Sunday, October 8, 2017

Plant Story--Ragweed, Ambrosia, an American Wildflower

Ragweed. Makes you think of allergies, right?

ragweed, Ambrosia
ragweed, Ambrosia
But my title says "an American wildflower."

And it is.

The ragweeds, the genus Ambrosia, in the sunflower family, Asteraceae, are all endemic to the Americas. Of the 22 species native to North America, only five are widespread (see USDA maps). But two species are in every one of the lower 48 states, annual ragweed Artemisia artemisifolia, and western ragweed, Artemisia psilostachya. They look very similar. The annual ragweed has always been a plant of disturbed areas, spending the winter as seeds, needing relatively open conditions to prosper during its one year of life. Human disturbances have given it many more places to grow. The other widespread ragweed, western ragweed, is a perennial, and is found in native ecosystems all across the United States.

In a project done in undisturbed tallgrass prairie in Kansas a few years ago, the only plant found in every plot, each about one square yard, was western ragweed. Across the continent, it is a similarly important component of native plant communities (link).

ragweed, Ambrosia
Green flowers all over this ragweed plant.
Deer, jackrabbits, and pronghorn are among the wildlife that eat both annual and western ragweed. Ragweed seeds are important fall and winter food for upland game birds. Apparently cattle find ragweeds unpalatable except early in the spring and don't eat them.

Ragweeds are native, they are common...and yet everyone hates them.

The hate is well-founded if you are allergic to ragweed pollen. Pollen from ragweed has distinctive chemistry to which people easily become sensitive. And those very common ragweed plants mate by lofting lots and lots of pollen into the air so that some comes down on the stigmas of another flower, leading to pollination and seed formation. It is hard to avoid breathing in ragweed pollen if you are outdoors in North America in late summer.

ragweed, Ambrosia
rageweed plant
It is easy to miss the plants though. They are small green herbs. The flowers are green. Like many wind-pollinated plants, the ragweeds don't need brightly colored flowers. They release their pollen into the air for pollination, they don't try to catch the eye of animal pollinators.

ragweed, Ambrosia
ragweed in full bloom
I'm posting about ragweed as a wild flower because it poses an important question. Do you love native plants? What about ragweed? Are only "nice" native plants worth conserving? We persecute weeds from elsewhere as invaders, but do we cherish all natives? Does the fact that we react to ragweed pollen make ragweed a plant to exterminate? It is just as native as the attractive blanketflower (Gaillardia) and purple cone flower (Ratibida). Even if you aren't impressed by green flowers.

Ragweed allergies are common and often awful, but not universal. Native Americans reportedly were not allergic to ragweed. Perhaps they were genetically resistant, perhaps their exposure from childhood protected them. Many modern Native Americans have ragweed allergies but a lot has changed since first contact. And allergies are irregularly distributed among Americans of European, Asian and African ancestry.  It is hard to know what causes having versus lacking allergies to ragweed.
Ragweed plants
I think they are called ragweeds because the leaves are divided and "ragged" looking. Ragged is defined as "having an irregular outline." Often "weed" was stuck on a plant name to indicate a plant of no value. European settlers had no use for it, even before they understood allergies.

The USDA Plants website calls western ragweed Cuman ragweed. Apparently that comes from  Ambrosia cumanensis which may--or may not--be the same as Ambrosia psilostachya. You may recognize that cumanensis means, "from Cumanรก" which is a province in nothern Venezuela. Almost everyone else calls Ambrosia psilostachya western ragweed.

Which brings me to the puzzle of the scientific name, Ambrosia, the genus of ragweeds. A quick google shows you that ambrosia means "the food of the gods," something heavenly to experience.  Ragweed? The botanical sources available to me agree the genus name means food of the gods but add "allusion unknown." Nobody knows why the ragweeds were given this particular name. They were named by Linnaeus in Sweden in the middle 1700s. Often Linnaeus gave plants the names they had in colloquial Greek or Latin. The ragweeds are American so that wasn't an option. For new plants, sometimes he recycled the colloquial name of a somewhat similar plant. For example, silphium was the Roman name of a Mediterranean-region plant that was overharvested to extinction (see recent BBC article link). Linneaus used Silphium as the scientific name of a group of sunflower relatives of central North America, for example compass plant link, creating substantial confusion. My Latin dictionary lists ambrosia as meaning, in addition to food of the gods, a food for the horses of the gods and an ointment that conferred immortality. None of these enlighten me as to why Linnaeus chose the name Ambrosia for ragweeds. I imagine Linnaeus sitting at his table considering a pressed, dried specimen of a ragweed that looked something like this (A click will magnify it. Specimen collected in 1884 in "North America" now in Harvard's Arnold Arboretum Herbarium). He would have had little information about it other than that it was from eastern North America and maybe that Indians there used it to prevent blood poisoning or to stop bleeding. Not knowing anything about its ability to induce allergies, he gave it an uplifting name. Why such a particularly nice name? No one knows. (Insight into this puzzle welcome!)

ragweed, Ambrosia

Ragweeds have been introduced to Europe and are causing allergies there. Europeans can certainly dislike the plant as both an exotic and a source of allergy!

Even though they are found in most habitats across North America, ragweeds are rarely serious threats to crops or other human endeavors. If they didn't cause allergies, they'd be nondescript common native plants. 

Bishop, L. T.  Treating Seasonal Allergies the Old School Indian Way. Revisited.  link
Drawer D. L. and T. W. Box. Forage Ratings for Deer and Cattle on the Welder
Wildlife Refuge. Contribution #118 Weldger Wildlife Foundation, Sinton Texas. 5610-5489-1-PB.pdf
Western Ragweed. Illinois Wildflowers. link
Western Ragweed. The Prairie Project. Oklahoma State University. link

Comments and corrections welcome.

Kathy Keeler, A Wandering Botanist
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My latest book tells stories about conspicuous wild plants NoCo Notables: 15 Northern Colorado Plants Worth Knowing Available at Amazon link

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