Ohio Noxious Weed Identification – Week 8 Canada Thistle

Canada Thistle

Family: Composite, Asteraceae.

Habitat: Pastures, crops, landscape areas throughout Ohio.

Life cycle: Perennial with creeping roots.

Growth Habit: 1-3 feet high, erect, branched; forming large patches.

Leaves: 3-8 inches long, alternate with spiny, crinkled margins; lower leaves are lobed.

Stems: Grooved and becoming hairy with age; not spiny; branched at apex.

Flower: Lavender flower heads about 1 inch wide and long. Flowers are surrounded by bracts without spiny tips.

Fruits: Seeds borne in white feathery structures, similar to dandelion, spread by wind.

Roots: A creeping root system allows this weed to spread aggressively. Hand-pulling and cultivation are often ineffective control mechanisms because new plants sprout from root pieces that snap off.

Similar plants: Stems of Canada thistle are not spiny in contrast to bull thistle (Cirsium vulgare) and nodding thistle (Carduus nutans).

The problem is…. An aggressive, spreading root system. Very competitive with field crops and forages. Canada thistle is also prolific in seed production at 700 seeds per stem. Seeds are dispersed by wind and birds.


Root System














Currently there are 21 weeds on the Ohio Prohibited Noxious Weed List:

  • Shattercane (Sorghum bicolor) – February 8
  • Russian thistle (Salsola Kali var. tenuifolia) – February 22
  • Johnsongrass (Sorghum halepense L. (Pers.))
  • Wild parsnip (Pastinaca sativa)
  • Wild carrot (Queen Anne’s lace) (Daucus carota L.)
  • Oxeye daisy (Chrysanthermum leucanthemum var. pinnatifidum)
  • Wild mustard (Brassica kaber var. pinnatifida)
  • Grapevines: when growing in groups of one hundred or more and not pruned, sprayed,cultivated, or otherwise maintained for two consecutive years. – February 15
  • Canada thistle (Cirsium arvense L. (Scop.)) – March 29
  • Poison hemlock (Conium maculatum)
  • Cressleaf groundsel (Senecio glabellus)
  • Musk thistle (Carduus nutans)
  • Purple loosestrife (Lythrum salicaria)
  • Mile-A-Minute Weed (Polygonum perfoliatum) – March 7
  • Giant Hogweed (Heracleum mantegazzianum) – March 14
  • Apple of Peru (Nicandra physalodes) – February 28
  • Marestail (Conyza canadensis)
  • Kochia (Bassia scoparia)
  • Palmer amaranth (Amaranthus palmeri)
  • Kudzu (Pueraria montana var. lobata)
  • Japanese knotweed (Polygonum cuspidatum)

Each week, for the next 21 weeks, I will post information and pictures on how to identify these invasive and harmful plants.

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