Tall Blue Lettuce

Tall Blue Lettuce

Lactuca biennis (Moench.) Fern.

Asteraceae (Sunflower Family)

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Lactuca biennis (Moench.) Fern., Tall Blue Lettuce: (Bayer Code: LACBI; US Code LABI)

  • Very tall, unbranched (until flowering) winter/summer annual or biennial U.S. native plant growing 2-8 feet tall; stems are smooth, green or tan or purple-streaked and hollow between the nodes
  • Leaves at base of plant can be up to 12 inches long; leaves are deeply lobed, with smaller teeth on lobes; terminal lobe is roughly triangular in shape
  • Upper stem leaves become increasingly smaller and more narrow
  • Leaves usually hairless on top side, often with short hairs on leaf underside midvein (not little spines, as with Prickly Lettuce (Lactuca serriola)
  • Inflorescence is an open panicle at the top of the stem; individual heads are about 1/4 inch diameter and have 15-30 disk florets with pale blue to white petals
  • Involucre of bracts surrounding florets is urn-shaped, 7-12 mm long at start of flowering, lengthening somewhat by fruiting; bracts of varying lengths, often tapering to a purplish tip
  • Flowers July-October
  • Fruit is small, oval, ridged achene, 4-5 mm long; numerous white hairs (pappus) 4-6 mm long are attached to a slender stalk at one end of the seed; stalk length less than 1/2 length of main fruit body
  • Leaves and stem have a milky-sap if broken
  • U.S. native more common north and east of Missouri; found in open woods, roadsides; not usually a serious weed problem
  • Similar species:
    • Woodland Lettuce, Tall Blue Lettuce, Florida Wild Lettuce (Lactuca floridana), is very similar to tall blue lettuce, but is a more southern-occurring species and can only be definitively distinguished by the number of florets per head inflorescence:
      • Lactuca floridana � has 10-15 florets per inflorescence
      • Lactuca biennis � has 15-30 florets per inflorescence
    • Perennial Blue Lettuce. (Lactuca tatarica), non-native creeping perennial with creeping roots that grows only 2-3 feet tall, often in open colonies; inflorescences have larger, showy, blue florets; more common in northern Midwest
    • Wild Lettuce, Tall Lettuce (Lactuca canadensis), is quite similar to Tall Blue Lettuce until flowering; wild lettuce has yellow ray flowers instead of blue to white, and tips of basal leaves end in long, tapered point; sap is also yellow, instead of white
    • Willowleaf Lettuce (Lactuca saligna), a non-native species that appears similar to Prickly Lettuce, but has more linear to lanceolate, slightly lobed leaves with smooth margins, and only a few hairs, but no spines, along midvein on lower side of the leaf; leaves often more blue-green in color than other Lactuca spp.
    • Prickly Lettuce (Lactuca serriola), a non-native species that can be readily identified by the row of stout hairs/prickles along midvein on lower side of the leaf and leaf margins, the lateral vertical orientation of stem leaves and yellow ray flowers; similar Willowleaf Lettuce has narrower leaves and no prickles along the lower leaf midvein

(Updated September 7, 2023)

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