Plant Assessment Form

Silybum marianum

Synonyms: Carduus marianus L., Carduus mariae Crantz, Cirsium maculatum Scop., Cathamus maculatum (Scop.) Lam., Silybum maculatum (Scop.) Moench, Silybum mariae (Crantz) Gray, Mariana lactea Hill

Common Names: milk thistle; variegated thistle; blessed milk thistle; Virgin Mary's thistle; Lady's milk; Holy thistle; spotted thistle; cabbage thistle; spotted thistle; St. Mary's thistle; white thistle

Evaluated on: 7/20/04

List committee review date: 27/08/2004

Re-evaluation date:

Evaluator(s)

Brianna Richardson
California Invasive Plant Council
1442-A Walnut St. #462, Berkeley, CA 94709
510.843.3902, 650.210.9453
brichardson@cal-ipc.org
Joseph M. DiTomaso
University of California
Weed Science Program, Robbins Hall, Davis, CA 95616
530-754-8715
ditomaso@vegmail.ucdavis.edu

List commitee members

Peter Warner
Joe DiTomaso
Alison Stanton
Jake Sigg
Cynthia Roye
John Randall

General Comments

No general comments for this species

Table 2. Criteria, Section, and Overall Scores

Overall Score? Limited
Alert Status? No Alert
Documentation? 3.5 out of 5
Score Documentation
1.1 ?Impact on abiotic ecosystem processes D Observational
Impact?
Four-part score DCDD Total Score
C
1.2 ?Impact on plant community C. Minor Reviewed Scientific Publication
1.3 ?Impact on higher trophic levels D. Negligible Reviewed Scientific Publication
1.4 ?Impact on genetic integrity D. None Other Published Material
2.1 ?Role of anthropogenic and natural disturbance in establishment C. Minor Reviewed Scientific Publication
Invasiveness?
Total Points
10 Total Score C
2.2 ?Local rate of spread with no management B. Increases less rapidly Reviewed Scientific Publication
2.3 ?Recent trend in total area infested within state C. Stable Reviewed Scientific Publication
2.4 ?Innate reproductive potential
(see Worksheet A)
A. High Reviewed Scientific Publication
2.5 ?Potential for human-caused dispersal C. Low Other Published Material
2.6 ? Potential for natural long-distance dispersal C. Rare Reviewed Scientific Publication
2.7 ?Other regions invaded C. Already invaded Reviewed Scientific Publication
3.1 ?Ecological amplitude/Range
(see Worksheet C)
A. Widespread Reviewed Scientific Publication
Distribution?
Total Score A
3.2 ?Distribution/Peak frequency
(see Worksheet C)
B. Moderate Observational

Table 3. Documentation

Scores are explained in the "Criteria for Categorizing Invasive Non-Native Plants that Threaten Wildlands".

Section 1: Impact
Question 1.1 Impact on abiotic ecosystem processes? D Observational
Identify ecosystem processes impacted:

None identified in literature. Dead skeletons remain standing throughout the summer, often along roadways or other human corridors. These skeletons may serve to carry human-ignited fires into neighboring grasslands or shrubs. This could increase fire frequency in some grassland areas. Mostly found in disturbed areas and along roadsides and fencelines. Not very common in wildlands. No mention of increases in fire frequency is made in the literature. My observations are just that.


Sources of information:

Personal observation, Brianna Richardson brichardson@cal-ipc.org.


Question 1.2 Impact on plant community composition,
structure, and interactions?
C Reviewed Scientific Publication
Identify type of impact or alteration:

Plants develop large (up to 3 feet in diameter) rosettes that block light to nearby vegetation and suppress germination and growth. Plants can reach 2-3 m in height, and skeletons continue to stand for several months, keeping an area bare of other vegetation. Infestations can be dense and dominate pasture. In nutrient enriched sites (ex. ruderal areas), where S. marianum grows more vigorously (than in un-enriched sites) species diversity can be considerably lower than where S. marianum grows less vigorously.
S. marianum may have a competitively suppressive effect upon companion species other than Avena and Hordeum. S. marianum does not appear to significanty affect growth of Avena sterilis, A. barbata, and H. spontaneum. This suppression is likely due to the rapid growth and large biomass of the S. marianum in nutrient-enriched sites.
In Australia, S. marianum becomes densely established and excludes most other species. Very competitive once established, and can eliminate most other plants by shading and competition for moisture and nutrients.
Often occurs in dense, competitive stands. When densely established, S. marianum can eliminate native and other species. However, disturbance is required for establishment and spread, so most often sites that are dominated by S. marianum will be limited in scale and will not eliminate all neighboring plant communities and species. Requires nutrient rich sites.


Sources of information:

1) Sindel, B.M. 1997. "The persistance and management of thistles in Australian pastures." New Zealand Plant Protection Society. Accessed July 19, 2004. www.hortnet.co.nz/publications/nzpps/proceedings/97/97_453.htm.
2) Tamar Valley Weed Strategy. date unknown. "Variegated thistle (Silybum marianum)." Accessed July 19, 04. www.weeds.asn.au/weeds/txts/var_thstle.html.
3) Gabay, R.; U. Plitmann; A. Danin. 1994. Factors affecting the dominance of Silybum marianum L. (Asteraceae) in its specific habitats. Flora v. 189: 201-206.
4) Parsons, W.T. 1973. Noxious Weeds of Victoria. Inkata Press, Melbourne.
5) Goeden, R.D. 1971. The phytophagous insect fauna of milk thistle in southern California. Journal of Economic Entomology. V.64, no 5: 1101-1104.
6) DiTomaso, J., E. Healy. unpublished. Weeds of California and Other Western States.
7) Observational, Peter Warner, Jake Sigg, John Randall, Joe DiTomaso, 2004.


Question 1.3 Impact on higher trophic levels? D Reviewed Scientific Publication
Identify type of impact or alteration:

Dense clumps provide shelter for rabbits. S. marianum invasion increases the food supply (seeds) for rodents and birds. More than 47 species of insect have been found to feed or reproduce on S. marianum in southern California. At least 26 of these insects are economically damaging crop pests. S. marianum can be toxic to sheep and cattle, especially under wet conditions or times of high soil moisture. Though potentially poisonous to grazers (deer) no mention was made of grazers eating S. marianum. It is believed to be unpalatable unless damaged by cutting or herbicide application. No other significant impacts on higher trophic levels were mentioned. Though S. marianum can push out native plant cover (thereby impacting wildlife) this is usually limited in scale [see 1.2].


Sources of information:

Question 1.4 Impact on genetic integrity? D Other Published Material

None. No native Silybums exist in California.


Sources of information:

Keil, D.J. 1993. Asteracea family. In: The Jepson Manual. Published by the University of Californa.


Section 2: Invasiveness
Question 2.1 Role of anthropogenic and natural disturbance
in establishment?
B Reviewed Scientific Publication
Describe role of disturbance:

Plants are larger, have more flowering heads, and a higher number of achenes per head in nutrient-enriched sites like waste places and ruderal zones, than those that grow in un-enriched sites. Plants are less likely to colonize and thrive in sites undisturbed by human activity.
Occurs in pastures, waste lands, irrigation banks.
Seedlings prefer disturbed soil, and don't establish in perennial pasture if the soil is well covered with vegetation during late summer and autumn.
Requires disturbance to spread. Infestations will remain localized unless disturbance becomes more widespread.
Grows on ant hills and rodent burrows. Requires human or animal disturbance to establish and spread.


Sources of information:

Question 2.2 Local rate of spread with no management? B Reviewed Scientific Publication
Describe rate of spread:

Rapidly spread through the central valleys of CA in the 1940's. Can spread quickly to dominate discontinuous habitat. Requires disturbance to spread. Infestations will remain localized unless disturbance becomes more widespread.


Sources of information:

Question 2.3 Recent trend in total area infested within state? C Reviewed Scientific Publication
Describe trend:

Increasing but not rapidly. Plants are larger, have more flowering heads, and a higher number of achenes per head in nutrient-enriched sites like waste places and ruderal zones, than those that grow in un-enriched sites. Plants are less likely to colonize and thrive in sites undisturbed by human activity. Requires disturbance to spread. Infestations will remain localized unless disturbance becomes more widespread. Though the plant is not usually targeted by management efforts, its requirement for disturbance and its preference for nutrient rich soils limits its spread.


Sources of information:

Question 2.4 Innate reproductive potential? A Reviewed Scientific Publication
Describe key reproductive characteristics:

Seeds can remain viable for at least 9 years. Dense stands in California can produce 1.4 million viable seeds per acre. Seed production can vary from 1.2 heads producing 42 seeds to 8.8 heads producing 876 seeds. One study found plants could produce as many as 6350 seeds/plant.
Can establish in areas dominated by annuals. Produces lots of seeds that are moved by animals, humans, and wind. Seeds are produced every year, and a plant reached reproductive maturity in one year. Seeds are viable for up to 9 years.


Sources of information:

Question 2.5 Potential for human-caused dispersal? C Other Published Material
Identify dispersal mechanisms:

Hay contamination is a common problem in Victoria, Australia. Seeds disperse long distances with human activities, often found along trail margins in grassland and chaparral. Long distance transport is probably rare though. Human dispersal does spread S. marianum, but the means (as a seed contaminate, by trail users) are less troublesome than if the plant were sold horticulturally or used in revegetation work.


Sources of information:

1) DiTomaso, J., E. Healy. unpublished. Weeds of California and Other Western States.
2) Wheatley, W.M. 1971. Thistles--Prickly problem of pasture improvement. The Agricultural Gazette of New South Wales.


Question 2.6 Potential for natural long-distance dispersal? C Reviewed Scientific Publication
Identify dispersal mechanisms:

Seeds equipped with a large pappus that enables effective though short-distance spread by wind. Seeds are consumed by birds, rodents, and insects. Seeds probably dipersed by water and soil movement. S.marianum spread rapidly through the central valleys of California during the 1940s. It spread to dominate a discontinuous habitat in a short period of time. Though evidence that the seeds are spread by birds and animals is somewhat lacking, and the seeds are too large to be wind-borne for long distances, this plant has historically spread quickly over large distances, and therefore may still be able to do so.


Sources of information:

Question 2.7 Other regions invaded? C Reviewed Scientific Publication
Identify other regions:

Invades pastures, ruderal areas, and ant hills in Australia, Tasmania, Israel. Also found in Alabama, Arizona, Arkansas, Connecticut, Indiana, Lousiana, Maryland, Massachusetts, Michigan, Nevada, New Hampshire, New Mexico, New York, Ohio, Oklahoma, Oregon, Pennsylvania, Tennessee, Texas, Virginia, Washington, West Virginia. S. marianum is an invader in other Mediterranean climates, but largely invades habitat similar to the habitat it is already found in here.


Sources of information:

1) Gabay, R.; U. Plitmann; A. Danin. 1994. Factors affecting the dominance of Silybum marianum L. (Asteraceae) in its specific habitats. Flora v. 189: 201-206.
2) Parsons, W.T. 1973. Noxious Weeds of Victoria. Inkata Press, Melbourne.
3) Plants Database http://plants.usda.gov


Section 3: Distribution
Question 3.1 Ecological amplitude/Range? A Reviewed Scientific Publication

Widely naturalized in California, particularly in coastal counties, but also inland. First recorded in CA in 1854. Common in coastal and inland valleys. Is found in at least 3 major ecological types and 7 minor ecological types.


Sources of information:

1) Klinger, Rob. 1999. Weed Report: Silybum marianum. TNC Wildland Weed Survey.
2) Goeden, R.D. 1971. The phytophagous insect fauna of milk thistle in southern California. Journal of Economic Entomology. V.64, no 5: 1101-1104.
3) Roche, Cindy. 1991. Weeds: Milk thistle. Pacific Northwest Extension Publication #382.
4)San Francisco Estuary Institute: www.sfei.org/wetlands/Reports/Final%20Draft%20Plant%20speci~000.pdf
5) Los Angeles CNPS chapter. Invasive Plant List: www.lacnps.org
6) Catalina Island Conservancy www.catalinaconservancy.org
7) Plants of the Kaweah River Delta Region www.kaweahoaks.com
8) Big Creek Reserve plants http://www.redshift.com/~bigcreek/index.html


Question 3.2 Distribution/Peak frequency? B Observational
Describe distribution:

I estimate it's in >50% of the grasslands in the SF Bay area. D.Chang estimates it's in 80% of the grasslands (rangelands) and oak woodlands in Santa Barbara County. Present in 21-50% of valley and foothill grasslands in CA.


Sources of information:

1) Personal observation: Brianna Richardson brichardson@cal-ipc.org
2) Personal communication: David Chang, Santa Barbara Co Agricultural Commissioner's Office dchang@co.santa-barbara.ca.us
3) Observational, Peter Warner, Jake Sigg, John Randall, Joe DiTomaso, 2004.


Worksheet A - Innate reproductive potential

Reaches reproductive maturity in 2 years or less Yes
Dense infestations produce >1,000 viable seed per square meter Yes
Populations of this species produce seeds every year. Yes
Seed production sustained over 3 or more months within a population annually Unknown
Seeds remain viable in soil for three or more years Yes
Viable seed produced with both self-pollination and cross-pollination Unknown
Has quickly spreading vegetative structures (rhizomes, roots, etc.) that may root at nodes No
Fragments easily and fragments can become established elsewhere No
Resprouts readily when cut, grazed, or burned No
Total points: 6
Total unknowns: 2
Total score: A?

Related traits:

Worksheet B - Arizona Ecological Types is not included here

Worksheet C - California Ecological Types

(sensu Holland 1986)
Major Ecological Types Minor Ecological Types Code?
Marine Systemsmarine systems
Freshwater and Estuarine lakes, ponds, reservoirs
Aquatic Systemsrivers, streams, canals
estuaries
Dunescoastal
desert
interior
Scrub and Chaparralcoastal bluff scrub
coastal scrubD, < 5%
Sonoran desert scrub
Mojavean desert scrub (incl. Joshua tree woodland)
Great Basin scrub
chenopod scrub
montane dwarf scrub
Upper Sonoran subshrub scrub
chaparralD, < 5%
Grasslands, Vernal Pools, Meadows, and other Herb Communitiescoastal prairieC, 5% - 20%
valley and foothill grasslandB, 20% - 50%
Great Basin grassland
vernal pool
meadow and seep
alkali playa
pebble plain
Bog and Marshbog and fen
marsh and swamp
Riparian and Bottomland habitatriparian forest
riparian woodlandD, < 5%
riparian scrub (incl.desert washes)D, < 5%
Woodlandcismontane woodlandD, < 5%
piñon and juniper woodland
Sonoran thorn woodland
Forestbroadleaved upland forest
North Coast coniferous forest
closed cone coniferous forest
lower montane coniferous forest
upper montane coniferous forest
subalpine coniferous forest
Alpine Habitatsalpine boulder and rock field
alpine dwarf scrub
Amplitude (breadth): A
Distribution (highest score): B

Infested Jepson Regions

Click here for a map of Jepson regions

  • CA Floristic Province
  • Cascade Range
  • Central West
  • Great Valley
  • Northwest
  • Sierra Nevada
  • Southwest
  • Mojave Desert