Horse thistle - Onopordum acaulon
EEffective: May 13, 2015
Resources:
Pest Tracker
CAPS Pest Datasheet
Screening Aids:
Taxonomic Position:
Asterales : Asteraceae
Pest Type:
Plants
Pest Code (NAPIS):
PBFCNDA
| No manual – See Host Matrix |
These Approved Methods are appropriate for:
2024, 2023, 2022, 2021, 2020, 2019, 2018, 2017, 2016, 2015
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 | Instructions | NAPIS Survey Method |
|---|---|---|
| Visual | Collect suspect plants. |
3031 - General Visual Observation |
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.
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.
Key Diagnostics
ID/Diagnostic: Morphological
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.
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
- 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.
- 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.
- 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.