Horse thistle - Onopordum acaulon
Effective: May 13, 2015
Taxonomic Position: Asterales : Asteraceae
Pest Type: Plants
Pest Code (NAPIS): PBFCNDA
These Approved Methods are appropriate for: 2025, 2024, 2023, 2022, 2021, 2020, 2019, 2018, 2017, 2016, 2015
Pest Information
CAPS Pest Datasheet: https://download.ceris.purdue.edu/file/4460
Pest Tracker: https://pesttracker.org/pest/PBFCNDA
Pest is vectored by:

This pest is is not known to be a vector and it is not vectored by any organism.

Survey

Approved Method(s)
Method Detail NAPIS Survey Method
Visual Collect suspect plants. 3031 - General Visual Observation

Survey Recommendations
The following are recommendations for executing the survey using the approved methods for pest surveillance. The recommendations are developed through literature review and consultation with subject matter experts.
Signs:

Found in pastures, roadsides, disturbed areas, irrigated vegetable crops, and cultivated land. Plants develop a large rosette that may spread to cover an area up to 24 in or more in diameter. The numerous whitish-green leaves are densely covered in woolly hairs. They are deeply divided with numerous spiny, waxy lobes. The flowers are borne in spherical flower heads 1.6 to 2.4 inches in diameter. Each head contains white or purple florets and is surrounded by sharp spines. Seeds are gray or brown in color and about 0.16 in long. They are four-sided with a ridged seed coat and ~1 inch long pappus (parachute) of pale barbed hairs.

Time of Year to Survey:

In Australia, this species germinates from March through August and flowers in October and November (DPIRD, 2020). In the northern hemisphere, it would be expected to germinate between September and February and flower in April and May.

Survey Site Selection:

Surveys should focus on open, disturbed areas such as pastures, roadsides, irrigated vegetable crops, and cultivated land. This species is drought tolerant, and infestations can occur in both high rainfall and semi-arid areas (DPIRD, 2020). Surveys should focus primarily on warm-temperate regions (less than 17.7 in of annual rainfall) with sandy soils, although it can be found on heavier calcareous soils. Additional surveys could be conducted in cooler and wetter areas (31.5 in annual rainfall), since this weed has also been found in similar weather in Australia (Briese et al., 1990).

Site Inspection:

Search for plants close to the ground, as they do not form a flowering stalk. The flowering heads are formed close to the ground as a cluster at the center of the rosette.

Key Diagnostic or Identification

Approved Method(s)
ID/Diagnostic:
Morphological: Mature flowering plant is readily identified.

The Federally Noxious Weeds Disseminules of the U.S. provides keys and fact sheets that can help identify seeds of Federally Noxious Weeds, including O. acaulon.
Mistaken Identities:

During seedling and rosette stages: Onopordum acanthium, Onopordum illyricum

References
  1. Lionakis Meyer, D. 2017. Lionakis Meyer, D. 2017. Identification Guide for Some Common and Noxious Thistle and Knapweed Fruits (Asteraceae, tribe Cardueae), California Department of Food and Agriculture Plant Pest Diagnostics Center, Seed Science Laboratory.
  2. Scher, J. L. and D. S. Walters. 2015. Federal Noxious Weed Disseminules of the U.S., Edition 2.2. California Department of Food and Agriculture, and USDA APHIS Identification Technology Program.
  3. Scher, J. L. and D. S. Walters. 2015. Federal Noxious Weed Disseminules of the U.S., Edition 2.2. California Department of Food and Agriculture, and USDA APHIS Identification Technology Program. Onopordum Fact Sheet.

If you are unable to find a reference, contact STCAPS@usda.gov. See the CAPS Pest Datasheet for all references.