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(updated September 7, 2023) 




Tall Blue Lettuce

Lactuca biennis (Moench.) Fern.

Asteraceae (Sunflower Family)


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Lactuca biennis (Moench.) Fern., Tall Blue Lettuce: 

Bayer Code:  LACBI


         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 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 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:

      o   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

      o   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

      o   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

      o   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.

      o   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




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