Prickly lettuce

Lactuca serriola L.

Prickly lettuce seedling
Prickly lettuce spines on midvein of leaf underside
Prickly lettuce plant

Images above: Upper left: Prickly lettuce seedling (Antonio DiTommaso, Cornell University). Upper right: Prickly lettuce spines on midvein of leaf underside (Antonio DiTommaso, Cornell University). Bottom: Prickly lettuce plant (Antonio DiTommaso, Cornell University).

Identification

Other common names:  wild lettuce, compass plant, milk thistle, horse thistle, wild opium, common wild lettuce, lobed prickly lettuce

Family:  aster family, Asteraceae

Habit:  Erect, taprooted, summer or winter annual herb.

Description:  Seedling cotyledons are round or oval shaped and 0.2-0.4” (0.5-1 cm) long by 0.1” (0.3 cm) wide, with a tapering base, hairy upper surfaces, and a notched tip.  Young true leaves form a basal rosette; they are club to oval shaped, widest at tip, pale green, hairy, and toothed or lobed; spines are present on the undersides of the prominent midveins and on leaf edges.  Mature plants bolt with erect, 1-6 ft (0.3-1.8 m) high, hollow, branched, somewhat woody stems.  Stems are pale green to white, sometimes with red flecking, and spines are present at the stem base.  Leaves are alternate, deeply lobed, blue-green, widest near the pointed leaf tip, 2-12” (5-30 cm) long by 1-4” (2.5-10 cm) wide, and often oriented vertically.  Lobes point backward, form round cavities, and clasp the stem at the leaf base.  Mature spines on lower leaf surface and leaf edges are larger and more yellow than those found on the seedling.  The entire plant, including the large, deep taproot, exudes a milky white sap when wounded.  Flower heads are yellow but turn blue as they wither. They are, 0.1-0.4” (0.3-1 cm) wide, composed of 5-15 ray flowers and have 3-4 rows of hairless, 0.5” (1.3 cm) long, green bracts covering the base.  Flowerheads occur in open pyramidal panicles of 13-27” (33-69 cm) at branch ends.  One seed is produced per ray flower, and enclosed in a dry, hard, grayish-yellow, 0.1-0.2” (0.3-0.5 cm) long, ribbed, fruit.  This dispersal unit is referred to as the seed below; it is attached by a 0.13” (0.3 cm) long stalk to a white, bristly pappus.

Similar species:  Sowthistle (Sonchus) species lack the characteristic midrib spines of prickly lettuce. Sowthistle leaves also remain parallel to the ground, unlike prickly lettuce.

Management

Prickly lettuce is primarily a problem in winter grains and reduced tillage cropping systems (Weaver and Downs 2003).  The species is particularly a problem in cereal grains because it usually flowers at harvest and the flower buds, which are difficult to separate from the grain, lower quality and raise moisture content.  Since established plants can re-sprout after winter grain harvest operations, postharvest control is advisable to prevent seed production (Weaver et al. 2006).

Spring tillage controls plants that established in the fall, and late spring tillage controls most of the spring-germinating individuals as well.  Tillage that mixes seeds deep into the soil helps eliminate the weed.  Since the seeds are small and short lived, few deeply buried seeds will return to emergence depth before they die.  Control of populations along field margins may be necessary to prevent re-population by wind dispersed seeds (Weaver et al. 2006).

The seedlings are weak and suffer high rates of natural mortality.  Consequently, they are susceptible to tine weeding or rotary hoeing.  They also can be controlled by flaming at the two- to four-leaf stage, but half of plants survive flaming at the six- to eight-leaf stage (Kitis and Gök 2013).  Because prickly lettuce often grows taller than the crop at some point in development, it is susceptible to weed pullers, electrocution weeders and raised mowers.

Incorporated white mustard seed meal has potential for allelopathically controlling prickly lettuce emergence and growth (Handiseni et al. 2011).  Soil solarization to 135 °F (57 °C) completely eliminates prickly lettuce (Linke 1994).

Ecology

Origin and distribution:  Prickly lettuce is native to the Mediterranean basin and western Asia.  It now occurs in northern Europe, central Asia, North America and Australia.  In North America, it occurs throughout southern Canada, the United States, and into northern Mexico, except the Florida peninsula (Weaver and Downs 2003, USDA 1970).

Seed weight:  0.45 mg (EFBI), 0.47-0.53 mg (Weaver and Downs 2003), 0.53 mg (Milberg et al. 2000), 0.5-0.6 mg (Alcocer-Ruthling et al. 1992), 0.62 mg (Jakobsson and Eriksson 2003).

Dormancy and germination:  Seeds have no primary dormancy and can germinate immediately following dispersal (Alcocer-Ruthling et al. 1992, Marks and Prince 1982).  Optimal temperatures for germination in England were 54-75 °F (12-24 °C), but seeds did not germinate below 46° F (8° C) and temperatures from 79 to 95 °F (26 to 35 °C) induced dormancy (Marks and Prince 1982).  Seeds from the Czech Republic germinated well at 50-86 °F (10-30 °C) temperatures (Mikulka and Chodová 2003).  Sunlight promotes germination of fresh seeds, but light filtered through a plant canopy inhibits germination (Marks and Prince 1982, Milberg et al. 2000).  After exposure to natural alternating temperatures and/or burial for at least 8 weeks, however, seeds germinate well in the dark (Marks and Prince 1982). 

Seed longevity:  Seeds in the soil normally do not survive longer than 3 years (Toole and Brown 1946, Weaver and Downs 2003).  In Idaho, prickly lettuce seeds on the soil surface did not survive longer than 12 to 18 months whereas those buried 6” (15 cm) survived up to 24 to 33 months (Alcocer-Ruthling et al. 1992).  Prickly lettuce seed viability was an estimated 15-37% after 12 months burial (Alcocer-Ruthling et al. 1992).  In England, the half-life of prickly lettuce seeds in soil was estimated to be 18 months and mortality after 12 months was estimated to be 40% (Marks and Prince 1982).

Season of emergence:  Most plants emerge in the late fall, but a few emerge in the spring (Marks and Prince 1981, 1982; Weaver et al. 2006).  Natural seedling mortality is much higher for fall germinating plants (Marks and Prince 1981).

Emergence depth:  Seeds emerge best when covered by no more than a thin layer of dust.  Emergence at 0.8” (2 cm) is less than 25% of that near the soil surface (Mikulka and Chodová 2003­).

Photosynthetic pathway:  C3

Sensitivity to frost:  Prickly lettuce is very frost tolerant, and commonly overwinters as a rosette, even in the northern parts of its range.  During midwinter in Illinois, leaves reached temperatures as much as 18 °F (10 °C) above air temperature (Weaver and Downs 2003), which allowed continued growth on sunny winter days.

Drought tolerance:  Prickly lettuce is highly drought tolerant (Weaver and Downs 2003).  Its taproot extends 3 ft (90 cm) or more, and it therefore can tolerate drying of the plow layer.  Prickly lettuce has greater taproot length per unit plant weight, number of lateral roots per unit taproot, and lateral number near the bottom of the taproot compared to cultivated lettuce (Johnson et al. 2000).  The leaves orient vertically in a north-south plane, which allows maximum photosynthesis early and late in the day while reducing overheating and water stress at mid-day (Weaver and Downs 2003).

Mycorrhiza:  Prickly lettuce is mycorrhizal (Pendleton and Smith 1983).

Response to fertility:  The limited information available seems to indicate that prickly lettuce is favored by additions of P and K but is out-competed by N responsive species when N or a balanced nutrient source is applied (Mamolos and Veresoglou 2000).

Soil physical requirements:  The species occurs on all soil textures from gravelly sand to clay (Weaver and Downs 2003).  It is most common on dry, well drained soils but it also occasionally occurs in wet sites.  

Response to shade:  Prickly lettuce, particularly in the rosette stage, is suppressed by a dense crop leaf canopy and suffers high mortality rates (Marks and Prince 1981, Weaver and Downs 2003, Weaver et al. 2006).

Sensitivity to disturbance:  If mowed during flowering, the plant will branch from the remaining stem and flower again.  Plants surviving winter wheat harvest in Ontario, produced 500 to 3000 mature seeds per plant by late October if left unmanaged (Weaver et al. 2006).

Time from emergence to reproduction:  Over-wintering plants begin to elongate in May, flower from July to September, and most seeds are shed during August and September (Alcocer-Ruthling et al. 1992, Prince et al. 1978, Prince and Carter 1985, Weaver et al. 2006).  Spring germinating individuals mature only 1 to 2 weeks later than plants establishing in the fall (Prince et al. 1978, Weaver et al. 2006).  Plants that began flowering in the last week of July produced viable seeds by August 17, indicating approximately 3 weeks were required for maturation (Alcocer-Ruthling et al. 1992).  Low temperature vernalization of rosettes at 40-50 °F (4-10 °C) is necessary to trigger flowering (Prince et al. 1978).

Pollination:  Prickly lettuce primarily self-pollinates, but insects accomplish some cross pollination (Weaver and Downs 2003).

Reproduction:  Seed heads typically contain from 15-22 seeds (Alcocer-Ruthling et al. 1992, Prince and Carter 1985, Weaver et al. 2006).  Seed production depends on plant height.  For plants growing in soybeans and grain stubble in Ontario, 1 ft (30 cm) plants produced about 330 seeds whereas 5 ft (152 cm) plants produced about 55,000 seeds.  Mid-sized plants of 40” (102 cm) produced 6,700 seeds (Weaver et al. 2006).  In Idaho, plants averaged 4,200 to 4,900 seeds per plant (Alcocer-Ruthling et al. 1992).  In a British study, fall germinating plants produced an average of 1,550-2,350 seeds whereas spring germinating plants produced 170-1,500 seeds (Marks and Prince 1981).

Dispersal:  The seeds have a clump of hairs at the top that provide buoyancy in air and assist in wind dispersal (Weaver and Downs 2003).  Although a few seeds may travel long distances, most fall close to the parent plant (Weaver and Downs 2003).  Seeds also disperse by water and can be introduced into fields with surface irrigation water (Kelley and Bruns 1975, Wilson 1980). 

Common natural enemies:  Downey mildew (Bremia lactucae) can infect a high proportion of prickly lettuce populations, but disease severity is usually low relative to that on cultivated lettuce (Petrželová and Lebeda 2004).  Powdery mildew (Erysiphe cichoracearum) can cause more extensive infection and may reduce seed production under favorable conditions (Petrželová and Lebeda 2004, Weaver et al. 2006).

Palatability:  Although prickly lettuce is the wild ancestor of domestic lettuce it is very bitter and unpalatable to people, even when young.  Cattle can develop emphysema from feeding on fresh, young plants, but dried or mature plants are apparently not toxic.  (Weaver and Downs 2003)

References:

  • Alcocer-Ruthling, M., D. C. Thill, and B. Shafii.  1992.  Seed biology of sulfonylurea-resistant and -susceptible biotypes of prickly lettuce (Lactuca serriola).  Weed Technology 6:858-864.
  • EFBI.  Ecological Flora of the British Isles.  http://ecoflora.org.uk/
  • Handiseni, M., J. Brown, R. Zemetra, and M. Mazzola.  2011.  Herbicidal activity of Brassicaceae seed meal on wild oat (Avena fatua), Italian ryegrass (Lolium multiflorum), redroot pigweed (Amaranthus retroflexus), and prickly lettuce (Lactuca serriola).  Weed Technology 25:127-134.
  • Jakobsson, A., and O. Eriksson.  2003.  Trade-offs between dispersal and competitive ability: a comparative study of wind-dispersed Asteraceae forbs.  Evolutionary Ecology 17:233-246.
  • Johnson, W. C., L. E. Jackson, O. Ochoa, R. van Wijk, J. Peleman, D. A. St. Clair, and R. W. Michelmore.  2000.  Lettuce, a shallow-rooted crop, and Lactuca serriola, its wild progenitor, differ in QTL determining root architecture and deep soil water exploitation.  Theoretical and Applied Genetics 101:1066-1073.
  • Kelley, A. D., and V. F. Bruns.  1975.  Dissemination of weed seeds by irrigation water.  Weed Science 23:486-493.
  • Kitis, Y. E., and Y. E. Gök.  2013.  Flame weeding effects on some weed species.  Proceedings of the 16th EWRS Symposium 2013, Samsun, Turkey.
  • Linke, K.-H.  1994.  Effects of soil solarization on arable weeds under Mediterranean conditions: control, lack of response or stimulation.  Crop Protection 13:115-120.
  • Mamolos, P., and D. S. Veresoglou.  2000.  Patterns of root activity and responses of species to nutrients in vegetation of fertile alluvial soil.  Plant Ecology 148:245-253.
  • Mikulka, J., and D. Chodová.  2003.  Germination and emergence of prickly lettuce (Lactuca serriola L.) and its susceptibility to selected herbicides.  Plant, Soil and Environment 49:89-94.
  • Marks, M., and S. Prince.  1981.  Influence of germination date on survival and fecundity in wild lettuce Lactuca serriola.  Oikos 36:326-330.
  • Marks, M. K., and S. D. Prince.  1982.  Seed physiology and seasonal emergence of wild lettuce Lactuca serriola.  Oikos 38:242-249.
  • Milberg, P., L. Andersson, and K. Thompson.  2000.  Large-seeded species are less dependent on light for germination than small-seeded ones.  Seed Science Research 10:99-104.
  • Pendleton, R. L., and B. N. Smith.  1983.  Vesicular-arbuscular mycorrhizae of weedy and colonizer plant species at disturbed sites in Utah.  Oecologia 59:296-301.
  • Petrželová, I., and A. Lebeda.  2004.  Occurrence of Bremia lactucae in natural populations of Lactuca serriola.  Journal of Phytopathology 152:391-398.
  • Prince, S. D., and R. N. Carter.  1985.  The geographic distribution of prickly lettuce (Lactuca serriola): III. Its performance in transplant sites beyond its distribution limit in Britain.  Journal of Ecology 73:49-64.
  • Prince, S. D., M. K. Marks, and R. N. Carter.  1978.  Induction of flowering in wild lettuce (Lactuca serriola).  New Phytologist 81:265-277.
  • Toole, E. H., and E. Brown.  1946.  Final results of the Duvel buried seed experiment.  Agricultural Research 72:201-210.
  • USDA.  1970.  Selected Weeds of the United States.  United States Department of Agriculture, Agricultural Research Service, Agriculture Handbook No. 366.  U.S. Government Printing Office: Washington, DC.
  • Wilson, R. G.  1980.  Dissemination of weed seeds by surface irrigation water in western Nebraska.  Weed Science 28:87-92.
  • Weaver, S., K. Cluney, M. Downs, and E. Page.  2006.  Prickly lettuce (Lactuca serriola) interference and seed production in soybeans and winter wheat.  Weed Science 54:496-503.
  • Weaver, S. E., and M. P. Downs.  2003.  The biology of Canadian weeds. 122. Lactuca serriola L.  Canadian Journal of Plant Science 83:619-628.