Bull thistle

Encyclopedia Article

Bull thistle          Cirsium vulgare (Savi.) Tenore

Family:  Asteraceae (Composite family)
Life cycle:   Biennial
Native status: Introduced
Habitat:  Pastures, roadsides, prairies

General description: First year plants form basal rosette up to 2 ft wide. Mature leaves deeply lobed with spine tipped lobes, lobes less prominent on young pants. Leaves are densely pubescent.  In second year flower stalk elongates up to 5 ft tall; numerous pink to purple flower heads up to 2 in wide; head surrounded by sharp bracts.

Key ID traits:  Individual lobes are narrow and tipped with prominent, sharp spine.  Leaves hairy on both sides

Similar species:  Musk and tall thistle are the primary biennial thistles in Iowa. While both produce basal rosettes, leaf characteristics are sufficiently different to simplify ID.  Deep lobes and pubescence distinguish bull thistle leaves from musk thistle.  FIeld thistle, a native prairie plant, has deeply lobed leaves that are not as pubescent as bull thistle, and the undersides of field thistle leaves are lighter colored than the upper surface.  Field thistle flowers in August/September, whereas bull thistle flowers in June. Flower heads of bull thistle are smaller than musk thistle.


Bull thistle basal rosette, the first several leaves are not as deeply lobed as leaves that develop later.

 


Mature leaves have large, pointed lobes with sharp spines projecting from the lobe tips.


The underside of bull thistle leaves are similar in color to the top, this is a key trait to differentiate from field and tall thistle, two native thistle species in Iowa.

 


Erect growth habit of bull thistle.


Flowerheads are approximately 1.5" in length, bracts at base of head are pubescent.

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