
  • Identify any native or desirable plants nearby, and take precautions to minimize and negative impact to them.
  • Dig individual plants or very young infestations.
  • Lay black plastic over infestations to solarize plants.
  • Chemical control: Canada thistle infestations should be mowed then followed up with herbicide application. This weakens the roots and increases the effectiveness of the herbicides.

Large Infestations:

  • Consider the land use practices on site.  Identify, and site specific considerations that should be taken into account before initiating control.
  • Be sure you can properly identify Canada thistle.  If you are unsure about your weed bring a sample to the Conservation District, and we can help to identify your particular weed.
  • Identify any native or desirable plants nearby, and take precautions to minimize and negative impact to them.
  • Mow or till populations and follow up with chemical applications on regrowth.
  • You may also be able to effectively control populations by tilling at 7-28 day intervals during the growing season for up to 4 years.

Fun Facts:

  • Canada thistle is an excellent pollinator plant supplying nectar to bees for honey.
  • Leaves can be chewed to relieve a tooth ache.
  • Roots are edible but often cause flatulence.
  • Plants host agricultural pests such as sod-web worm, bean aphid, stalk borer, and cucumber mosaic virus.

Additional Information:

References:

  1. DiTomaso, J.M., G.B. Kyser, S.R. Oneto, R. G. Wilson, S. B. Orloff, L. W. Anderson, S. D. Wright, J.A. Roncoroni, T.L. Miller, T.S. Prather.  2013.  Weed Control in Natural Areas in the Western United States.  Davis, CA: UC Weed Research and Information Center.  pp 119-121.
  2. Bossard, C. C., J. M. Randall, & M.C. Hoshovsky.  2000.  Invasive Plants of California’s Wildlands.  Berkeley, CA: University of California Press.  pp 106-111.
  3. Kaufman, S. R. & W. Kaufman.  2007.  Invasive Plants: Guide to identification and the Impacts and Control of Common North American Species.  Mechanicspurg, PA: Stackpole Books. pp 301-303.
  4. King County Noxious Weed Control Program.  2011.  King County: Canada Thistle Fact sheet http://your.kingcounty.gov/dnrp/library/water-and-land/weeds/Brochures/CanadaThistle_factsheet.pdf  (Retrieved Jan. 7, 2015).
  5. Oregon Flora Project. 2013.  Oregon Plant Atlas: Rubus bifrons.  http://www.oregonflora.org/atlas.php.  (Retrieved May 22, 2013)
  6. Peachy, E., D. Ball, A. Hulting, T. Miller, D. Morishita, P. Hutchinson. eds.  2013.  Pacific Northwest Weed Management Handbook: http://pnwhandbooks.org/weed/other-items/control-problem-weeds/thistle-canada-cirsium-arvense-selective-control-crops (Retrieved Jan. 7, 2015).
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