Ontario Weeds: Giant Ragweed
Table of Contents
- Other Names
- General Description
- Stems and Roots
- Flowers and Fruit
- Similar Species
- Related Links
Name: Giant ragweed, Ambrosia trifida
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.
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.
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
Caution: The pollen of Giant ragweed is
an important cause of hay fever during August and September, especially
in southwestern Ontario.
... on general Weed
... on weed identification, order OMAFRA
Publication 505: Ontario Weeds
... on weed control, order OMAFRA
Publication 75: Guide To Weed Control
to the Ontario Weeds Gallery |