Common name

Milk thistle, blessed milk thistle,  Mary thistle, holy thistle, variegated thistle 

Scientific name: Silybum marianum

History & Introduction 

Native to the Mediterranean region, milk thistle has since been introduced to parts of Europe, Australia, New Zealand, South America, and North America. Today, it can be found in many temperate regions throughout the world and it is established throughout the western United States, where it is classified as an invasive species. The noxious weed is found in pastures, wetlands and irrigation banks. It is suspected that it was introduced via contaminated livestock feed; Milk thistle likely found its way to New York through commercial trade, contaminated crop seed, and/or intentional cultivation in medicinal and ornamental gardens. Milk thistle has historically been used to cure a number of ailments.

Identification 

Outside its native range, milk thistle can either be an annual or biennial. When germinated early in a growth season, it can complete its life cycle within a year, behaving as a winter annual. Seeds that germinate later in the season will usually overwinter as a rosette and produce stems the following spring, making them biennials in that case. Milk thistle is an herbaceous, flowering plant that grows approximately 2 to 6 feet tall growing out of a deep taproot. 

Flat rosettes form first, developing either in the late autumn and over winter, or emerge in the early spring. Rosette leaves are deeply lobed, obovate, with spiny edges. Each individual leaf can reach up to 10 inches wide and 20 inches long, resulting in some cases where the broad basal leaves can reach up to 3 feet in diameter. The surface of the leaf has a distinctive white marbling along the veins, giving a milky appearance which inspired its common name – milk thistle. In the spring, the rosette will give rise to stems, with alternating, clasping leaves that get smaller the further up the stem. 

Flowering starts in May and continues into August in the northeast temperate regions. The flowerheads sit on the tops of terminal compound stems reaching approximately 2 inches in diameter, with bright pink to purple disk florets. Each flower head is held together by a broad, leathery, spine-tipped (meaning it has prickly points) involucral bract (surrounding the flower).

Milk thistle reproduces entirely by seed. Flowers mature in mid June and continue throughout July. Then, the flower heads transition into dense, fluffy seed heads. Each flowering head produces 100 seeds, allowing for an individual plant to produce between 1,000 to 5,000 seeds per season. The seeds are dark brown to black and are approximately ½  inch long. These seeds can remain viable in a seed bank for up to 9 years once buried. These seeds are denser than other seeds which limits their dispersal via wind, usually the seeds will drop near the parent plant, eventually creating dense colonial patches. Birds and other wildlife can transport the seeds far distances. 

Look -alike species:

North America is home to roughly 62 native thistle species, primarily found across the western prairie regions. The more widespread invasive thistles can be found throughout New York but milk thistle can be distinguished from common invasive thistle look-alikes by its dramatic, striking white marbling on its leaves and its spine-tipped involucral bracts. Milk thistle can be confused with: Canada thistle (Cirsium arvense), musk/nodding thistle (Carduus nutans), Scotch thistle (Onopordum acanthium), and bull thistle (Cirsium vulgare). 

Damage and Concerns 

When left unmanaged, it forms dense, impenetrable thickets along roadsides, grazing pastures, and disturbed areas. The sharp spines can cause harm to people, pets, and grazing animals. 

Milk thistle is allelopathic; it is able to release chemicals into the soil that will suppress the germination and the growth of surrounding plants, greatly impacting native populations. A fully formed bud will continue producing viable seeds even if removed from the root. 

Milk thistle is also a nitrate accumulator, it concentrates potassium nitrate at toxic levels to grazing animals like cattle and sheep. Upon ingestion the nitrate converts to nitrite, which binds to hemoglobin limiting oxygen transportation throughout the body leading to death. This alone poses a major threat to the United States cattle industry and ranchers

Prevention and Management  

Practicing PlayCleanGo when leaving wooded areas by cleaning and removing seeds from boots, outerwear, and gear to prevent accidental transport. 

If working to manage the plant manually, clip and bag all flowering heads in heavy-duty contractor bags; do not compost, flower heads will continue producing mature seeds after being severed. Because the seeds persist for up to nine years in the soil, monitoring and spot treatment of any emerging rosettes may be needed.

Reporting 

LIISMA is currently monitoring milk thistle as part of our Early Detection and Rapid Response Program. As reports of the species are collected, LIISMA is working with partners to delineate and manage any early emerging infections before becoming widespread. Milk thistle is currently a Tier 2 species within LIISMA, prioritized for eradication. Report suspected populations using iMapInvasives or iNaturalist to help us track the local distribution of milk thistle.