The fatal flowers of Tall Thistle (Cirsium altissimum)

Tall Thistle (Cirsium altissimum); Durham Co, NC (9/14/15)

I find Tall Thistle (Cirsium altissimum) to be one of our most interesting natives.  Perhaps this is because I often root for the underdog, and so many gardeners, farmers, and even "conservationists" despise thistles. Hopefully their aversion may change if they come to understand the wildlife uses of native thistles and more particularly the benefits for native bees and butterflies. A fairly impressive, preliminary list of these has been documented using Tall Thistle, including over 20 native bees and nearly as many butterflies (1)

Hummingbird Clearwing Moth (Hemaris thysbe) visiting Tall Thistle; Durham Co, NC (9/14/15)

Tall Thistle occurs widely across the eastern US, from the prairie regions eastward, but is largely absent from the southeastern coastal plain. In North Carolina it has been reported mainly from western Piedmont and mountain counties; Durham County appears to be the easternmost location in the state.

Predominately white flowering Cirsium altissimum with a scattering of pink flowers  (Durham Co, NC - 9/14/15)

The majority of plants I have seen in North Carolina flower white, with a lesser intermixing of pink flowers (elsewhere in the range flowers seem to be more typically pink or purple and less commonly white). Tall Thistle is "monocarpic" meaning individual plants die shortly after flowering and setting seed (the classic case of monocarpism is the Century Plant). Cirsium altissimum is sometimes listed as biennial, but one group of authors (2) documented individual plants taking up to 4 years to emerge from young rosettes (such as the one shown below) into flowering stems.

Densely pubescent leaves of C. altissimum

In comparison to most thistle species, Cirsium altissimum has relatively soft and prickle-free leaves. Lower leaf surfaces are covered with dense white pubescence and stems are obviously hairy to the naked eye. Large leaves, a foot or more long, may develop on heavily shaded plants (see below). Some flowering stems exceed 10' in height and most produce multiple blooms.

Large shade leaves of C. altissimum, almost free of thorns

The large natural range of Tall Thistle and frequent occurrence in many regions suggest this is not, generally, a species whose survival we would need to be overly concerned about. HOWEVER, in a strange twist of fate, Tall Thistle may now be threatened by efforts in Midwestern prairie regions to eliminate truly problematic and invasive thistle species. A "bio-control" agent (weevil) deliberately introduced to control non-native thistles actually uses Tall Thistle "as frequently and intensively as it uses the targeted, exotic host plant" (3).

Cirsium altissimum seems to thrive only in relatively open habitats.  Ironically, the site where Tall Thistle occurs in Durham Co, NC also happens to be both nutrient rich and moist, creating growing conditions very conducive to dense woody plant growth. This apparent conundrum seems to suggest, at least in our area, that Tall Thistle requires regular disturbance and is likely another in the growing list of fire benefiting species. In fact, the currently large Durham population only appeared after burning. Thus far, we have been able to keep a portion of the site open with a combination of prescribed fire, selective chainsaw removal of trees, and invasive plant control.

Tall Thistle (Cirsium altissimum) flowering in open, "savanna"; Durham Co, NC (9/25/15)

References:
(1) http://illinoiswildflowers.info/flower_insects/plants/tall_thistle.htm
(2) http://www.esajournals.org/doi/pdf/10.1890/ES11-00096.1
(3) http://www.researchgate.net/publication/26278359