Ambrosia trifida (Giant Ragweed)
|Also known as:
|Great Ragweed, Horseweed
|part shade, sun; disturbed soil; fields, along roads, edges of woods
|July - September
|3 to 12 feet
|Wetland Indicator Status:
|GP: FAC MW: FAC NCNE: FAC
|MN county distribution (click map to enlarge):
|National distribution (click map to enlarge):
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Slender spike-like racemes, often branched at the base, 3 to 8 inches long, at the tip of the central stem and branches. Male (staminate) flowers are yellow to greenish with no petals, about 1/8 inch across, hanging down on short stalks along the spike. Female (pistillate) flowers are indistinct, hidden in clusters of short leafy bracts at the base of the cluster, along the stem or in the axils. Clusters are initially densely packed but spread out as the plant matures.
Leaves and stems:
Leaves are opposite, up to 12 inches long and 8 inches wide, the larger leaves divided into 3 or 5 deep lobes, the lobe divisions oval lance shaped similar to the unlobed smaller leaves. Edges are finely toothed, surfaces are variably hairy, smooth to covered in fine, stiff pubescent hairs, the leaf stalks more frequently hairy. Stems are coarse, mostly smooth in the lower plant with spreading hairs in the upper, the central stem branching from nodes along the entire length, the upper branches often reaching heights equal to the main stem.
Ragweed is an early pioneer species of disturbed soils, has been introduced worldwide and is now a common weed in both agricultural and urban sites. With high population densities and prolific production of highly allergenic pollen, it is a major contributor to the agony of hayfever sufferers around the globe.
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Photos by K. Chayka taken in Ramsey County. Photos courtesy Peter M. Dziuk taken in Anoka and Ramsey counties.
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