Photo: Loke T. Kok, Virginia Polytechnic Institute and State University, Bugwood.org, bull thistle, Cirsium vulgare
The foliage of bull thistle has stiff hairs on top and a woolly appearance on the bottom; while Canada thistle is smooth on top and either smooth or hairy on the underside. Bull thistle reproduces only from seeds which germinate in the spring. During the first growing season, the plant forms a rosette. Newly emerged leaves are egg-shaped and have spines along the edge. The leaves that follow are longer with pronounced spiny lobes. Bull thistle overwinters in this stage. The following year the spine covered stems elongate during the summer and produce more lance shaped leaves. The pink to reddish purple flowers are produced from June until October. Bull thistle can reach a height of 3 feet. Comparatively, Canada thistle is a perennial that reproduces through underground rhizomes and wind-blown seeds.
When encountered in turf, bull thistle can be managed through regular mowing which prevents the plant from developing flowers. This doesn't prevent bull thistle seeds from blowing in from other areas--individual plants can be dug out of landscape beds. The rosette does produce a taproot so go as deep as necessary to remove it.
Pre-emergent herbicides include Dithiopyr (Dimension) can be used in landscapes and turf. Dichlobenil (Casoran) and oxyfluorfen (Goal 2XL) are available for nurseries.
Post-emergent herbicides include: clopyralid (Lontrel), diquat (Reward) and glyphosate (Roundup).
Remember to always read the label for proper application sites and rates.