Michael Shephard, USDA Forest Service, Bugwood.org
Bull thistle is a noxious, biennial weed that grows in gardens, lawns, roadsides, crop sites and more. It splays from the soil with numerous leaves, each one having spines and hairs. This plant grows erect with red to purplish flowers surrounded by spiny bracts. It's a very invasive weed and requires immediate control upon detection; otherwise, it can spread and take over a landscape.
Applying an herbicide before bull thistle produces seeds is the best way to manage the problem. It reproduces by seed, so early detection and herbicidal application will help to stem its growth. For the best control, apply a postemergent herbicide with triclopyr or clopyralid to the thistle when it's in the seedling or rosette stage.