Kill the Thistles... Right?

In early July I made a trip to a northwest Indiana location to see a thistle.  "Why would anyone travel to see a thistle," you ask?  Thistles are common weeds in almost every old field, right?

 

Not this one.  To the untrained eye, the plant in these photographs may look a bit like Bull Thistle (Cirsium vulgare), but you can see at the bottom of the above photograph that the stem below the flowering head is not winged and spiny, as it would be in Bull Thistle.  In addition, this thistle would be dwarfed by Bull Thistle, as it grows to just over half a meter tall at the most.  No, I didn't make the hour-plus drive to see a weed; I was visiting a known site for one of the rarest thistles in North America, a Great Lakes endemic, Hill's Thistle (Cirsium hillii).  I had planned my trip for early July because Hill's Thistle should have been in peak bloom, but as a result of the weird weather year that was 2012, all but one of the plants in the small population had finished blooming, and the one that still was flowering was well past its peak.  Hill's thistle was first collected back in 1890 by Rev. E.J. Hill, not more than a few miles from the location where I saw it in 122 years later.

Hill's Thistle is only known from six states and one Canadian province surrounding the Great Lakes; it is listed as endangered or threatened in three of them and special concern in others, but more importantly, it is considered globally vulnerable.  Not only is it already rare, but populations of Hill's Thistle are declining as a result of habitat loss, in part due to fire suppression and the litter accumulation and succession from prairie to shrubby habitat that comes as a result.  In addition, Hill's thistle is a short-live perennial species, with a life span of two to five years, and it only produces flowers in the final one to three of those years; if the seed isn't successful in germinating in those years, the plant simply doesn't reproduce.  Aside from locations close to the lakes, Hill's Thistle is focused in counties along the Mississippi River.  This rare thistle has been found in habitats including prairies, savannas, barrens, and open woodlands, as well as in limestone pavement alvars.  Hill's Thistle is sometimes treated as as variety of Pasture Thistle (Cirsium pumilum), a plant of the eastern (mostly northeastern) United States.