Blessed Milkthistle (Silybum marianum)

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:

• WeedWise: Priority
• State of Oregon: Class B
• State of Washington: Class A
• Four County CWMA: Class A
• Columbia Gorge CWMA: Class A

Origin:

Southern Europe

Present in Clackamas County:

Yes

Links:

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