What a difference a year makes It was one year ago that we reported a large infestation of milkthistle (Silybum marianum) growing in a twenty-acre horse pasture outside of Wilsonville. At that time, the WeedWise program was approached by the resident for assistance in helping her to control the milkthistle. As a priority invasive weed […]
Where do you start when you are dealing with 20 acres of pasture land heavily infested with Blessed milkthistle? The first step is to call the WeedWise program! We were recently contacted by a landowner with this impressive stand of Blessed milkthistle (Silybum marianum) growing in horse pasture lands. The landowner was feeling very overwhelmed […]
blessed milkthistle, milk thistle, Marian thistle, Mary thistle, Saint Mary’s thistle, Mediterranean milk thistle, variegated thistle
Silybum marianum (Syn. Cardus marianus)
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.
Up to 6 feet.
Purple
April to October
None
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.
Southern Europe
Yes
Oregon Noxious Weed Pprofile
Washington Noxious Weed Profile
Invasive.org profile
CABI Invasive Species Compendium
Blessed milkthistle, Milk thistle, Marian thistle, Mary thistle, Saint Mary’s thistle, Mediterranean milk thistle, Variegated thistle
Silybum marianum. (Syn. Carduus marianus)
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.
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. Leaf margins are tipped with spines up to 1/2 inch in length. Large rosettes can reach 3 feet in diameter.
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. Plants flower from April to October.
Dark brown, relatively heavy hairless achenes, about 1/2 by 1/4 inch, form in autumn.
Plants grow from long white taproots.
Reproduction is from seeds. Plants are typically biennial in our area, so they die after going to seed. Seeds are relatively heavy and dispersal is usually by animals, equipment, vehicles or contaminated hay. Plants can produce more than 6,000 seeds per year which can remain viable in the soil for at least 9 years, where disturbance will cause them to germinate. Seeds often germinate following the first rains of the season but may continue to germinate through the winter.
Blessed milkthistle can be found in full sun or part shade. They typically grow in heavily grazed 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.
Native to southern Europe, blessed milkthistle was probably introduced to the US as a medicinal plant by early colonists. It has become common in Canada and throughout the southern United States.
Blessed milkthistle is relatively rare in Clackamas County, with populations associated with disturbed and unmanaged sites. Milkthistle is occasionally cultivated as an herbal supplement.
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’s), which outlines the best approach for controlling a weed particular infestation.
Blessed milkthistle responds well to manual control for small outbreaks. Dig plants whenever you find them, prior to flowering. Once they are flowering, be sure to carefully bag and dispose of plants to prevent seed set. Viable seeds will continue to form after plants are dug up and it can be difficult to contain mature seed heads without spreading seeds. In this case, carefully remove and bag all flower and seed heads before digging plants. Where mature plants are removed search carefully for small rosettes or germinating seeds. Return to the same location in spring and fall, and continue to monitor for several years. For best control, sow native plant seeds on disturbed soils to suppress milkthistle germination.
Cultivation and tillage can be effective control options for seedlings, but soil disturbance will increase germination in the soil seed bank. Mowing will not eradicate milkthistle. Plants are able to re-sprout and flower in the same season after being mowed. Plants can persist as perennials in a regular mowing cycle and may flower below the level of the mower, spreading seeds to new areas. Mowing may also increase the amount of toxic material ingested by grazing animals, as milk thistle becomes more palatable as it wilts. If you do mow near a milkthistle infestation, be sure to clean your mower to prevent spreading seeds to new locations.
Good grazing practices and management for grasses and forage species will reduce the disturbance regime and decrease the opportunity for milk thistle to become established. Minimize soil disturbance and re-vegetate to prevent infestations. Burning is not an effective control and can encourage seed germination and establishment.
Herbicides should only be used at the rates and site conditions specified on their label.
For control of large milk thistle infestations, apply a selective broadleaf herbicide and surfactant in the spring and again in the fall. Infested areas should not be mowed until the herbicide has had a chance to work. Continue to monitor areas in spring and autumn for several years following treatment.
Before you Start:
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.
Product Names: Many products
Rate: 3-4 pt product/acre (1.43 to 1.9 lb a.e./acre)
Time: Apply post emergence in spring or fall to young, growing plants.
Comments: 2,4-D is a broadleaf-selective herbicide without residual soil activity. Treat rosettes in the fall. Use spring treatments prior to flower stalk elongation. Treat annually for several years to control seedlings.
Product Names: Milestone
Rate: 3-5 oz product/acre (0.75 to 1.25 oz a.e./acre)
Time: Apply post emergence in spring or to early summer to rosettes and bolting plants. Apply in fall to seedlings and rosettes.
Comments: Aminopyralid is a broadleaf-selective herbicide with moderate soil residual activity. A non-ionic surfactant can enhance control under adverse environmental conditions.
Product Names: Transline
Rate: 0.25-1 pt product/acre. (1.5-6 oz a.e/acre) Spot treatment: 1-2% v/v solution
Time: Apply post emergence from the seedling to bud stage, when plants are rapidly growing
Comments: Generally safe for grass but will harm members of Asteraceae and Fabaceae
Product Names: Curtail
Rate: 1-5 qt product/acre.
Time: Apply post emergence to basal rosettes from spring until bud stage.
Comments: This is a non-selective product and neither compound has soil activity. Replant sprayed area to prevent re-establishment of milk thistle.
Product Names: Banvel, Clarity
Rate: 1-2 pt product/acre (0.5-1 lb a.e./acre).
Time: Apply post emergence to basal rosettes from spring until bud stage.
Comments: This is a non-selective product and neither compound has soil activity. Replant sprayed area to prevent re-establishment of milk thistle.
Product Names: Campaign
Rate: Broadcast foliar treatment: 1-2 pt product/acre. Spot treatment: 1-2% v/v solution
Time: Apply post emergence in spring to rosettes, or in fall before a freeze.
Comments: This is a non-selective product and neither compound has soil activity. Replant sprayed area to prevent re-establishment of milk thistle.
Product Names: Telar
Rate:1 oz product/acre (0.75 oz ai/acre)
Time: Apply post emergence, to rapidly growing plants
Comments: Chlorsulfuron is primarily active against broad-leaf plants and generally will not harm grasses. It has long soil activity. Do not apply to dry, powdery, or sandy soils unless rain is expected.
Product Names: Escort
Rate:1 oz product/acre (0.6 oz ai/acre)
Time: Apply post emergence, to rapidly growing plants
Comments: Metsulfuron is primarily active against broadleaf plants and generally will not harm grasses. It has some soil activity. Use a non-ionic or silicone-based surfactant to increase effectiveness.
Blessed milkthistle is impacted by Rhinocyllus conicus, a seed head weevil first released in 1979. Rhinocyllus conicus is widespread in Oregon and feed heavily on blessed milkthistle. The presence of Rhinocyllus conicus offered excellent control of blessed milkthistle but is not recommended. Rhinocyllus conicus targets many native thistles species, and its redistribution is discouraged. Interstate distribution of this biological control is prohibited.
Bag all flowers and seed heads and dispose of in the garbage. Seeds will continue to mature after a plant is pulled. Do not compost seeds or flower heads.
Sites should be monitored for several years after the removal of mature plants. Viable seeds can last at least 9 years in the soil.
