Ambrosia trifida (Giant Ragweed)
| Also known as: | Great Ragweed, Horseweed |
|---|---|
| Genus: | Ambrosia |
| Family: | Asteraceae (Aster) |
| Life cycle: | annual |
| Origin: | native |
| Status: |
|
| 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): |  |
| National distribution (click map to enlarge): |  |
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Detailed Information
Flower: 
![[photo of flowers]](/udata/r9ndp23q/green/giant-ragweed-100_2636.jpg)
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:
![[photo of leaves]](/udata/r9ndp23q/green/giant-ragweed-100_2641.jpg)
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
Giant Ragweed plant
a wall of Giant Ragweed
early growth, with Common Ragweed sprouts
more flowersPhotos 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?
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.
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.
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.
on: 2019-07-17 18:26:50
I found this plant growing in a small garden bed next to my Morning Glories and Sweet Pea flower vines. I'd never seen anything like it and when it got about 4ft tall a Master Gardener helped me identify it and advised me to pull it up as soon as possible. After reading how it causes terrible allergies- I couldnt pull it out fast enough..I'd been experiencing the worst allergies ever this year- Giant Ragweed probably the culprit.
on: 2019-08-04 18:03:21
First saw it 6-8 years ago ...... can be hard to keep up with.
on: 2020-08-06 18:18:13
Saw this plant today at Turtle Lake Park in Shoreview. It's HUGE. The patch looked similar in size to pic #4 above. It's on the edge of a native plant garden between the boat launch & the public restrooms.
on: 2021-08-17 22:47:51
It is growing in our new garden in St. Paul. Oh well - maybe that's why the kids are sneezing a lot! Grew amazing 6-8 feet in a few weeks. Deer eating most of it but some plants grew next to the house and in good company with Abutilon theophrasti/velvet leaf.