Description: This herbaceous plant is monocarpic, taking several years to develop, until it finally flowers, and then it dies down (Isaac, 2000). Most of the time, this plant consists of a loose rosette of basal leaves that develops during the spring and withers away by early to mid-summer. These basal leaves are typical 3-8" long and about one-half as much across; they have long petioles. The blades and petioles of the basal leaves are light green and either hairless or slightly short-hairy. They typically develop from rhizomes and often form colonies of immature plants up to several feet across. In any given year, a small minority of plants will bolt and develop a flowering stalk about 2½-6' tall. This central stalk is light green or purplish green, terete (circular in cross-section), and mostly glabrous, becoming more or less pubescent underneath the infloresence. It branches sparingly above, making more than one inflorescence possible. The leaves alternate along their stems. The blades of the lower to middle leaves are 3-10" long and about one-half as much across; these blades are broadly ovate, ovate-triangular, sagitate-ovate, or hastate-ovate in shape. They often have a pair of basal lobes and their margins are coarsely dentate. The upper surfaces of these blades are medium to dark green and glabrous, while their lower surfaces are light to medium green and glabrous. The alternate leaves have long petioles that are up to two-thirds the length of the blades; they are channeled above and partially winged. The upper leaf blades are smaller in size (about 2-3"), lanceolate-elliptic in shape, and less conspicuously toothed; they often have short sparse pubescence on their upper and lower surfaces.
The inflorescence has a panicle-like structure; it is about ¾-1½' tall and similarly across. Inflorescences that occur at the apex of any lateral upper stems tend to be smaller in size. The branches of the inflorescence are light green to purplish green, terete, and sparsely to densely pubescent with white hairs. Flowerheads occur at the tips of secondary or tertiary branches of the inflorescence; they are either solitary or occur in small clusters of 2-3. Individual flowerheads are about ¾-1" long and similarly across when they are mature and fully open. The base of each flowerhead consists of an involucre that is 12-16 mm. long, light green to purple (turning brown or black after forming mature seeds), and short-cylindrical in shape; the involucre has 10-15 linear bracts that are appressed together. The involucre is sparsely hairy to densely hairy. Above the involucre are 20-30 ray florets; the petaloid rays of these florets are more or less cream-colored (yellowish white), linear in shape and truncate with 5 tiny teeth. Each floret produce a narrowly linear reproductive column consisting of a whitish bifurcated style at its tip (which soon withers away) and a brownish stamen below. The blooming period occurs from late summer to mid-autumn, or even later in the absence of a hard frost; it lasts about 1-2 months. Afterwards, the ray florets are replaced with narrowly bullet-shaped achenes about 5-6 mm. long that are yellowish brown to brown. A sessile tuft of tawny hairs about 6-8 mm. across occurs at the apex of each achene. As a result, the seeds are readily dispersed by wind and less by often water. The root system of flowering plants consists of a fleshy taproot that is somewhat tuberous in shape. This plant reproduces clonally by rhizomes and by reseeding itself.
Cultivation: The preference is partial sun and light shade, and rich soil containing loam or silt with organic matter. This plant will tolerate medium shade, but it will persist only as immature rosettes of basal leaves.
Range & Habitat: The native Great White Lettuce is a relatively uncommon plant that is found in scattered locations all over Illinois (see Distribution Map). Habitats consist of moist to mesic woodlands, woodland borders, woodland openings, slopes of creeks in wooded areas, slopes of wooded ravines, small meadows in wooded areas, and disturbed woodlands. This plant is typically found in higher quality habitats, but it also occurs in degraded wooded areas.
Faunal Associations: The flowerheads are cross-pollinated mainly by bees, including bumblebees, green metallic bees (Halictidae), and an Andrenid bee (Andrena simplex). Other visitors to the flowerheads include a flower fly (Epistrophe emarginata) and the Goldenrod Soldier Beetle (Chauliognathus pennsylvanicus); see Robertson (1929) and Isaac (2000). A very small number of insects reportedly feed destructively on Great White Lettuce and other white lettuce species (Prenanthes spp.). This includes: larvae of a gall wasp (Aulacidea nabali), that forms galls near the base of stems; larvae of a gall midge (Cystiphora canadensis), that forms small purplish leaf galls; an aphid (Hyperomyzus nabali), that sucks sap from the stems, leaf undersides, and inflorescence branches during the summer; and larvae of the Gray-banded Leafroller Moth (Argyrotaenia mariana), that feeds on the leaves (Felt, 1917; Isaac, 2000; Pepper, 1965; Hottes & Frison, 1931; Natural History Museum, accessed 2010). However, by far the biggest threat to Great White Lettuce is the White-tailed Deer. This mammalian herbivore avidly browses on both the basal leaves of immature plants and the foliage of mature flowering plants (Isaac, 2000; personal observation).
Photographic Location: A woodland at Chief Shemauger Park in Urbana, Illinois.
Comments: This is one of several white lettuce species (Prenanthes spp.) in Illinois. Most of these species are woodland plants, but one of them, Rough White Lettuce (Prenanthes aspera) is a prairie plant, and another one, Glaucous White Lettuce (Prenanthes racemosa), is a wetland plant. Both of these 2 species have cylindrical-shaped inflorescences that don't branch like the woodland species of white lettuce in Illinois (Prenanthes alba, Prenanthes altissima). Great White Lettuce (Prenanthes crepidinea), differs from these other woodland species by being considerably more hairy along the branches of its inflorescence and its floral bracts (phyllaries) are hairy too. This species also spends more time in a dormant state, occurring primarily as loose rosettes of basal leaves during the spring. All of these species have been reassigned to another genus (Nabalus), and so Great White Lettuce is often referred to as Nabulus crepidineus. Another common name of this plant is Nodding Rattlesnake Root.