Sunday, August 19, 2012


pollen-producing male ragweed flowers on spike-like racemes
photographed with Canon MP-E 65mm 1-5x macro

       The weather segment of the local evening news sometimes gives the pollen  information for allergy sufferers. The other night I noticed that the "Tree" and "Grass" categories were absent, but "Weeds" were high, with ragweed and Artemisia (mugwort) listed as the main culprits.

        It's the peak of allergy season for ragweed victims, and according to weed ecologists, the problem is getting worse. Increased carbon dioxide in the atmosphere and warmer temperatures are producing a bumper crop and higher-than-normal pollen counts. In other words, global warming is having an immediate impact close to home. Each large ragweed plant is capable of producing a billion pollen grains over the course of its flowering period and they all travel on the wind or a breeze.

Common Ragweed is also known as bitterweed, carrot-weed, tassel-weed and hayfever-weed (probably the most appropriate of them all). Its Latin name is Ambrosia artemisiifolia. Ambrosia now refers to the mythic food of Greek gods that allowed them to live forever, but originally it meant simply "immortal." This is probably a testimonial to the tenacity of a weed that's almost impossible to irradicate, except perhaps on a temporary local basis. The second (specific) part of its scientific name indicates its relationship, both in appearance and genetics, to Artemisia, the genus of wormwoods and mugworts. They all are prolific powdery-pollen producers in the Aster (Daisy) Family. The relationship is based on technical characteristics of the flowers, even though these particular species lack showy petals because there's no need to attract insects for pollination.

Ragweed flower detail cropped from 100mm macro shot at 1.6x
[allowing for sensor-size scale] and using a flash
against natural sky background while shielding from the wind

Instead, the wind does the pollinating. The tiny bell-like flowers have green hoods covering the yellowish male stamens. On ragweed, these blossoms are lined up along the top of the stem in a spike-like raceme. (See the opening photo.) Female flowers are even less conspicuous and grow in the leaf axils where the leaves attach to the main stem. The green leaves themselves are deeply dissected and remotely fern-like.

Common Ragweed leaf photographed with 100mm macro
after creating a shadow to hide the background

During the winter, persistent ragweed seeds -- rich in oil -- are eaten by a number of resident songbirds and upland game birds.  To humans, the seeds are bitter tasting, hence the alternate name bitterweed.  In the summer, a few ragweeds may be eaten by the caterpillars of several butterfly and moth species.

Ragweed is an annual plant that grows in disturbed ground and road shoulders.  It goes through its entire life cycle in one growing season and then dies.  From the human perspective its useless if not outright obnoxious and undesirable.  From an ecological point of view, it is a native pioneer plant that colonizes bare patches of ground after some sort of disturbance, either natural or manmade.

        Ragweed's miniscule arrowhead-shaped seeds are covered with spines that cling to any passing animal brushing by the ripe plant. Eventually they drop off and a few may land in suitably dry, sunny or partly-shaded locations.

        By contrast, dandelion and thistle seeds arrive on the wind. Goldenrod seeds have smaller fuzzy "parachutes" and travel shorter distances.  The weeds stabilize the soil and reduce erosion, paving the way for shrubs to move in.  Eventually trees gain a foothold and given enough time, a forest will regenerate itself in what was once an open field.  By then the annual ragweeds and perennial goldenrods are long gone.  They lost out in the battle for sunlight.

        Goldenrods often get blamed for allergies because they flower at the same time in the same habitats as ragweeds and mugworts, but their pollen is sticky and heavier.  Goldenrods require insects for pollination, not wind.  There's more on my Goldenrod blog.

Corrections, comments and questions are always welcome at  Other articles can be found on a parallel blog at  More nature photos can be found at

A small Common Ragweed plant; they can grow much larger and more prolific of pollen!

No comments: