Spear Thistle (Cirsium vulgare)

Spear Thistle, Black thistle or Bull Thistle, spreads quickly via wind blown seeds. It is an Autumn and Winter germinating biennial herb, has a two year life span, and a taproot up to 70 cm long.

The flowers, stems and leaves of the Bull Thistle can irritate the skin upon contact, and cause redness and minor wounds.

For more information, please check out our weed ID chart. The distribution map for Spear Thistle in Australia is courtesy of the Atlas of Living Australia.

Spear Thistle is a good indicator weed of compaction, and heavy, acidic soils. 

 After you finish reading this, you will be able to:

• Identify Spear Thistle, Black Thistle or Bull Thistle.
• Know the habitat of Spear Thistle.
• Know the best cultural and chemical options to control Black Thistle.

How to Identify Spear Thistle.

The Spear or Bull Thistle spreads very quickly by wind blown seeds.

Category: This weed is an Autumn and Winter biennial dicot.

Photosynthetic Pathway: C3 Weed.

Flower: Flower heads are red to purple and 1.2 to 4 cm wide. In poor growing conditions, Spear Thistles can flower when as small as 30 cm tall.

Leaves: The dark green leaves of the Spear Thistle are rough and hairy on their upper surface. Each lobe terminates in a sharp rigid spine, and the leaves are deeply divided.

Height: It grows up to 60 to 120 cm, and sometimes reaches up to 1.5 metres.

Reproduction: Bull Thistle’s only reproduce by seed, and each plant can produce up to 8,000 seeds in a year. The seeds are short-lived on the soil surface, but can remain dormant for many years in the soil.

• Seed Longevity: >5 years.
• Weight: 2.5mg.
• Seeds/head or capsule: 100.
• No/plant: 8,000.

Comments: At first Black Thistle grows as a rosette. Then as it matures, it produces upright branched spiny stems. The rosette leaves at the base of the plant are up to 30 cm long, and the stem leaves have no stalk and are 4–25 cm long.

Habitat: Spear Thistle is a common species of wet or Summer-moist land, waste-land, pastures and high N soils. It prefers heavy soils, in direct sunlight with good fertility. It tolerates some degree of salinity, and occasional waterlogging, but does not survive prolonged flooding.

Spear Thistle or the Black Thistle is not a strong competitor. This means that well maintained lawns and turf areas tend not to have issues with Spear Thistles. Basically this weed does not invade areas that have good turf cover in the Summer and Autumn.

So the basis of cultural control of this weed is to mow at the right height, fertilize appropriately, and water as and when required for the turf species present.

If you mow regularly before Spear Thistle sets seed it will reduce the population over time and prevent further seed production. However, be aware that even if you mow frequently this does not kill first year rosette.

Hand removal of Spear Thistle works well (wear good gloves) if you cut the weed below the crown or the rosette of the plant. Make sure that you dig up all the roots, and you bag all vegetative material up. Its important that you then dispose of this material and the flower stems off-site, as this prevents them from forming viable seeds.

• The most cost-effective way to control the Spear or Bull Thistle is to spray young thistles
• Thistles are still small in the Autumn and Winter so you will get the best results if you spray then.
• You can spot spray in the Spring and Summer for larger thistles.

2,4-D and MCPA provides good top growth control on young plants before they flower. Spear Thistles in the rosette stage are very sensitive to herbicides when when you use these early in the growing season.

Dicamba herbicides work well on larger Bull Thistles, but be aware that you cannot use this herbicide on Buffalo grass. Other options on larger thistles include Contra M, Weed Blast MA and Warhead Trio. The latter two are safe to use on Buffalo.