Ontario Weeds: Giant Ragweed

Table of Contents

1. Name
2. Other Names
3. Family
4. General Description
   • Photos and Pictures
5. Stems and Roots
6. Flowers and Fruit
7. Habitat
8. Similar Species
9. Caution
10. Related Links

Name: Giant ragweed, Ambrosia trifida L.,

Other Names: grande herbe à poux, Great ragweed, Kinghead, Tall ragweed, ambrosie trifide

Family: Composite or Aster Family (Compositae) 

General Description: Annual, reproducing only by seed.

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Photos and Pictures

A. Plant in flower. B. Young plant before flowering.

Giant ragweed. A. Seedling, top and side views.

B. Top of flowering plant. C. Lower portion of flowering branch with 3 stalkless hads, each having 1 female flower.

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Stems & Roots: Stems erect, 0.4-4 m (16 in.-13 ft) tall, usually much-branched and somewhat hairy; leaves 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); leaves large, rounded in outline and 3- to 5-lobed, the lobes smooth or coarsely toothed; uppermost small leaves usually not lobed; in some plants most or all leaves not lobed but ovate with acuminate tips and only shallowly toothed; leaf surfaces usually rough (like medium sandpaper).

Flowers & Fruit: Flower heads unisexual as in Common ragweed; male (pollen-producing) flower heads and female (seed-producing) flower heads similar to those described for Common ragweed except that the female head is much larger and usually 2 to 4 are grouped together; seed much larger than Common ragweed, 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.

Habitat: Giant ragweed occurs in river valleys, meadows, roadsides and occasionally in cultivated fields in the southwestern portion of southern Ontario.

Similar Species: It is distinguished by its very tall stature (up to 4 m or 13 ft), its large, lobed but not divided leaves, its long, slender spikes of pollen-producing flower heads and its large, angular seeds with spines around the upper shoulder.

Caution: The pollen of Giant ragweed is an important cause of hay fever during August and September, especially in southwestern Ontario.

Related Links

... on general Weed topics
... on weed identification, order OMAFRA Publication 505: Ontario Weeds
... on weed control, order OMAFRA Publication 75: Guide To Weed Control

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