What Good is a Ragweed?

Ragweed is notorious for causing seasonal allergies. In late summer and early fall an estimated 23 million Americans suffer runny noses, itchy eyes and respiratory congestion caused by pollen of ragweed.  This seasonal allergy, also called hay fever, is caused by an inappropriate immune response.  Our immune system identifies and eliminates foreign material like an infectious bacterium or an invading virus.  In some cases, the immune system mounts a strong attack on a non-pathogenic agent like ragweed pollen.  This overly vigorous response includes the release of a powerful molecule called histamine that causes many of the hay fever symptoms mentioned above.  Treatments for seasonal allergy usually contain an anti-histamine to counter the effects of histamine.  

Giant Ragweed Ambrosia trifida 

But ragweed has a life of its own, beyond being the cause of seasonal allergy, and an interesting life it is.   Two species of ragweed are common in North Carolina, Common Ragweed (Ambrosia artemisiifolia) and Giant Ragweed (Ambrosia trifida).  These two ragweed species are in the same family as sunflowers, the Asteraceae.  The name of the genus of ragweed, Ambrosia, is from the Greek and means food of the gods.  Ragweed seeds are highly nutritious, containing high concentrations of protein and oil.  Native Americans grew ragweed as a food crop then abandoned its cultivation when they adopted corn based agriculture.  The specific name of Common Ragweed, artemisiifolia, suggests the leaves of this plant resembles the highly dissected leaves of Artemisia, wormwood or sagebrush.   The specific name of Giant Ragweed is trifida and that refers to the three-lobed leaves of the plant. 

Common Ragweed grows to a height of about 2 feet.  It has dissected leaves and produces small, green flowers.  The flowers, which do not look like sunflowers despite being in that family, are borne on vertical, candelabra-like inflorescences.  Each inflorescence has dozens of male flowers that bear pollen-producing stamens.  Near the base of the inflorescence are a few female flowers that have the egg-producing carpel.  The flowers of ragweed are small, inconspicuous and colored green.  Plants that have brightly colored flowers use them to attract pollinators like insects or birds.  Ragweed does not depend on animal pollinators but uses wind to spread its pollen. Wind pollinated plants tend to have dull flowers.  Each ragweed plant produces about a billion tiny pollen grains in a season.  This pollen floats on the wind and a few grains pollinate female flowers.  This strategy of producing lots of pollen and casting it to the winds contributes to ragweed’s importance as an allergy health hazard because some of that pollen is inhaled by sensitive people.  Imagine billions of plants each producing a billion pollen grains and you can see the impact of this plant on human health. Each ragweed plant has a few female flowers at the base of each inflorescence where they receive the free-floating pollen.  Pollen lands on stigmas extending from the female flower and a pollen tube grows down into the flower to fertilize the egg within.  Each flower makes a single, dark brown seed that is a favored food of wildlife including quail.  

The dissected leaves of Common
Ragweed, Ambrosia artemisiifolia

Male, pollen producing flowers
of Common Ragweed

Female flowers of Common Ragweed
with their extended stigmas to collect pollen

Giant Ragweed is much taller than Common Ragweed with plants reaching 13 feet in height.  The stems bear the large, three-part leaves that give the plant its scientific name.  At the tops of stems are the inflorescences that are similar to those of Common Ragweed.  The green flowers, both male and female, carry out the same type of reproduction as the Common Ragweed with massive amounts of pollen released and brown seeds produced. 

The three-part leaf of Giant Ragweed, Ambrosia trifida

Male flowers of Giant Ragweed with yellow,
pollen bearing stamens visible.  

Female flowers of Giant Ragweed with their
stigmas extended.

 Brown fruits developing from the female flowers of Giant Ragweed.

Ragweed plants produce large amounts of pollen because wind pollination is inefficient.  Other wind-pollinated plants, like Bermuda Grass and Oaks, also produce vast numbers of pollen grains and contribute hay fever suffering.  Ragweed pollen has a molecule on its surface that makes it a potent allergen that stimulates the human immune system.  The molecule is a protein, an enzyme called pectinase.  This enzyme breaks down pectin, a complex carbohydrate called pectin found in plant cell walls.   Pectinase helps the pollen deliver the sperm nucleus to the egg, allowing fertilization and more ragweed for next year.  Remember, ragweed is not trying to make us miserable, it is not trying to give us runny noses, it is not trying to clog our sinuses.  But, in the fall of the year these things do happen because of the reproductive strategy followed by Ambrosia artemisiifolia and Ambrosia trifida. 

Back to the question, what good is a ragweed?  People often ask this question of living things they view as problems.  What good is a mosquito?  What good is a tick?  What good is a ragweed?  All these organisms had an evolutionary history before people appeared on the scene.  They lived, reproduced and adapted.  The two ragweed species in this area are highly successful.  Ambrosia artemisiifolia and Ambrosia trifida fix carbon, provide food for a variety of animals, were once crop plants and because of the shape of a protein on the surface of their pollen, cause allergies.   None of these traits was the result of design for a human, utilitarian end. With the gene editing technology CRISPR it is now theoretically possible to deliberately cause the extinction of a whole species.  This very thing, intentional extinction, has been discussed to rid the world of a species of a mosquito that transmits one type of malaria.  I hope humanity has better sense than to eliminate even more species than we have already done, be it the vector for a terrible disease or the cause of hay fever.