Ambrosia trifida (Giant Ragweed)

Plant Info
Also known as: Great Ragweed, Horseweed
Genus:Ambrosia
Family:Asteraceae (Aster)
Life cycle:annual
Origin:native
Status:
  • Weedy
Habitat:part shade, sun; disturbed soil; fields, along roads, edges of woods
Bloom season:July - September
Plant height:3 to 12 feet
Wetland Indicator Status:GP: FAC MW: FAC NCNE: FAC
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

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Detailed Information

Flower: Flower shape: indistinct Cluster type: raceme Cluster type: spike

[photo of flowers] 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: Leaf attachment: opposite Leaf type: lobed Leaf type: simple

[photo of leaves] 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.

Notes:

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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More photos

Photos by K. Chayka taken in Ramsey County. Photos courtesy Peter M. Dziuk taken in Anoka and Ramsey counties.

Comments

Have you seen this plant in Minnesota, or have any other comments about it?

Posted by: Elizabeth M - Lino Lakes -Anoka County
on: 2009-08-10 14:09:56

We have an abundance of this plant at our property. The property borders protected wetlands and is mostly wooded and shady with sandy soil. This particular plant grows along the tree lines.

Posted by: Alex - Proctor - southern St. Louis County
on: 2010-08-16 09:51:49

When we left for vacation two weeks ago, it was not quite identifiable or noticeable. When we got back last Sunday, it was dark. Monday arrived, and we noticed the nine-foot tall plants in our gooseberry and currant garden...seems to be this plant.

Posted by: Jake - Morrison County
on: 2017-07-02 21:02:21

I have a small corner wildflower garden near the alley and this plant appeared last year...got to be about 6 feet tall before I cut it down. This year, I removed 4 plants in the spring and now there are another 6 plants. Apparently, the seed packet that I used had these seeds mixed in because the rest of my flower beds don't have this plant.

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