Ohio Perennial and Biennial Weed Guide

Family:

Sunflower Family (Asteraceae)

Other Names:

Lactuca scariola, compass plant, horse thistle, milk thistle, wild lettuce, wild opium.

Origin and Distribution:

Originally from Eurasia, prickly lettuce immigrated to North America from Europe around 1860. Later, it spread through southern Canada and over much of the U.S. except for areas in extreme northern Maine and southern Florida. Currently, it is naturalized in about 3/4 of the counties in Ohio. Prickly lettuce grows along roads, railroads, and sidewalks and in alleys, vacant lots, waste areas, pastures, orchards, and cultivated fields. The plant prefers dry conditions, although it tolerates and can often be found growing on moist ground such as that in low areas or irrigated fields. Prickly lettuce grows most abundantly on nutrient-rich soils.

Plant Description:

Prickly lettuce is an erect biennial (rarely an annual) that grows as a rosette of basal leaves during its first year. Each rosette gives rise to a solitary stem that is usually erect and sometimes branched, especially the top portion where small, daisy-like, yellow flowers are borne. Stem leaves are irregularly-lobed and have prickly edges and a distinctive row of stiff, sharp prickles on the underside of midribs. Nearly half of the length of each seed consists of a beak having a tuft of silky white hairs (pappus) at the tip. All plant parts exude a milky juice when cut or broken. The plant reproduces only by seeds.

• Root System:

  Plants produce a large white taproot that exudes a milky juice if cut or crushed.

• Seedlings and Shoots:

  Emerging first are two seed leaves (cotyledons) that are round and have short hairs scattered on the upper surface, lower surface, and edge. Subsequent leaves are club-shaped, light green above, paler green beneath, and have a few short hairs on the upper surface, lower surface, and midvein. Short, knob-tipped hairs develop along the leaf edge. Young leaves form a rosette resembling that of dandelion.

• Stems:

  Flowering stems are 2 to 7 feet tall, stiff, hollow, and filled with milky juice. Stems are usually leafy and may bear a few prickles on the lower portion. The stem surface is covered with a waxy coat giving it a whitish cast and it is sometimes flecked with red. Stems are typically erect, usually solitary, and frequently branched throughout the upper portion that becomes dotted with small, daisy-like, yellow flowers.

• Leaves:

  Leaves are alternate (1 leaf per node), 6 to 12 inches long, coarse, and variable in size and shape. Basal leaves are larger than stem leaves. Leaves located on the lower part of the stem may be deeply or irregularly lobed or may lack lobes. Leaf bases typically consist of ear-like lobes that clasp the stem. The upper leaf surface is hairless, but there are prickles along the edge, and the midrib on the lower leaf surface is lined with sharp prickles. Prickles are generally absent from upper stem leaves, which are small, lance-shaped, and lack lobes. Leaves taste like lettuce and exude a milky juice when cut or crushed.

• Flowers:

  Many small flower heads, each borne singly at the end of a short branch, are grouped in open terminal clusters. Unopened flower heads resemble a tight green teardrop. Opened flower heads are less than 1/2 inch wide and consist of 5 to 12 yellow ray flowers that often fade to blue as they dry.

• Fruits and Seeds:

  The single-seeded fruits are oblong, about 1/10-inch long, brownish, and end with a slender beak that is nearly as long as the fruit. A soft white tuft of bristles (pappus) arises at the end of the beak.

Similar Species:

Prickly lettuce can be confused with sowthistles (Sonchus spp.), which have prickly leaf margins but smooth midribs. Tall lettuce (Lactuca canadensis) and tall blue lettuce (Lactuca biennis) look similar to prickly lettuce except they have leaves with smooth edges and midribs without prickles.

Biology:

Prickly lettuce is a highly variable plant that may be difficult to identify until the row of prickles on the underside of the leaf margin is evident. Experts sometimes distinguish two varieties of prickly lettuce based on the presence or absence of leaf lobes. The plant blooms from July until frost. Only about a third of the flower heads open at a time. Flowers mature into small, fuzzy seed heads from which pappus-bearing seeds disperse. The overall appearance of the reproductive plant resembles a tangled, feathery web.

Toxicity:

Regrowth of plants in autumn has caused poisoning in cattle, but mature and dried plants appear to be harmless.

Facts and Folklore:

• 'Lactuca' refers to the white milky juice contained in this plant, which has been investigated as an alternative source of rubber.

• Basal leaves often twist and face the sun such that leaves point north and south, hence the origin for the common name 'compass plant'.

• The juice in prickly lettuce is reported to have narcotic properties and said to be useful for sunburn.