Silphium integrifolium Michx.

Silphium integrifolium plant

Family - Asteraceae

Stems - Stout, erect, herbaceous, to +2.5m tall, branching in the apical 1/2, single or multiple from the base, green but often purplish in strong sun, terete to slightly 4-angled (the angles rounded), villosulous to villous. The hairs of the stem with pustulate bases which create a slightly scabrous feel to the stem.

Silphium integrifolium stemStem color in strong sun.

Leaves - Opposite, decussate, sessile, ovate, acute to acuminate, to +15cm long, +9cm broad, entire or with coarse shallow teeth (the teeth with minute whitish apices), scabrous above, sericeous below. Hairs on the upper surface of the leaves with pustulate bases, those of the lower surface pilosulose. Veins around the base of the blade impressed adaxially, expressed abaxially.

Silphium integrifolium leaves

Inflorescence - Terminal cymose arrangement of flower heads on stems. Each division of the inflorescence subtended by a pair of reduced foliaceous bracts. Peduncles villosulous, with multicellular trichomes.

Involucre - To 2.5cm broad, +1.5cm tall (long). Phyllaries imbricate, not tightly appressed, broadly ovate to orbicular, acute to slightly acuminate, to 1.5cm long and broad, green, glandular pubescent and with some antrorse appressed hairs at the base externally, glandular and antrorse appressed pubescent in the apical half internally (glabrous basally), with ciliolate margins.

Silphium integrifolium involucreInvolucre.

Ray flowers - +/-15 per flower head, fertile, pistillate. Ligule yellow, +3cm long, +7mm broad, with two adaxial vertical grooves, with a single notch at the apex, sparsely pubescent at the base. Corolla tube to 3mm long, pilose, pale yellow. Style yellowish-brown, divided to below the middle, glabrous, +/-7mm long. Achenes compressed, orbicular to broadly ovate, +/-7mm in diameter in flower, retrorse appressed pubescent, with two basal projections. Projections pointed, to 2-3mm long.

Silphium integrifolium flowers

Disk flowers - Disk +2cm broad. Flowers staminate. Corolla tube yellow, 8-9mm long, contracted in the basal 1/5, white at the base, glabrous basally, with some pubescence at the apex, glabrous internally, 5-lobed. Lobes triangular, acute, +/-1mm long, erect. Stamens 5, adnate at the apex of the contracted portion of the corolla tube. Filaments yellow, -2mm long. Anthers brown, 5mm long, partially exserted, connivent around the style. Style yellow (pale in basal 1/2), to +1.5cm long, glabrous in the basal 1/2, pubescent apically, undivided. Achene white in flower, terete, with retrorse hairs, 6mm long, -1mm in diameter, tapered to the apex. Receptacle small, mostly flat or slightly convex. Chaff partially enclosing the achenes, +/-1.5cm long, -2mm broad, white in the basal 2/3, green apically, glabrous internally, pubescent (ciliate) externally at least on the midrib and at the apex, glabrous at the base.

Silphium integrifolium diskArrow shows undivided style.

Flowering - July - September.

Habitat - Rocky and dry open woods, prairies, glades.

Origin - Native to U.S.

Other info. - This big species can be found throughout Missouri. The plant is variable in stem and involucre pubescence but despite this, it is still an easy species to ID in the field. The broad rounded phyllaries and undivided styles are good characteristics for proper identification.
Steyermark breaks the species up into two varieties based on stem pubescence. Variety laeve T. & G. has glabrous and glaucous stems and involucres. Variety integrifolium has pubescent stems and involucres. This latter variety is much more common in the state.

Photographs taken off Hwy 60, Shannon County, MO., 7-19-03.


Back