Cirsium arvense (L.) Scop.
Canada thistle, field thistle, lettuce from hell thistle, California thistle, corn thistle, cursed thistle, green thistle, hard thistle, perennial thistle, prickly thistle, setose thistle, small-flowered thistle, way thistle, stinger-needles
Canada thistle, the most common American name (of many!) for this species, is not actually Canadian in origin. It is native to Europe, western Asia, and northern Africa. It is a serious invasive species in many habitats, including in 43 U.S. states.
Plants: 12–59″ (30–150 cm) tall, heavily branched, with hairless stems and branches.
Leaves: Alternate, lanceolate or elliptic, festooned with pale yellow spines, lobed, and wavy. Leaf upper surfaces are hairless, but lower surfaces may be somewhat hairy. Leaves are sessile, clasping the stems. They are 6–8 ⨉ ¾–1″ (15–20 ⨉ 2–3 cm).
Flowers: Flowerheads are compound cymes. Each cyme is ⅜–¾″ (1–2.2 cm) in diameter, and pink-purple in color. Although most aster family member flowers are compound, with both disk and ray florets, Cananda thistle flowers have up to 100 identical filament-like florets that don’t fall neaty into either category. Each branch has 1 to 5 flowers. Male and female flowers are produced on separate plants. Only the female flowers have fragrance, and produce seeds.
Bracts beneath the flowerheads form a “vase” for each flower. Individual bracts are pointed, but lack spines, and may be purple tinged.
Fruits: Seeds are ⅛–3/16″ (4–5 mm) long, light brown, with a pappus of long white hairs for wind distribution.
Habitats: Moist places, grasslands, mountain slopes, flooded lands, disturbed sites, at elevations of 328–14108′ (100–4300 m). They prefer full sun, and clay/loam soils.
Online References:
7/18/2020 · Woodward Point Preserve, Brunswick, Maine · ≈ 4 × 5″ (10 × 13 cm) 
9/15/1995 · Brier Island, Nova Scotia · By Constance B. Kent 
7/18/2020 · Woodward Point Preserve, Brunswick, Maine · ≈ 5 × 4½″ (12 × 12 cm) 
7/25/2025 · River Point Conservation Area, Falmouth, Maine · ≈ 4½ × 6″ (11 × 14 cm) 
8/20/2025 · Peaks Island, Maine · ≈ 11 × 9″ (28 × 22 cm) 
8/26/2014 · El Malpais Information Center, National Parks Service, El Malpais, New Mexico · ≈ 7 × 10″ (16 × 25 cm) 
8/28/2014 · Eagle Nest Lake, Eagle Nest, New Mexico · ≈ 11 × 7″ (28 × 18 cm) 
7/25/2025 · River Point Conservation Area, Falmouth, Maine · ≈ 5 × 8″ (13 × 20 cm) 
7/25/2025 · River Point Conservation Area, Falmouth, Maine · ≈ 4 × 3″ (10 × 8.3 cm) 
7/18/2020 · Woodward Point Preserve, Brunswick, Maine · ≈ 9 × 6″ (21 × 16 cm) 
7/2/2010 · Stansbury Mill Rd, Phoenix, Maryland · ≈ 4½ × 5″ (12 × 13 cm) 
8/28/2014 · Eagle Nest Lake, Eagle Nest, New Mexico · ≈ 11 × 7″ (28 × 18 cm) 
7/17/2021 · Wilbur Preserve, Cox’s Head, Phippsburg, Maine · ≈ 4½ × 3½″ (12 × 9 cm) 
7/26/2025 · River Point Conservation Area, Falmouth, Maine · ≈ 6 × 10″ (16 × 24 cm) 
7/17/2021 · Wilbur Preserve, Cox’s Head, Phippsburg, Maine · ≈ 6 × 3½″ (14 × 9.4 cm) 
8/26/2014 · El Malpais Information Center, National Parks Service, El Malpais, New Mexico · ≈ 7 × 6″ (16 × 16 cm) 
7/18/2020 · Woodward Point Preserve, Brunswick, Maine · ≈ 4½ × 7″ (12 × 18 cm) 
7/17/2021 · Wilbur Preserve, Cox’s Head, Phippsburg, Maine · ≈ 4½ × 3½″ (12 × 9.2 cm) 
8/26/2014 · El Malpais Information Center, National Parks Service, El Malpais, New Mexico · ≈ 7 × 8″ (18 × 20 cm) 
7/2/2010 · Stansbury Mill Rd, Phoenix, Maryland · ≈ 6 × 9″ (14 × 22 cm) 
7/18/2020 · Woodward Point Preserve, Brunswick, Maine · ≈ 4½ × 5″ (12 × 13 cm) 
7/26/2025 · River Point Conservation Area, Falmouth, Maine · ≈ 11 × 14″ (27 × 36 cm) 
8/28/2014 · Eagle Nest Lake, Eagle Nest, New Mexico · ≈ 10 × 7″ (25 × 16 cm) 
| Kingdom | Plantae | Plants, but not fungi, lichens, or algae (from Stearn's Botanical Latin) |
| Subkingdom | Tracheobionta | Vascular plants—plants with a “circulatory system” for delivering water and nutrients |
| Division | Magnoliophyta | Flowering plants, also known as angiosperms |
| Class | Magnoliopsida | Dicotyledons—plants with two initial seed leaves |
| Subclass | Asteridae | A large class that encompasses asters |
| Order | Asterales | Flowering plants with a central disk flower and surrounding petals, like daisies |
| Family | Asteraceae | The aster family, which also includes daisies and sunflowers; from the Greek ἀστήρ, “star,” for the star-shaped flowers |
| Genus | Cirsium | From Greek kirsion, “a kind of thistle” |
| Species | arvense | Of the fields |
Cirsium arvense description by Thomas H. Kent, not updated.
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