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Ragweeds
The Good, The Bad and The Ugly
(Ambrosia spp.)
By Ann Black and Jeff Sparks

Ragweeds are native, annual and perennial forbs found throughout North America. There are over 43 native species of ragweed in North America with several occurring in Texas. Ragweeds belong to the family Asteraceae, tribe Heliantheae. The species name Ambrosia is derived from Greek and means “food of the gods that is exceptionally pleasing to taste or smell”. Ragweeds are a early successional species and can commonly be found in disturbed areas, such as plowed fields, overgrazed pastures, road sides, borrow pits, and other waste places. They also compete with food crops such as soybean and other grains by using soil resources and water and rapidly outgrowing the cultivated crops. The most common species found in Texas are Common Ragweed (A. artemisiifolia), Giant Ragweed (A. trifida), and Western Ragweed. (A. psilostachya). 

Ragweeds have rough, hairy stems and usually lobed or divided leaves. The flowers of ragweeds are greenish and concealed in small heads on the bloom spike. Ragweeds have imperfect flowers and are monoecious, that is the male and female flower parts occur on separate flowers. The male (pollen-producing) flowers are bell-shaped, hang upside down and are packed with stamens. The female (seed-producing) flowers face upward and are usually farther down on the bloom spike. Ragweed flowers are small and not too attractive to pollinators; therefore they are wind-pollinated. One plant produces millions of pollen grains, which become airborne very easily. Approximately 250,000 tons of ragweed pollen are released into the air annually in the United States, causing considerable misery to some 25 million hay fever sufferers. The allergic response is initiated when pollen grain proteins (antigens) attach receptors on antibodies linked to immune system cells. This results in he immune cells releasing histamines, which are the molecules directly responsible for the symptoms known as hay fever (Diggs et al. 1999).  

Giant Ragweed (Ambrosia trifida) is an annual ragweed, that grows from a fibrous root system to a height of > 10 feet in soils with proper moisture and fertility such as found near streambeds and roadsides. It may become troublesome in fields with moist soil where it can form vast pure stands. Giant ragweed will generally begin pollination 2 to 3 weeks before common ragweed, and will produce significantly greater amounts of pollen compared with common ragweed because of its larger size. The male flower heads and female flower heads are similar to those of Common Ragweed except that the female head is much larger and usually 2 to 4 are grouped together; the seed is much larger than Common Ragweed, 1/5-2/5 in. long with several, prominent, lengthwise ridges ending in short blunt spines around the upper shoulder of the seed. Giant Ragweed flowers from August to October.

Common Ragweed (Ambrosia artemisiifolia) is an upright annual that grows from 4 to 30 inches in height. Its stems may be highly branched and may be hairless or hairy. The twice-compound leaves are finely divided and show lots of variation; usually lacy in appearance, sometimes hairy below, and are smooth above. The early leaves are opposite, but older leaves become alternate on the stem. Common ragweed blooms from August to October. The flower heads are small, green, and inconspicuous. Although individual flowers are either male or female, both types are found on the same plant. The small seeds are a dry, hard achene with several short sharp spines. Common ragweed is one of the most abundant weeds of cultivated land. It is also found as a weed in gardens, flower borders, poorly kept lawns, edges of sidewalks, roadsides, fence lines, waste places, and in disturbed areas in pastures and meadows.

Western Ragweed (Ambrosia psilostachya) is a perennial forb found in many habitats throughout the western U.S. It grows 10 to 30 inches in height and can spread by rhizomes. Its leaves are opposite, long, deeply lobed and lanceolate. The flower heads are small and green. The male flowers are borne on a terminal spike, while the female flowers are borne farther down along the stem. The genus name psilostachya is derived from the Greek words psilos, "bare," and stachys, "a spike," and refers to the bare flower spike. Seed production is of secondary importance for western ragweed since it is a perennial and produces new growth from rootstocks. During the first growing season, seedlings will not flower or produce additional shoots. However, in the second year, additional shoots will develop from rhizomes and form a colony. Western ragweed is a good wildlife plant; however, due to its high nitrate content, it may cause bitter-tasting milk in livestock. Cattle producers often think it of as the “scourge of the weed world”, and it is an indicator of poor grazing management. On deep soils, the roots of western ragweed can grow to a depth of 5 or 6 feet and compete with more shallow-rooted dominant grasses. On shallow soils, western ragweed roots are able to penetrate the clay pan while those of grasses cannot. For this reason, ragweed may be beneficial to an overgrazed site by breaking the soil down over time. Young foliage of western ragweed is eaten by livestock and whitetail deer. Quail and other birds consume the seeds.