Weed Seed: Blessed milkthistle/Milk thistle (Silybum marianum)

Family

Asteraceae

Distribution in Canada

Found in BC, AB, MB, ON, QC, NB, NS

Duration of life cycle

Annual or biennial

Seed/Fruit type

Achene - a one-seeded, dry fruit

Seed identification features

Size

  • length: 6.5–7.5 mm
  • width: 2.0–2.5 mm
  • thickness: 1.7 mm

Shape

  • in outline: oblong (oval with straight sides)
  • in cross-section: slightly curved, oval

Surface texture

  • smooth with a sheen
  • slight cross-wise wrinkles under magnification

Colour

  • overall dark brown, with occasional golden streaks
  • golden-coloured ring at top of seed
  • middle of seed can be greenish-brown
  • occasionally, seeds have large, light-brown patches

Other structures

  • achene has two ends
    • top is a lobed peg, ringed by thin rim (a remnant of the floral parts)
    • base is a small slit (seen only under magnification)
  • a long white pappus may be present on freshly harvested seeds

Similar species

Nodding thistle (Carduus nutans)

  • found across Canada (except in NU, YT, NT) and the United States (except in Florida)
  • one-half the length and width of blessed milkthistle
  • similar to blessed milkthistle in shape, satiny surface texture, and the presence of prominent peg at top of achene
  • top peg is not lobed, and is a lighter colour than blessed milkthistle

Photos

Photo - Blessed milkthistle (Silybum marianum) achene (side view)
Blessed milkthistle (Silybum marianum) achene (side view)
Photo - Blessed milkthistle (Silybum marianum) achene (edge view)
Blessed milkthistle (Silybum marianum) achene (edge view)
Photo - Blessed milkthistle (Silybum marianum) achene (close-up of peg at top)
Blessed milkthistle (Silybum marianum) achene (close-up of peg at top)
Photo - Blessed milkthistle (Silybum marianum) achene (top-down view)
Blessed milkthistle (Silybum marianum) achene (top-down view)
Photo - Blessed milkthistle (Silybum marianum) group of achenes
Blessed milkthistle (Silybum marianum) group of achenes

Similar species

Photo - Similar species: Nodding thistle (Carduus nutans) achene (side view)
Similar species: Nodding thistle (Carduus nutans) achene (side view)
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