As usual, it didn't take long to get a correct answer. Tom correctly identified the quiz plant as Senecio vulgaris, Common Groundsel. According to the USDA Plants database, the common name of this plant is Old Man in the Spring (USDA NRCS 2009). The Latin name Senecio comes from senex, meaning "old man," which is a reference to "the hoariness of many species [in the genus], or perhaps to the white hairs of the pappus" (Fernald 1950). The specific epithet vulgaris means "common" (Fernald 1950).
As I mentioned in my quiz post, the plant was in bloom on Thanksgiving 2009. It has one of the longest flowering periods of any plant in the Chicago Region, being known to bloom from April into December (Swink & Wilhelm 1994). It is, in fact, considered a winter annual (Oregon State 2009), which is a plant that germinates and grows in fall or winter but that does not live longer than a year. This weed of gardens and waste areas is native to Eurasia and has spread throughout nearly all of North America with the exception of the Canadian Arctic north of the Hudson Bay (Barkley 2006). This member of the Asteraceae lacks ray flowers and has only disk flowers.
Fernald, M.L.. (1950). Gray's Manual of Botany, Eighth (Centennial) Edition. New York: American Book Company.
Swink, F. & G. Wilhelm. (1994). Plants of the Chicago Region. Indianapolis: Indiana Academy of Science.
Oregon State. 2009. Weed Management in Nursery Crops (http://oregonstate.edu/dept/nursery-weeds/weedspeciespage/common_groundsel/common_groundsel_page.html, 28 December 2009).
USDA, NRCS. 2009. The PLANTS Database (http://plants.usda.gov/, 28 December 2009). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.