This huge plant is probably an allergy sufferer's worst nightmare. It has some ecological value to various moths, but otherwise is less important than Ambrosia artemesiifolia (Common Ragweed). Giant Ragweed can be distinguished from other Ambrosia spp. (Ragweeds) by its palmately lobed leaves; other Ragweeds have leaves that are pinnatifid or bipinnatifid. The name of this genus of plants refers to ambrosia, "the food of the gods" in antiquity. This seems like a strange name for a group of unattractive plants, unless it refers to the value of the seeds of certain species from a bird's point of view. Return
This is a native annual plant from 3-12' tall, branching occasionally. The green stems are covered with white hairs. The opposite leaves are up to 12" long and 8" across. The larger leaves are divided into 3 or 5 lobes, usually serrated along the margins, and have long petioles that are sometimes winged. The smaller leaves near the base of an inflorescence are lanceolate and often hairy underneath. Many of the upper stems terminate in a cylindrical spike of flowers, about 3-6" long, with one or more smaller spikes near its base. The small flowers are are yellowish green and devoid of petals and sepals. They occur in small drooping clusters less than ¼" across on short pedicels, and are densely arranged all around each spike. The fine pollen of the male flowers is easily carried aloft by the wind. This typically occurs during late summer or early fall. The seeds are large, tough-coated, and remain viable in the soil for several years. The root system is fibrous. Cultivation
Ambrosia trifida is a species of flowering plant in the aster family, Asteraceae. It is native to North America, where it is widespread in Canada, the United States, and northern Mexico. It is present in Europe and Asia as an introduced species, and it is known as a common weed in many regions. Its common names include great ragweed, Texan great ragweed, giant ragweed, tall ragweed, blood ragweed, perennial ragweed, horseweed, buffaloweed, and kinghead.
This is an annual herb usually growing up to 2 meters tall, but known to reach 6 meters in rich, moist soils. The tough stems have woody bases and are branching or unbranched. Most leaves are oppositely arranged. The blades are variable in shape, sometimes palmate with five lobes, and often with toothed edges. The largest can be over 25 centimeters long by 20 wide. They are borne on petioles several centimeters long. They are glandular and rough in texture. The species is monoecious, with plants bearing inflorescences containing both pistillate and staminate flowers. The former are clustered at the base of the spike and the latter grow at the end. The fruit is a bur a few millimeters long tipped with several tiny spines.
This species is well known as a noxious weed, both in its native range and in areas where it is an introduced and often invasive species. It is naturalized in some areas, and it is recorded as an adventive species in others. It grows in many types of disturbed habitat, such as roadsides, and in cultivated fields. Widespread seed dispersal occurs when its spiny burs fall off the plant and are carried to new habitat by people, animals, machinery, or flowing water. The plant is destructive to native and crop plants because it easily outcompetes them for light.
There is also great interest in preventing the spread of this plant because its pollen is a significant human allergen. It is one of the most familiar allergenic ragweeds, and residents of different regions begin to experience allergic symptoms as the plant spreads into the area.
Giant Ragweed is a common plant that occurs in every county of Illinois (see Distribution Map). It can be found in disturbed areas of moist to mesic black soil prairies, especially along the margins near woodlands or fields. Other native habitats include disturbed areas of moist clay prairies, meadows in woodland areas or near rivers, thickets, and woodland borders. In more developed areas, it occurs in vacant lots, cropland, abandoned fields, poorly drained waste areas, areas along roadsides and railroads, and fence rows. Faunal Associations
The name Ambrosia ×helenae Rouleau applies to hybrids between A. artemisiifolia and A. trifida. Hybrids between A. bidentata and A. trifida have been recorded. Ambrosia trifida may be no longer extant in British Columbia.