Annual Sow-thistle Sonchus oleraceus Asteraceae—Aster family by Tom Reaume © 2010 Nature Manitoba Grant:
An annual wildflower 20–130 (–200) cm tall, singly or in patches, from a taproot 10–20 cm long by 2–10 mm thick; in cultivated fields and disturbed sites, ditch banks, bottom- lands, city lots and alleys; juice milky. l FLOWER HEADS yellow, blooming February–October; inflorescence a corymb of several heads, terminal and along branches; floral branches several, glaucous, ascending, 4–67 cm long, the bases reddish purple; peduncles 0.5–11 cm long by 1–2 mm thick, some glandular-hairy; flower heads 1.5–2.8 cm wide by 10–15 mm long, in clusters of 5–15, open for 1–2 days; involucre 9–15 mm long by 5–7 Several Annual Sow-thistle plants in bloom (yellow heads) mm wide, the flower head buds often with congested white and with fruiting heads (white) along the wall of a commer- cial building in Winnipeg, Manitoba basal hairs resembling frost; involucral bracts in 3 or 4 series, 25–33 per head, imbricate, 3–13 mm long by 0.7–2.6 tooth mm wide, margins hyaline, tips pointed, with a few scattered often gland-tipped hairs, the midnerve thickest at the base pappi of fruit of the outer bracts, descending to spreading with fruit; disc ligule florets absent; ligulate florets perfect, 110–180 per head, 10–18 mm long; ovary white, shiny, hairless, flattened, faintly ribbed, 1.1–1.8 mm long by 0.5–0.7 mm wide; pappi of fine white bristles 5–7 mm long; corolla tube pale yellow to white, 6–9.6 mm long by 0.2–0.3 mm wide, with curly white hairs along its upper half; ligules yellow, 3.5–7.5 mm long stigma by 1.2–1.7 mm wide, the outer ligules the longest, each with 5 apical teeth 0.1–0.4 mm long; stamens 5; anthers yellow, anthers forming a tube 1–2 mm long; style exserted 0.8–1.7 mm past Flowering and fruiting heads c. 2.5 tips of anthers; stigmas 2-parted, each part c. 0.7 mm long; cm wide corolla fruiting heads white, round, 2.2–2.8 cm across by 1.3–2.2 tube cm tall; flower to fruit c. 7 days. l FRUIT an achene, 1-seeded, medium to dark brown, dull, ligule pappus flattened, 2.5–3.5 mm long by c. 1 mm wide by 0.3–0.5 mm bristle thick, not beaked, thin walled, rough with shallow transverse wrinkles, the faces 3- to 5-ribbed; pappi of numerous white bristles 5–8 mm long; receptacles 5–11 mm wide, round, hairless, concave in middle; seeds gray, smooth, c. 2.3 mm long by c. 0.8 mm wide by c. 0.3 mm thick. l LEAVES basal and stem, simple, often lobed and toothed, involucre glabrous and glaucous; basal leaves petiolate, to 30 cm long (including petiole) and to c. 12 cm wide, early deciduous, ovary auricles lacking or smaller than those of stem leaves; peti- oles (of stem leaves) winged, 0–9 cm long by 5–7 mm wide, Flower head beginning to open; a Ligulate reduced above; stem leaves alternate, with 1–5 lobes per wide pink stripe colors the back of floret x7; each ligule side, teeth ending with short weak, but not hurtful, prickles, ventral view upper blades arched downward, terminal lobes 3.2–12 cm long by 1–14 cm wide; blades 4–35 cm long by 1.5–15 cm petiole wide; auricles pointed, lobed and toothed, 1–3 cm long and flat, extending well past the stem, usually not clasping. terminal l STEM erect, hollow, glabrous and glaucous, bluntly lobe 5-angled, usually branched, bisected near the top; 3–15 mm thick near the reddish smooth base. l RANGE: (CAN) 11 provinces; (USA) 49 states; natural- ized in North America. Basal leaf 25 cm long by 7 cm wide involucre involucre
Flower head from the side and Annual Sow-thistle flower head; Flower head 2.2 cm wide; above below; with a few green aphids below
involucre
pappi
Cross-section x2; of lower hol- 2 ligule 1 3 low stem with 4 5 fruit 6 walls c. 1.6 mm wide
Left to right: (1) flower head bud (2) flower head starting to open (3) open flower head 2.5 cm wide (4) post blooming with ligules erect and wilted (5) fruit ripening inside the involucre (6) fruit ready for dispersal by wind
receptacle peduncle branch root midrib
involucre auricle bract taproot side lobe Two views of involucre and bare concave recep- Subtending glaucous leaf, c. 22 cm long; tacle c. 8 mm wide after fruit has departed of upper branch Taproot 12 cm long
side lobe
petiole
terminal Involucral bracts x3; left, the in- lobe nermost and thinnest, and right, a short outer thick bract; the Basal leaf 30 cm long by 10 cm wide longest is 13 mm receptacle pappus, partial
pappi
involucral bract
Concave receptacle 9 mm wide; fruit dispersed by wind Seed x10 rib Annual Sow-thistle fruiting head 2.2 cm wide; pappi hiding fruit below Fruit x14; a 1-seeded achene 3 mm long by 1 mm wide by 0.5 mm thick
wilted ligules
involucral bract
involucre
peduncle peduncle hairs
bract at base tooth Fruit ripening inside of peduncle closed involucral bracts x3 Cluster of several flower heads showing frost-like hairs at base of some buds Margin of terminal lobe
side lobe wing
auricle
sinus
midrib midrib
Pointed horizontal auricles 2.5 cm long at base of leaf blade Midrib, lobes, wing and sinuses of leaf blade