Aster family (Asteraceae)
This is a biennial plant that becomes 2-5' tall during its second year.
During the first year, a rosette of spreading to ascending basal leaves
develops that is up to 1' across. Individual basal leaves are up to 6"
long and 2" across; they are elliptic-oblanceolate to obovate in shape,
while their margins are unlobed to shallowly pinnatifid, occasionally
sinuous (horizontally wavy), and dentate-prickly. During the second
year, this plant develops an erect
central stem that is unbranched below, while branching occasionally
above. The stems are light green, terete, glabrous, and often glaucous.
Alternate leaves occur along the entire length of these stems, becoming
gradually smaller above; their blades are widely spreading to
ascending. The blades of these alternate leaves are up to 8" long and
2" across; they are elliptic-oblanceolate to oblanceolate in shape,
while their margins are unlobed to moderately pinnatifid, occasionally
sinuous, and dentate-prickly.
The upper surface of both basal and
alternate leaves is yellowish green to medium green, glabrous and
sometimes glaucous, while
their lower surface is light green, glabrous, and sometimes glaucous.
In addition, abundant prickles occur along the central vein of the
lower leaf surface, while sparse prickles sometimes occur along the
major lateral veins. The bases of alternate leaves clasp their stems
with short rounded auricles (ear-like basal lobes) that are
dentate-prickly and occasionally sinuous. The tips of these leaves
are bluntly angled to acute. Both the stems and leaves of this plant
contain a brown latex. The central stem and sometimes upper lateral
stems terminate in panicles of flowerheads; these panicles are variable
in size, but on larger plants they become up to 1½' long and 1' across.
The branches of each inflorescence are more slender than the stems,
otherwise they share similar characteristics. Small bractlets occur
where these branches divide. The flowerheads often fail to open, but
they are self-fertile. Those flowerheads that do open (on bright sunny
days during the morning), can vary considerably in size, depending on
the local ecotype; their diameter varies from 4-6 mm. in some areas to
12-18 mm. in other areas. Each
flowerhead has 20-55 widely spreading ray florets and no disk florets.
The petaloid rays of the flowerhead are narrowly oblong in shape; their
tips are truncate and 5-toothed. The petaloid rays are usually light
but sometimes they are white to yellow. The tips of the petaloid rays,
where the teeth occur, are often pale bronze.
The bases of
flowerheads (involucres) are up to 15-20 mm. long and
cylindrical-lanceoloid to lanceoloid in shape; they achieve their
maximum size shortly after blooming. Around the base of each
flowerhead, the floral bracts (involucral bracts) are arranged in
several series; inner floral bracts are longer than outer floral
bracts. They are linear-lanceolate in shape (upper-inner bracts) to
ovate in shape (lower-outer bracts) and appressed together; these
bracts are light green with white to purplish red
margins, and they are glabrous. The blooming period occurs during
the summer and early autumn, lasting 1½–3 months. Afterwards, the
flowerheads are replaced by dense clusters of achenes. Mature achenes
are about 4 mm. long, broadly ellipsoid-flattened in shape, and
black mottled with brown; each side of an achene has a single
central rib (less often 2-3 ribs). At the apex of each achene,
there is a white thread-like beak about 4 mm. long, and attached to the
apex of this beak is a tuft of white hairs spanning about 15-20 mm.
(individual hairs are 7-10 mm. long). These achenes are distributed by
the wind. The root system consists of a taproot. This plant spreads by
Cultivation: The preference is full sun, mesic
to dry conditions, and soil containing sandy loam,
loam, clay-loam, or loess. First-year rosettes of basal leaves may
die down during winter, but new leaves are regenerated from the taproot
during the spring. Open ground with exposed topsoil facilitates
germination of the seed. Cultivation of this plant is not difficult,
but it is rather weedy-looking and its flowerheads often fail to open.
& Habitat: The native Prairie Lettuce is
rare in Illinois and
state-listed as 'endangered.' It has been found primarily in the
northern and west-central sections of the state (see Distribution
Illinois lies along the eastern range-limit of this species; it is more
common in many areas of central and western United States. Populations
of Prairie Lettuce have declined in Illinois, where it is found in such
habitats as mesic to dry prairies and hill prairies. Disturbed areas of
high quality natural habitats (mainly prairies) are preferred. This
wildflower probably benefits from the disturbance provided by
occasional wildfires and grazing by
American Bison; this animal prefers to graze on grasses rather than
Faunal Associations: Little is known
the relationships of Prairie Lettuce to various fauna. The flowerheads
of this plant are probably cross-pollinated by various long-tongued and
short-tongued bees. Insects that feed destructively on both cultivated
various species of wild lettuce (Lactuca spp.)
of aphids and larvae of polyphagous moths. The webmaster observed
unidentified blackish aphids feeding on the upper stems and
inflorescence branches of Prairie Lettuce. Both White-tailed Deer and
the Cottontail Rabbit occasionally browse on the foliage of this plant;
the latter animal fed on some of the basal leaves of Prairie Lettuce
was being cultivated in a garden by the webmaster.
The wildflower garden of the webmaster in Urbana, Illinois.
There are several native and non-native lettuce species (Lactuca
that exist as wildflowers in Illinois. Most of these species are
similar in appearance and they can be difficult to distinguish; the
presence of both flowerheads and achenes are often required for
correct identification. Prairie Lettuce (Lactuca ludoviciana)
distinguished from many lettuce species by the brown latex of its
foliage (rather than white latex). The rays of its flowerheads are
often blue, rather than yellow, and the involucres (bract-covered
bases) of these flowerheads are usually larger in size (up to 15-20 mm.
long). The achenes of Prairie Lettuce have single-ribbed sides (less
often 2-3 ribbed) and they have long thread-like beaks that are
attached to relatively large tufts of hair (15-20 mm. across). A common
Eurasian species, Prickly Lettuce (Lactuca serriola),
Prairie Lettuce by having flowerheads with pale yellow rays only
(never blue), shorter involucres (up to 10-12 mm. long), and achenes
with 5-7 ribs per side, to which are attached smaller tufts of white
hair (only 7-10 mm. across).
A common native species, Lactuca
canadensis (Wild Lettuce), differs by having leaves that lack
stiff prickles along their margins, flowerheads with yellow to
orange-yellow rays (never blue), shorter involucres (up to 10-15 mm.
long), and achenes with smaller tufts of white hair (10-14 mm. across).
Another native species, Lactuca biennis (Tall Blue
shorter involucres (up to 8-10 mm. long), achenes with 5-6 ribs per
side, very short or no beaks on the achenes, and smaller tufts of
hair (10-12 mm. across) that are light brown, rather than white.
Another native species, Lactuca florida (Woodland
differs by having leaves with triangular-shaped terminal lobes, shorter
involucres (up to 10-12 mm. long), achenes with 5-6 ribs per side, very
short or no beaks on its achenes, and smaller tufts of white hair (8-10
mm. across). An uncommon native species, Lactuca hirsuta
Lettuce), differs from Prairie Lettuce by having hairy stems, hairy
leaves with ciliate margins, and flowerheads that sometimes have
reddish rays, rather than yellow.