Cirsium vulgare / bull thistle

Adjectives: disturbed, dramatic, edible, exposed, highly visible, nasty, non-native, ruderal, showy, weed, winter botany

Also known as: spear thistle, common thistle, Scots thistle, Scottish thistle
See also: Carduus nutans (musk thistle), Cirsium arvense (Canada thistle)

First and foremost, this is clearly a thistle. If it has started to bolt and make flowers, the stems, leaves, involucre bracts, and disk florets are all “armed and dangerous”. This plant has serious spines! And like many other thistles, the flower heads are purple and consist only of disk florets.

Second, while all thistles have a lot in common, it is possible to tell them apart. In the case of bull thistle, the distinguishing field marks are: serious stem spines; an involucre of narrow, spine-tipped bracts below the flower; the involucre is more often pear- or egg-shaped than globe shaped. When only the rosettes are present and you have a choice between bull thistle, musk thistle and Canada thistle, the musk thistle rosettes tend to be larger with wider leaves and the Canada thistle rosettes are numerous (rather than isolated) due to the clonal spread of that species.

Bull thistle is a biennial, with a really nasty rosette and long taproot developing in the first year. The leaves of the rosette and stem (after bolting starts) are deeply lobed and grey-green, possibly hairy. And did I mention they are nasty?

The flower heads sit above a pear- or egg-shaped involucre of short, spiny bracts. The fruit contain relatively long (1/4″) seeds attached to a downy pappus. The pappus hairs have feathery side hairs (plumes) which distinguish the Cirsium thistles from the Carduus (plumeless) thistles. Bull thistle spreads only by seed, but like musk thistle, the bulk of the seeds do not blow away when their pappus does and largely fall to the ground with the rest of the seedhead.

Bull thistle, like the other exotic invasive thistles, is commonly a ruderal species, colonizing bare disturbed ground. It also persists well on heavily grazed land as it is unpalatable to most grazing animals and it really loves high levels of nitrogen. In the intermountain region and the Valley, it is also found in seepage areas or along streams. Distrubance is nonetheless absolutely the big thing, whether from fire, construction, grazing or even gopher mounds… perhaps even vole holes?

Despite all the disparaging remarks here, this plant is not all bad. For one thing, it provides a great deal of nectar – more than most plants – for pollinators. The seeds are eaten by goldfinches, and the Painted Lady butterfly uses it as a host plant (i.e. the caterpillars eat it). Monarchs (the Idaho state butterfly) visit for nectar. The thistle down is used as nesting material by birds. It is really only a problem when it manages to form a monoculture somewhere other than a roadside or wasteland… and, thankfully, that isn’t often.