Milk Thistle

Milk Thistle

Silybum marianum
Milk_Thistle011_JLeekie

Status in Squamish:

PREVENTION-WATCHLIST

Status in Whistler:

PREVENTION-WATCHLIST

Status in Pemberton:

PREVENTION-WATCHLIST
Vectors of Spread:
Synonyms

Blessed Milk Thistle, Spotted Thistle, Variegated Thistle

ID Characteristics

General: Milk Thistle is an annual or biennial herb.

Flowers: Large reddish-purple flowers have long spines on the bracts. Flower heads are between 2 – 6 cm in diameter and are mildly scented. Flower bracts are hairless with long spines.

Stem: Mature plants range from 0.5 – 2.0 m in height. Large Milk Thistle has hollow stems. Both stems and leaves exude a milky sap when cut.

Leaves: Are shiny and green with distinct white marbling patterns along the veins on deeply lobed leaves. Basal leaves are 15 – 70 cm long with spiny margins. The clasping stem leaves are much smaller.

Fruit: The seeds are black and brown and 8 mm long, slightly flattened with a ring of bristles at one end.

Roots: Taproot.

Similar Species
  • Native:

Wavy Leaf Thistle (Cirsium undulatum) by R.W Smith

Wavy Leaf Thistle has distinctive wavy leaves, and the upper leaf surface lacks spines. White hairs on the leaves make it appear grey.

 

 

 

 

 

 

 

 

  • Non-Native:

Bull Thistle (Cirsium vulgare)

Bull Thistle (Cirsium vulgare) has lobed hairy leaves and very sharp spines on the stem.

 

 

 

 

Scotch Thistle (Onopordum acanthium) by J. Leekie

Scotch Thistle (Onopordum acanthium) is very tall, reaching 2m. It has a woolly appearance and spiny wings on the stem.

 

 

 

 

 

 

 

 

 

Another distinguishing feature is that Milk Thistle has distinctive white marbling on the leaves.

Report

Please report any sighting of Milk Thistle by clicking here.

Habitat and Origin

Milk Thistle is native to the Mediterranean region, western Asia, and Russia. Milk Thistle was originally imported as an ornamental or medicinal plant.

Milk Thistle grows well in areas with full sun and moist soils. It prefers nutrient-rich soils with high nitrogen levels. It can be found in disturbed soils such as roadsides, ditches, and pastures.

Current Distribution

Propagation & Vectors of Spread

Milk Thistle reproduces by seed. Each plant can produce up to 6,000 seeds per year. The seeds have a large pappus (tuft of hair) which allows it to be carried by the wind. Germination occurs in the fall. Milk Thistle seeds can stay viable for more than 9 years.

Milk Thistle spreads by seed. The heavy seeds do not fall far from the parent plant but they are often transported by rainwater or streams. Animal and human activity also spreads Milk Thistle.

Ecological, Economic, & Health Impacts

Ecological: 

  • Toxic to wildlife and livestock.
  • Creates dense stands that are difficult to cross, reducing habitat and creating barriers.
  • Reduces biodiversity.

Economic:

  • Reduces livestock foraging areas.
  • Reduces crop value.

Health:

  • Has large woody thorns that can pierce footwear.
  • Forms dense stands that are very uncomfortable for people to move through, even on horseback.
What Can I Do?

Milk Thistle is NOT currently found throughout the Sea to Sky Corridor, so the best approach to controlling its spread is by PREVENTION.

 

Learn to identify Milk Thistle: use the images presented in this profile page to learn how to identify Milk Thistle.

What to do if you spot it: You can report any Milk Thistle sighting by clicking here.

DO:

  • Regularly monitor properties for weed infestations.
  • Ensure soil and gravel are uncontaminated before transport.
  • Check wildflower mixes to ensure that they do not contain Milk Thistle.
  • Ensure that plants are disposed of in a garbage bag if found in floral arrangements to prevent seeds from spreading.

 

DO NOT:

  • Do not unload, park or store equipment or vehicles in infested areas; remove plant material from any equipment, vehicles, or clothing used in such areas and wash equipment and vehicles at designated cleaning sites before leaving infested areas.
  • Do not plant Milk Thistle in a garden, no matter how well-contained its enclosure may seem.
  • Do not move soil that has been contaminated with Milk Thistle.
  • Do not compost.

 

Control

Mechanical

  • Dig or excavate Milk Thistle, replacing any divots created to reduce soil disturbance.
  • Regular mowing/tilling to reduce weed progression is an option and material can be left on-site to decompose; if the plants have already reached the post-flowering stage, all plant parts must be bagged and deeply buried at the landfill.
  • When tilling, be sure to prevent machinery from spreading root fragments to new sites.
  • The best time to remove Milk Thistle is April and May before the plant goes to seed.
  • Wear protective clothing when removing to avoid puncture wounds from the spines.

Chemical

  • Triclopyr, 2,4-D, aminopyralid, and glyphosate are effective herbicides.
  • We recommend that any herbicide application is carried out by a person holding a valid BC Pesticide Applicator Certificate. Before selecting and applying herbicides, you must review and follow herbicide labels and application rates; municipal, regional, provincial and federal laws and regulations; species-specific treatment recommendations, and site-specific goals and objectives.

Biological

There is no biocontrol available for this plant.

References