Bull thistle, a member of the Asteraceae family, is yet another herbaceous biennial on our list of invasives. Unfortunately, bull thistle can flourish in an array of sites ranging from meadows, forests, roadsides, ditches, and most disturbed sites.
- basal rosette during first year
- reaches heights ranging from 1-5 feet second year
- very hairy, deeply lobed leaves with narrow teeth and lobes tipped with spines (coarse on top of leaf, soft underneath)
- purple flower heads are 1-2 inches wide, with bracts tipped in long, stiff spines
- often singular flowers, bloom July-August, typically a few weeks after other thistles
- stems are winged and spiny, similar to that of European marsh thistle
- long taproot reaching lengths upwards of two feet
Like other thistles, bull thistle is armed with a great number of spines that can and will penetrate exposed skin.
For more information visit Midwest Invasive Species Information Network (MISIN).
KISMA Management Practices
A common invasive found throughout the continental United States, bull thistle poses many ecological threats such as reducing quality wildlife forage and the loss of species diversity. Like many other biennial invasives, bull thistle is able to produce a large seed bank that can remain viable for up to 10 years! As a result, the plant can easily spread and form dense stands in disturbed sites. See the following steps for proper bull thistle management.
- wear protective clothing - pants, long sleeve shirts, gloves, and potentially safety glasses
- when working with smaller populations, hand pulling with gloves and a shovel is the preferred option (if root cannot be fully extracted, try to cut it at least one inch below the soil)
- when working with larger infestations, mowing or weed whipping can be done, although repeated cuts - one early season before flowers bloom, and one late season will be needed to suppress the plant (be sure not to mow infestations that are flowering as this will distribute seed)
- after removal, ensure proper disposal through bagging/piling
Note: Like many other invasives, early detection and management of small populations is key to bull thistle control. Because of the plant’s ability to produce a large seed bank, complete eradication will require several years of continued management and monitoring. Furthermore, management practices which target the plant's roots such as hand pulling are preferred, but not necessary as bull thistle can only reproduce through seed production via its flowers.
If you do happen to remove bull thistle on your property, it is never a bad idea to plant some natives in its absence! Since bull thistle prefers an array of disturbed sites, it would be best to use natives that also grow well in these conditions. A few great options would be goldenrod (Solidago spp.), sunflowers (Helianthus spp.), wild lupine (Lupinus perennis), swamp milkweed (Asclepias incarnata), native marsh thistle (Cirsium muticum), and native asters (Asteraceae family). In addition to promoting wildlife diversity, these species also grow well in meadows, field edges, and forest edges.