The tiny male flowers are arranged in clusters near the top of the plant. They produce the irritating pollen that produces misery for seasonal allergy sufferers.
The tiny female flowers are arranged in small clusters situated in the leaf axils below the male flower clusters.
Common ragweed is a coarse, rather ugly roadside weed that gets 1-4 ft (30-120 cm) tall with the upper part of its hairy stem much branched. It has deeply dissected leaves, 2-4 in (5-10 cm) long, that are opposite on the lower stem, and then alternate above. From July through October ragweed spews clouds of yellow pollen from tiny green male disc florets which are borne in flowerheads carried in 2-4 in (5-10 cm) spikelike clusters at the tips of the branches. The female florets are also greenish and tiny, but don’t produce pollen and are borne in small clusters in the leaf axils. Ragweed does not have ray florets. Female florets eventually turn brown as they mature and develop countless seeds which may be retained on the plants into the winter.
Location Ambrosia artemisiifolia is native to North America, but exactly where is not known. It has expanded its range and today occurs in almost all of Canada and in every U.S. state except Alaska. Common ragweed flourishes on roadsides and in fields and waste places, virtually anywhere the soil has been disturbed. It sneaks into gardens and crop fields and has become an invasive pest in southern and central Europe, Africa, Japan and Australia.
Common ragweed can grow in just about any soil, from hard packed clay to loose sand. It actually does best on poor soils because there is less competition from other plants. Light: Common ragweed thrives in full sun and does pretty darn well in partial shade, too. Moisture: Common ragweed is relatively drought tolerant and is usually found in drier sites. However, it also grows in wetlands. Hardiness: USDA Zones 3 - 10. Propagation: Seeds can remain viable for five years or more. Ragweed is pretty good at propagating itself.
Common ragweed is a serious agricultural pest in soybean and grain operations, where it must be controlled with herbicides.
It isn’t goldenrod (Solidago odora is one species) that causes the hay fever that millions suffer in late summer and autumn. Ragweed has tiny pollen grains that are transferred from the male florets to the female florets by gravity and wind. Goldenrod, on the other hand, has larger pollen grains that are transferred from its male florets to female florets by insects. Goldenrod pollen is too big and heavy to float on the wind like ragweed pollen. Note that goldenrod’s flowers are conspicuous and showy, all the better to attract insects, whereas the flowers of ragweed are barely noticeable.
Ragweed sprawls onto a parking lot in late summer.
Shake these male flower clusters to release a cloud of agony-producing pollen.
Ragweed seeds are rich in oil and relished by upland game birds, song birds and small mammals. In winter, the elongated seed clusters sometimes protrude above the snow, serving up easy access to hungry birds. Ragweed foliage is eaten by several kinds of moth caterpillars, but is apparently unpalatable to mammalian herbivores.
Native Americans used infusions of ragweed foliage externally for skin infections, rashes and injuries, and internally to control diarrhea, and as an emetic to cause vomiting.
Ambrosia – the food of the Greek gods. Consuming ambrosia confers immortality. Or is ambrosia a totally delightful dessert made with sugar, cream, marshmallows, pineapple, cherries, pecans and coconut (probably not conferring immortality)? Or is ambrosia the curse of hay fever sufferers every autumn? Carolus Linnaeus, the 17th century father of plant taxonomy, gave ragweed the name, Ambrosia. Certainly he knew his Greek mythology, if not modern culinary indulgences. I cannot imagine why he named this weed as he did. The specific epithet, artemisiifolia, noting the similarity of the leaves to those of absinthe (Artemisia absinthium), makes more sense.
Pollinosis, or an allergy to pollen, affects about 20% of the population of industrialized countries. In spring, pines, oaks and other trees spew windblown pollen that causes suffering. Then in summer various flowering grasses continue the morbid tradition. Finally, in autumn, comes ragweed pollen, perhaps the worst offender of all. We get a break in winter.
There are 40 or more species of Ambrosia, all native to the New World. Probably most can contribute to the airborne pollen load and the dismay of those who put up with pollinosis.