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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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 […]
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. 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.
Height of mature plants
Up to 6 feet.
April to October
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.
- 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
Present in Clackamas County: