Description: This native or adventive perennial plant is usually 1-2' tall, but sometimes considerably higher; it often branches frequently in the upper half. The stems are covered with white hairs, and have fine lines that run vertically along their lengths. The leaves are more or less sessile and up to 5" long and 2" across. They are lanceolate, but have widely spaced pinnate lobes that are bluntly pointed. Their surface is covered with a fine white pubescence, especially on the lower side, providing them with a whitish green appearance.
Upper stems terminate individually in a
spike-like inflorescence about 1-4" long, which is covered with fine short
hairs. This inflorescence is covered all around with small green flowers about
1/8" long on short drooping pedicels. The tiny reproductive parts of these
flowers can be white or yellow; eventually, the flowers turn brown and form
achenes. The pollen of the male flowers is very small and easily dispersed by
the wind. The blooming period occurs from mid-summer to fall, and lasts 1-2
months. There is no floral scent. The achenes are without tufts of hair. The
root system is fibrous, and produces rhizomes. This plant is a strong
colonizer, in part because the root system exudes chemicals that inhibit the
germination and development of other plants (allelopathy).
Cultivation: The preference is full sun, dry
conditions, and poor soil. Although this plant can grow perfectly well in
moist, fertile soil, it usually grows in dry soil that contains significant
amounts of clay, sand, or gravelly material, as this reduces competition from
surrounding plants. This plant is easy to grow, and can become aggressive in
sunny, dry situations. It doesn't appear to be bothered much by foliar
disease.
Range & Habitat: Western Ragweed occurs
occasionally in northern and western Illinois, and is uncommon or absent
elsewhere (see
Distribution
Map). It is unclear if this plant is adventive from the west, or native to
Illinois. Habitats include dry areas of prairies, old cemeteries, areas along
railroads and roadsides, abandoned fields, and miscellaneous waste areas. It is
more common in disturbed areas, especially if the soil is sandy or gravelly.
Western Ragweed may show up unexpectedly in prairie restorations if seed from
western areas is used; this is another reason to use seed from local
ecotypes.
Faunal Associations: The
flowers are wind-pollinated and rarely attract pollen or nectar-seeking
insects. The caterpillars of various moths feed on the foliage of this and
other ragweeds, including Bird-Dropping moths, the Wavy-Lined Emerald Moth,
Ragweed Flower Moth, and others (see
Moth Table). Many grasshoppers
feed on the foliage of Western Ragweed (see
Grasshopper Table),
which are eaten by insectivorous songbirds and upland gamebirds. Like
Ambrosia artemesiifolia (Common Ragweed), the seeds of Western Ragweed
are popular with many kinds of songbirds and upland gamebirds (see
Bird Table). The seeds are
nutritious and remain available through the winter months. Some rodents eat the
seeds, including the Thirteen-Lined Ground Squirrel and Prairie Vole. Mammalian
herbivores eat the bitter foliage of this and other ragweeds only to a limited
extent. It is possible that the seeds are only semi-digestible, and may be
distributed to some extent by these animals.
Photographic Location: Photographs were taken at the
Windsor Road Prairie in Champaign, Illinois.
Comments: Like other ragweeds, the airborne pollen
of Western Ragweed can cause allergic reactions in people during the late
summer and fall. This plant has high ecological value to grasshoppers, moths,
and birds. It resembles Common Ragweed; however, the leaves of Western Ragweed
are simply pinnate, while the leaves of the latter plant are often doubly
pinnate and more deeply lobed.