Senecio glabellus Poir. - Butter Weed

Senecio glabellus plant

Family - Asteraceae

Stems - From many fibrous roots, erect, herbaceous, carinate, hollow, glabrous, often purplish, +1cm in diameter, +1m tall, mostly simple except in the inflorescence, easily broken.

Senecio glabellus stem

Leaves - Alternate, petiolate basally, sessile above, pinnately divided. Lowest leaves to +20cm long, +5cm broad, with a shallow adaxial groove along the rachis, glabrous. Terminal division of the leaf rounded, shallow-coarse serrate, larger than the lateral divisions. Lateral leaf divisions opposite, spatulate, toothes as the termianl leaflet. Leaves reduced upward, few on the stem, with stipules. Stipules lobed.

Senecio glabellus leaves

Inflorescence - Terminal corymbiform cluster of flower heads to +/-20cm broad. Divisions of the inflorescence subtended by small attenuate bracts. Bracts with dense arachnoid pubescence at their bases adaxially, often purplish. Larger bracts clasping, with toothed margins. Peduncles and pedicels of the inflorescnece glabrous and carinate.

Involucre - 4-6mm long, 3-4mm in diameter, cupulate to (later) urceolate. Phyllaries uniseriate, glabrous, green, acute, linear, 4-5mm long, .8mm broad, united for most of their length.

Senecio glabellus involucreInvolucre.

Ray flowers - Flowers pistillate, +/-10 per head. Ligule yellow, glabrous, -1cm long, +/-2mm broad, 2-3-toothed at the apex. Corolla tube 4-5mm long, greenish-white, glabrous. Style glabrous, 5mm long, barely exserted, bifurcate in the apical -1mm, whitish basally, yellow apically. Pappus and achenes as in the disk flowers.

Disk flowers - Disk 4-5mm broad. Corolla tube 5mm long, glabrous, white in the basal 2/3, yellow in the apical 1/3, contracted in the basal 2/3, expanded in the apical 1/3, 5-lobed. Lobes erect to spreading, .8mm long, .5mm broad at the base, acute. Stamens 5, adnate at the apex of the contracted portion of the corolla tube, mostly included. Filaments glabrous, translucent, thin in the basal 2/3, expanded and whitish in the apical 1/3, 1-1.5mm long. Anthers yellow, connate around the style, -1.5mm long, partially exserted. Style 1, glabrous, to 6mm long, exserted just beyond the anthers, white basally, yellow apically, bifurcate in the apical 1mm. Pappus of white, uniseriate capillary bristles to 4mm long. Achene white in flower, glabrous, cylindrical, 1mm long. Achenes in fruit +/-2mm long at maturity, brown, 4-5-angled, glabrous or with retrorse hairs on the angles. Receptacle flat, naked.

Senecio glabellus flowers

Senecio glabellus fruitsFruiting head.

Flowering - April - June.

Habitat - Low wet ground along streams, pond margins, swamps, ditches, low fallow fields, wet woods.

Origin - Native to U.S.

Other info. - This striking species can be found throughout Missouri in nearly every county bordering a major riverway. The plant is very easy to identify becasue of its distinctive leaves, hollow, easily broken stems, and bright yellow flowers. It is a common weed of moist to wet fallow agricultural land.
A modern synonym is Packera glabella (Poir.) C. Jeffery

Photographs taken somewhere in Tennessee, 5-10-04.


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