Know Your Natives – Tall Thistle

Tall thistle (Cirsium altissimum) of the Aster, Sunflower or Composite (Asteraceae) family is a tall biennial thistle with weak spines and pink to lavender flower heads. The genus name derives from a Greek root for “swollen vein” in reference to past use of the plants to reduce swelling. The specific epithet is Latin for “tallest.” In the U.S., occurrence is concentrated in an area extending from eastern Oklahoma and Arkansas northward into Minnesota to western Pennsylvania, along with scattered populations from eastern Texas into western South Carolina. In Arkansas, tall thistle occurs statewide. Preferred habitats include prairies, woodlands, wood borders, and disturbed sites, on mesic, well-drained, sandy, rocky, and loamy soils.

Plants in their first year have a rosette of long-petiolate ascending elliptical leaves with minute pubescence overall and margins lined with minute spines. First year plants have long (to 6+ inches) slender taproots with a swollen segment along the lower portion. New growth for the second year begins as a rosette of rugose, ground-hugging leaves with long pubescence and stout marginal spines. Mature second year plants have shallow, sinewy, radiating roots as well as remnants of the first year’s tap root.

A second year plant, growing to a height of 5 ft (shady sites) to 10 ft (sunny sites), has straight, erect, hollow stems, with a cross-section to ⅝+ inch. Mature stems become hardened and tough. Larger plants have a few relatively short (to 2 ft long) branches along their upper portion. Stems are densely covered with short dense hispid pubescence which extends onto branches as dense pilose pubescence. While pubescence of stems may be reddish, that of branches is clear. Slight longitudinal ridges extend down from the leaf petioles so that the lower stem becomes 8-sided in cross-section. With drying soils, plants are subject to the loss of lower leaves. Dried leaves hang down and persist. As the bloom period ends in late September, the entire plant quickly dies, though typically remaining erect into spring.

Photo 1: First year plants have ascending petiolate elliptical leaves. As shown, soil has been removed from plant at right to expose a 6-inch taproot with a swollen segment. Photo – October 8.
Photo 2: New basal leaves of this second year plant are well armed with spines and long pubescence. Photo – February 20.

Stem leaves are alternate, green above and prominently white beneath. They become gradually smaller up-stem and toward the branch tips, with those at the base of flowerheads becoming bract-like. On mature plants, the largest leaves occur lower on the stem where they may be 1 foot long and 5 inches wide, often drooping with age. Larger leaves are sessile, the blades narrowing at the base. The upper leaf surface may be glabrate (hairless) or densely short hispid; the lower surface is white with a dense mat of minutely short appressed wooly hairs. Leaves are generally lanceolate (larger leaves) or elliptic (smaller leaves) in overall outline, varying from uncut to deeply pinnately lobed from plant-to-plant and even on the same plant. Leaves on sunny-sites tend to be lobed whereas plants in the shade tend to bear uncut leaves. Lobed leaves have about 5 alternate pairs of narrowly triangular lobes (incision does not extend to midrib) and a similar apical lobe. Lobe margins are mostly smooth (entire) with a large terminal spine and a few smaller lateral spines; margins of unlobed leaves are mostly serrate with small spines at the tips of the teeth. In general, the leaf outline is consistent for an entire plant, but upper leaves of a lobed plant may become gradually unlobed. Venation, recessed above and expressed below, is pinnate, with secondary veins of deeply lobed leaves terminating as spines at the lobe tips.

Photo 3: This plant has produced a stout stem with large lobed leaves. Leaves will droop with age. Photo – May 1.
Photo 4: Plants are leafy, but lower leaves wither with drying soil. Plant at center has varying leaf outlines. Undersurface of leaves is white. Photo – June 15.

The inflorescence, in August to September, consists of single composite flowerheads growing on peduncles from the uppermost leaf axils. Involucres of the flowerheads are ball-shaped in bud and become vase-shaped in bloom. They comprise tightly imbricated, lanceolate phyllaries (bracts), spirally arranged in about ten series. Phyllaries are medium green along their upper exposed half, typically with a central whitish stripe. They terminate in slender straight piercing spines. Spines lower on the involucre project outward or downward while upper spines tend to be ascending and may be up-hooked at their tips. There may be a dark spot at the base of the spine. Phyllaries are broader at the base of the involucre and become lanceolate to linear at the rim, with the linear phyllaries to ⅝ inch long.

Photo 5: Serrate leaf margins are spiny to a greater or lesser degree. Venation is pinnate. (These four stems developed after the original stem apex was damaged.) Photo – June 30.
Photo 6: Involucres are composed of tightly imbricated phyllaries which may have a dark spot at the base of the spine. Leaves below flowerheads may be few or clustered. Photo – August 4.

Flowerheads are composed of 100+ perfect disk florets, each with five stamens and a pistil. Florets are densely packed within the confining involucre so that marginal florets “mushroom” over the edge, broadening the flowerheads to 2 inches across. Florets, in pink to lavender shades (rarely white), have slender tubular corollas with 5 linear lobes, 1 positioned below the style and 4 above. Corolla tubes attach to the tops of the stubby-elongate inferior ovaries, each encircled by a pappus of inch-long, white bristles. As florets approach anthesis (blooming centripetally from involucral margin inward), corollas elongate beyond the pappus. The 5 stamens, with fuzzy white filaments, are tipped with elongate anthers fused into a ring. The anther ring is exserted beyond the corolla tube. As the style exserts through the anther ring, it picks up and carries the white pollen well beyond the anthers, making it available to foraging pollinators. As the anthers fade and the pollen is dispersed, the darker colored, minutely bilobed stigma at the tip of the style is fully exserted. Individual florets are about 1 inch long with a ¾ inch slender tube (hidden within the involucre) and ¼ inch lobes.

Photo 7: Arrows on flowerhead at left identify: white anther tubes with purple style tips just emerging (red arrow), white anther tube (yellow arrow) and emerged, elongated style and stigma (white arrow). Note pappus covering center of flowerhead at left and changes of phyllary size and shape of flowerhead at upper right. Photo – September 4.
Photo 8: Florets are densely packed within the involucre. The slender corolla tubes are tipped with five lobes, as can be seen at far left, along with anther ring and emerging style. Note the white pollen.
Photo 9: Single disk floret removed from receptacle. Arrows: ovary (white), corolla tube (blue), corolla lobes (pink), style (green), pubescent filaments (red), and anther ring (orange). (Style has not yet split to reveal two minute terminal lobes.)

As early flowerheads pass anthesis, the heads fade while others are still in bloom. With completion of flowering in late September, the entire plant dies. As the fruits (achenes) within an involucre mature, a mass of achenes and pappus expands. Some clumps, especially in wet weather, drop near the parent plant while some individual achenes becoming airborne. The light brown, slightly ribbed, bullet-shaped achenes (flat top and pointed base) are <¼ inch long with apical collars that allows the inch-long pappus to easily separate.

Photo 10: Bullet-shaped 1-seeded achenes are forced out of the involucre as they enlarge. Photo – October 1.

For a garden or natural area, tall thistle’s droopy lower leaves tend to die so that plants may become unattractive. However, when the showy flowerheads appear, it reasserts itself as a striking plant. Nectar is favored by butterflies and the achenes are a valuable food source for finches and other small birds and small mammals. Tall thistle is somewhat shade tolerant. Plants may self-seed too well, but young plants can be removed while in the rosette stage. It is not eaten by deer.

Photo 11: Tall thistles provide nectar to Pipevine Swallowtails (Battus philenor, shown here) and other butterflies.

Tall thistle is one of nine species (two non-native) in the Cirsium genus in Arkansas, all having pink to lavender flowerheads. Of the other eight species, the ones (all native) with the most similar leaf outline are Carolina thistle (Cirsium carolinianum), field thistle (Cirsium discolor), swamp thistle (Cirsium muticum), Englemann’s thistle (Cirsium engelmannii), and Nuttall’s thistle (Cirsium nuttallii). Tall thistle has generally less lobed leaves, or when lobed, more broadly so, than the other species. It also has leafier upper stems and peduncles. Like many large Composite genera, though, the species distinctions are not always clearly marked.

In addition to the nine Cirsium species, six additional “thistles” in the Composite family occur in Arkansas, namely, three in the genus Carduus (lavender to purple flowers on heavily spined stalks), one in Centaurea (yellow flowers), and two in Sonchus (“sow-thistles”; yellow flowers).

Article and photographs by ANPS member Sid Vogelpohl

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