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Plant of the Week
Common Ragweed
Latin: Ambrosia artemissifolia
It’s ragweed season so sniffling and sneezing have become my daily
companions. Were I alone in my suffering my discontent would be heard far and
wide. But all of us allergy sufferers endure the floristic injustice in silence
while we await either a good rain to wash the air or that first killing frost.
For most of us the main culprit this time of year is common ragweed. This weed
is a problem throughout the entire country but most concentrated in the eastern
half of the United States.
Ragweed is a member of the daisy family, but unlike most members of the group,
it lacks the showy strap-shaped ray petals that give the daisy its
characteristic appearance. Three ragweed species are encountered in the eastern
states but A. artemissifolia is the most common in Arkansas. It is an annual
weed that grows in pastures, roadsides and about anyplace except in the shade of
trees. Plants reach 12 to 16 inches tall with dissected greenish-pubescent
leaves up the erect stems.
The insignificant appearing flowers are produced at the ends of the branches in
6 inch long spikes with the flowers in clusters up the spike. Flowers at the
base of the spike are usually female and produce next years seeds while those
towards the ends are the male flowers that produce all of the pollen. Ragweed is
a plant with wind borne pollen with flowers appearing from July through October.
Its pollen is small enough - between 20 to 50 microns in size - to float long
distances without settling to the earth. Pictured here is giant ragweed, A. trifida which is an introduced weed that can grow six to eight feet tall.
Some people seem highly sensitive to allergens and are bothered by a wide
variety of plant pollens while others are specialists and only respond to one or
a few kinds of pollen. On the first exposure a sensitive individual’s immune
system reacts to the proteins in the pollen and produces antibodies specific to
those proteins. The next time the pollen grains are inhaled and embed themselves
in the mucous of the nasal lining, the immunoglobulin antibodies of the immune
system recognize the foreign protein and produce chemicals such as histamines in
response to the insult to the immune system. These histamines cause dilation of
capillaries, contraction of the nasal muscles and an excess secretion of nasal
fluids. Walla - all of the symptoms of a hay fever attack.
Many fall blooming plants - but especially goldenrod - are incorrectly blamed
for causing allergies. Goldenrod has the misfortune of blooming at the same time
as the ragweed. Because ragweed flowers are not at all showy, they are often
overlooked as people scan the roadsides for the cause of their discomfort.
Goldenrod is largely insect pollinated. The pollen grains are too heavy for wind
transport and must be moved by bees or other insects as they work the flowers.
By: Gerald Klingaman, retired
Extension Horticulturist
- Ornamentals
Extension News -
August 27, 1999
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