Plant Assessment Form

Onopordum acanthium

Common Names: Scotch thistle; cotton thistle; woolly thistle; winged thistle; jackass thistle; heraldic thistle

Evaluated on: 1/14/05

List committee review date: 08/07/2005

Re-evaluation date:

Evaluator(s)

Gina Skurka
California Department of Food and Agriculture
1220 N Street, Room A-357, Sacramento, CA 95814
(916) 654-0768
gskurka@cdfa.ca.gov
Joseph DiTomaso
University of California-Davis
Dept. Plant Sci., Mail Stop 4, Davis, CA 95616
530-754-8715
jmditomaso@ucdavis.edu

List commitee members

Carla Bossard
John Randall
Carri Pirosko
Dan Gluesenkamp
Gina Skurka
Brianna Richardson

General Comments

No general comments for this species

Table 2. Criteria, Section, and Overall Scores

Overall Score? High
Alert Status? No Alert
Documentation? 3 out of 5
Score Documentation
1.1 ?Impact on abiotic ecosystem processes B Other Published Material
Impact?
Four-part score BAAD Total Score
A
1.2 ?Impact on plant community A. Severe Other Published Material
1.3 ?Impact on higher trophic levels A. Severe Other Published Material
1.4 ?Impact on genetic integrity D. None Other Published Material
2.1 ?Role of anthropogenic and natural disturbance in establishment B. Moderate Reviewed Scientific Publication
Invasiveness?
Total Points
13 Total Score B
2.2 ?Local rate of spread with no management A. Increases rapidly Other Published Material
2.3 ?Recent trend in total area infested within state B. Increasing less rapidly Other Published Material
2.4 ?Innate reproductive potential
(see Worksheet A)
A. High Other Published Material
2.5 ?Potential for human-caused dispersal C. Low Other Published Material
2.6 ? Potential for natural long-distance dispersal C. Rare Other Published Material
2.7 ?Other regions invaded C. Already invaded Observational
3.1 ?Ecological amplitude/Range
(see Worksheet C)
B. Moderate Other Published Material
Distribution?
Total Score B
3.2 ?Distribution/Peak frequency
(see Worksheet C)
C. Low 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? B Other Published Material
Identify ecosystem processes impacted:

The fleshy taproot that may extend down 1 foot or more, would tend to be deeper than native grasses so this will contribute to less organic matter near the soil surface, increased soil erosion than native grasses' fibrous roots, and may affect soil water content.. Thick taproots of this weed may decompose more slowly than fine roots of grasses, lowering the annual input of organic matter ot the soil. Large infestations could have effects on evaporation, temperature, and erosion of the exposed soil surface. Moderate alteration of an ecosystem process.


Sources of information:

CDFA Encycloweedia, Jan 6, 2005. <>.
Hildreth, D. 2003. Scotch Thistle (Onopordum acanthium). <>.


Question 1.2 Impact on plant community composition,
structure, and interactions?
A Other Published Material
Identify type of impact or alteration:

Severe infestations can form tall, dense, impenetrable stands, especially in fertile soils. Scotch thistle reduces productivity and strongly competes with native plants for resources. Scotch thistle can suppress the highly competitive annual grasses, medusahead, and down brome, when the three are found in the same plant community. It drastically reduces productive rangeland by out competing desirable forage species. Formation of stands dominated (>75% cover) by this species. Severe alteration of plant community composition, structure and interractions.


Sources of information:

Kadrmas, T. and W.S. Johnson. Managing Scotch Thistle. University of Nevada, Reno, Cooperative Extension. Fact Sheet FS-02-57, <>.
CDFA Encycloweedia, Jan 6, 2005. <>.
Young, J.A. and R. A. Evans. 1968. Control and Ecological Studies of Scotch Thistle. Weed Science.


Question 1.3 Impact on higher trophic levels? A Other Published Material
Identify type of impact or alteration:

The growth habit is such that it is capable of excluding cattle from heavily infested areas, as well as crowding out native rangeland forage or plants. Scotch thistle is rejected by stock because of the dense spines. If it is eaten, the spines cause physical damage, particularly around the mouths and eyes of animals as well as effecting the quality of wool. Major reductions in foraging sites. Injurious components, spines, that damage the mouth and gut of native wildlife and livestock.


Sources of information:

Draft Action Plan for Scotch Thistle Eradication in Modoc and Lassen Counties, 1994. CDFA-IPC internal document from files at Redding Field Office.
Hildreth, D. 2003. Scotch Thistle (Onopordum acanthium). <>.


Question 1.4 Impact on genetic integrity? D Other Published Material

No native Onopordum in California.


Sources of information:

Hickman. 1993. The Jepson Manual.


Section 2: Invasiveness
Question 2.1 Role of anthropogenic and natural disturbance
in establishment?
A Other Published Material
Describe role of disturbance:

Invades rangeland disturbed by grazing and fire. Moderate invasive potential. This species may occasionally establish in undisturbed areas but can readily establish in areas with natural disturbance.


Sources of information:

Young, J.A. and R. A. Evans. 1968. Control and Ecological Studies of Scotch Thistle. Weed Science.


Question 2.2 Local rate of spread with no management? A Other Published Material
Describe rate of spread:

A single half acre infestation near Adin, Modoc County is known to have increased to 300 agregate acres within a 2000 acre area. This increase occurred from 1942 to 1968. Increases rapidly, doubling in less than 10 years.


Sources of information:

Proposed Scotch Thistle Program on Bureau of Land Management Lands. 1973. CDFA-IPC files, internal document, Redding field office.


Question 2.3 Recent trend in total area infested within state? B Other Published Material
Describe trend:

As of 1993, Scotch thistle has been detected in 28 of the 58 counties in California.Scotch thistle can spread rapidly. For example, it was first found in Utah in 1963. By 1981, it covered approximately 6070 hectares in 17 counties. Eight years later, it had spread to cover more than 22,540 hectares in 22 counties.


Sources of information:

Information about scotch thistle, Updated April 2000, viewed 1/4/2005 <>.


Question 2.4 Innate reproductive potential? A Other Published Material
Describe key reproductive characteristics:

Kadrmas, T. and W.S. Johnson. Managing Scotch Thistle. University of Nevada, Reno, Cooperative Extension. Fact Sheet FS-02-57, <>.
CDFA Encycloweedia, Jan 6, 2005. <>.
Hildreth, D. 2003. Scotch Thistle (Onopordum acanthium). <>.
Schuhardt, S. Noxious Weeds - The Silent Invader. The Creekside Almanac, Prescott Creeks Preservation Association. Spring 2001, Volume 7, Issue 1. <>.


Sources of information:

human activities disperse the seeds. Scotch thistle is still sold as an ornamental plant, but this is rare. Moderate - human dispersal occurs, but not at a high level.


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

human activities disperse the seeds. Scotch thistle is still sold as an ornamental plant, but this is rare. Moderate - human dispersal occurs, but not at a high level.


Sources of information:

Kadrmas, T. and W.S. Johnson. Managing Scotch Thistle. University of Nevada, Reno, Cooperative Extension. Fact Sheet FS-02-57, <>.
Onopordum acanthium. Thompson and Morgan Online Seed and Plant Catalogues. <>.


Question 2.6 Potential for natural long-distance dispersal? C Other Published Material
Identify dispersal mechanisms:

Wind, water, wildlife, and livestock disperse seeds, but the vast majority of seed fall below parent plant. Occasional long-distance dispersal by animals or abiotic mechanisms.


Sources of information:

Kadrmas, T. and W.S. Johnson. Managing Scotch Thistle. University of Nevada, Reno, Cooperative Extension. Fact Sheet FS-02-57, <>.


Question 2.7 Other regions invaded? C Observational
Identify other regions:

Occupies similar habitat in other areas outside of California.


Sources of information:

DiTomaso, observational.


Section 3: Distribution
Question 3.1 Ecological amplitude/Range? B Other Published Material

Scotch Thistle has a wide range of elevations in North America where it has been found, from 200 feet to 7,000 feet. Inhabits wet meadows, pastures, small grain fields, dry alfalfa, as well as disturbed areas in open rangeland. Grows best on the slope between arid rangeland and wet meadows along streams. Observed to invade undisturbed sage brush areas. The first mention of Scotch Thistle in CA in the literature is an article by T.C. Fuller in 1958, stating that Modoc County Ag Comm submitted the first sample for identification in 1957. The major area of infestation at the time in Modoc County was the Hung Estate Ranch which had 500 - 600 acres. According to that reprt, Scotch thistle was present at that location in 1942. In 1958, plants were found in Lassen and Lake Counties. Earliest reference to this plant being in the US is the 5th edition of Grays Manual dated 1878. Infests disturbed and neglected lands. It prefers sites near ditch banks and rivers, but also infests pastureland, crops, rangeland, and roadsides.


Sources of information:

Draft Action Plan for Scotch Thistle Eradication in Modoc and Lassen Counties, 1994. CDFA-IPC internal document from files at Redding Field Office.
Kadrmas, T. and W.S. Johnson. Managing Scotch Thistle. University of Nevada, Reno, Cooperative Extension. Fact Sheet FS-02-57, <>.


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

Most common in Great Basin graslands.


Sources of information:

DiTomaso, observational
Carri Pirosko, California Dept. of Food and Agriculture, pers. obs..


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 No
Seeds remain viable in soil for three or more years Yes
Viable seed produced with both self-pollination and cross-pollination Yes
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 Yes
Total points: 8
Total unknowns: 0
Total score: A?

Related traits:

A single plant can produce over 30,000 seeds, which have a high degree of dormancy. Scotch thistle can flower and produce seed at less than 12 inches tall. Scotch thistle reproduces only by seed.

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 scrub
Sonoran desert scrub
Mojavean desert scrub (incl. Joshua tree woodland)
Great Basin scrub
chenopod scrub
montane dwarf scrub
Upper Sonoran subshrub scrub
chaparral
Grasslands, Vernal Pools, Meadows, and other Herb Communitiescoastal prairie
valley and foothill grasslandD, < 5%
Great Basin grasslandC, 5% - 20%
vernal pool
meadow and seepD, < 5%
alkali playa
pebble plain
Bog and Marshbog and fen
marsh and swamp
Riparian and Bottomland habitatriparian forest
riparian woodland
riparian scrub (incl.desert washes)
Woodlandcismontane woodland
piñon and juniper woodlandD, < 5%
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): B
Distribution (highest score): C

Infested Jepson Regions

Click here for a map of Jepson regions

  • CA Floristic Province
  • Cascade Range
  • Central West
  • Great Valley
  • Northwest
  • Sierra Nevada
  • Southwest
  • Modoc Plateau
  • Sierra Nevada East