Prickly Lettuce is an introduced and widely naturalized erect annual or biennial plant (usually annual), growing from 1 to 5+ feet high on leafy stems that contain a milky juice. The stem is light green, smooth, except the base may have prickles, and unbranched below the floral array.
The leaves are alternate, up to 12 inches long, entire to toothed to pinnately divided with narrow to arrow shaped bases. Edges are spiny and wavy and the underside of the whitish-green mid-rib has spines. Each leaf has a pair of angular lobes that clasp the stem; lobe edges tend to curl backwards toward the stem in a north/south arrangement when the plant grows in the open sun - hence the alternate common name of Compass Plant although that name more properly belongs to Silphium laciniatum. This north/south leaf tendency of certain plants was suggested by Dr. Asa Gray, (1810-1888 - American botanist), as for the purpose of avoiding the direct rays of the sun in order to check too great a loss of water by transpiration when plants grow in hot sun. Larger leaves are toward the base of the stem. Like the stem, the leaves contain a milky juice.
The floral array is a long conical loosely branched spreading cluster of individual panicles. The panicles contain very small leaves. Each panicle has clusters of flower heads, each head is only 1/2 inch long.
The flowers each have 12 to 20 pale yellow bisexual ray florets which have blunt tips with 5 small teeth. The involucre has several series of overlapping slender, hairless, blue-green phyllaries. These number 5 to 13+, mostly equal in size, with tips that are erect or slightly reflexed when the head is in fruit. The filaments and anthers of the five stamens of each ray flower are tightly appressed around the style. Flower rays absorb moisture and dissipate rapidly (are deliquescent) and thus are open only one day, but the flower buds do not open all at once so the bloom time of a large plant can last about a month.
Seed: Fertile flowers produce a dry oblong cypsela (seeds in composite plants that resemble an achene) that has a long beak with tufts of white hair (pappus). The cypsela has 3 to 9 nerve lines on each side - which is a distinguishing characteristic. Dispersion is by the wind.
Habitat: Prickly lettuce is found in fields and disturbed places. It grows from a taproot in a variety of soils that are mesic to dry. It requires full sun. It regenerates from seed.
Names: The genus Lactuca, is Latin for 'milk' referring to the milky juice of the plant. The species, serriola, is also Latin for 'in ranks" referring to the way the leaves line up directionally. The author name for the plant classification, 'L.' refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. The alternate common name of 'Compass Plant' is explained above. 'Milk Thistle' refers to the milky sap and 'English Thistle' refers to the plant having been introduced, thought to be is seed from England. The 'thistle' part refers to the resemblance of the leaves to thistle leaves.
Comparisons: Similar appearing plants are Wild Lettuce, L. canadensis, and the sow thistles, particularly the Prickly Sow Thistle, Sonchus asper. Wild Lettuce leaves lack the prickles and spiny mid-rib. Prickly Sow Thistle lacks the spiny mid-rib and the yellow flower is much larger.
Above 1st photo - The panicles of the floral array fully extended. 2nd photo - a full size plant - click image for larger photo. 3rd photo - Leaf lobes tend to align in a compass direction.
Below: 1st photo - Cypselae are a gray to tan-brown color with 3 to 9 nerve lines on the side with a long beak to which is attached white pappus for wind dispersion. 2nd photo - A seed head with fully developed cypselae.
Below: 1st photo - Leaves have prickly edges (upper surface shown) and prickles along the mid-rib underside (2nd photo). Two angular lobes clasp the stem (photo above and below).
Below and Above: Each flower has up to 20 pale yellow ray florets which have blunt tips with 5 small teeth. The involucre (below 1st photo) has several overlapping slender, hairless, blue-green phyllaries which can have purplish color tinges.
Notes: Prickly Lettuce is not indigenous to the Garden. It is a fairly recent arrival, the 2009 census being the first to list it. It is believed to have arrived in North America in the late 19th century in Massachusetts. It has since naturalized itself across the continent in all the 48 states and the lower Canadian Provinces. The pathway was its inclusion in other seed mixtures that were not pure to the intended species. (Ref. #6b). There are five species of Wild Lettuce in Minnesota including L. serriola. The other four are native: Biennial (Tall) Blue Lettuce, L. biennis; Canada Wild Lettuce, L. canadensis; Florida Wild Lettuce [reported as historical only], L. floridana; and Louisiana Lettuce, L. ludoviciana.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"