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Ministry of Agriculture, Food and Rural Affairs

Ragweeds - COMMON or GIANT

There are 2 types of ragweed found in sweet corn; Common ragweed and Giant ragweed.

Scientific Names: Common ragweed, Ambrosia artemisiifolia L.; Giant ragweed, Ambrosia trifida L.

Other Names: Common ragweed is also called petite herbe à poux, Short ragweed, ambroisie a feuilles d’armoise. Giant ragweed is also called grande herbe à poux, Great ragweed, Kinghead, Tall ragweed, ambroisie trifide

Family: Composite or Aster Family (Compositae)

General Description: Annual, reproducing only by seed.

Habitat: Common ragweed is one of the most abundant weeds of cultivated land throughout southern Ontario, but is rare or absent in northern and north-western parts of the province. It also occurs in gardens, flower borders, poorly kept lawns, edges of sidewalks, roadsides, fence lines, waste places, and in disturbed areas in pastures and meadows. Giant ragweed occurs in river valleys, meadows, roadsides and occasionally in cultivated fields in the south-western portion of southern Ontario.


Common Ragweed Giant Ragweed
  • Cotyledons are thick, dark green, rounded above, narrowed near the base, sometimes with purple spots near the margins
  • First leaves are alternate; opposite by the fourth node
  • Cotyledons are thick, round to oblong, 3- 4 times larger than common ragweed
  • Young leaves are opposite
  • First pair of leaves are unlobed; following leaves have 3 large lobes


Common Ragweed Giant Ragweed
  • Erect
  • 15- 150 com (6- 60 in.) high
  • Usually much-branched
  • Hairless or hairy throughout
  • Erect
  • 0.4- 4 m (16 in.- 13 ft)
  • Usually much-branched
  • Somewhat hairy


Common Ragweed Giant Ragweed
  • Lower leaves opposite (2 per node); becoming alternate (1 per node) higher on the plant 
  • Bright green to slightly yellowish-green on young plants; becoming greyish-green on older plants
  • Compound and finely divided, the final divisions usually coarsely toothed
  • Opposite (2 per node) throughout the plant except towards the ends of the smaller branches at flowering time where they may be alternate (1 per node)
  • Large, rounded in outline and 3 to 5-lobed; lobes smooth or coarsely toothed
  • Uppermost leaves usually not lobed
  • Leaf surfaces usually rough


Common Ragweed Giant Ragweed
  • Heads small, not showy
  • 2- 5 mm (1/12- 1/5 in.) across
  • Individual florets either male or female
  • Male- flowers in race-like elongated clusters at ends or branches, each head hanging downwards on a short stalk
  • Female- heads in axils of short, narrow green bracts near the base of each long cluster of male heads
    • Each head with a single flower and producing a single, hard, somewhat triangular or diamond-shaped seed with several short, sharp spines around the upper shoulder
    • Seed 3- 5 mm (1/8- 1/5 in.) long 
  • Flowers from August to October
  • Heads not showy
  • Individual florets either male or female
  • Male heads similar to common ragweed
  • Female- similar to common ragweed except is much larger and usually 2 to 4 are grouped together
  • Seed much larger, 5- 10 mm (1/5- 2/5 in.) long with several prominent lengthwise ridges ending in short blunt spines around the upper shoulder of the seed
  • Flowers from August to October

Often Confused With
Goldenrod (Goldenrod has a very conspicuous bright yellow inflorescences during the ragweed hay fever season of late summer and autumn)

Common ragweed is the most important cause of hay fever during August and September. Although inconspicuous and not recognized by most people, the tiny male flower beads hanging on their slender stalks produce hug quantities of very light pollen. As the pollen falls from these hanging flowers, it is caught by the wind and may be carried for distances greater than 200 km (125 miles). Hay fever sufferers, therefore, may be affected by pollen from ragweed plants far away.

Common ragweed flower Common ragweed Common ragweed seedling Giant ragweed Giant ragweed  seedling

Click to enlarge.