Giant Ragweed | Ambrosia trifida L.

California Pest Rating for
Giant Ragweed | Ambrosia trifida L.
Family: Asteraceae
Pest Rating: B


Initiating Event:

This plant has been included on the CDFA noxious weed list.

History & Status:

Background: Giant ragweed is a large (to 2 meters tall), annual herb.  The stems have black, longitudinal lines and are covered with hairs.  Leaves are palmately 3- to 5-lobed and are sparsely covered with tiny, stiff hairs.  Fruits (“burrs”) are 6-12 mm long and are tapered and blunt on one end and widened with 5-8 teeth on the other end.   Giant ragweed grows best in disturbed areas with moist, fertile soil, and it is an important weed.  Ragweed pollen is a major cause of allergies, and the pollen of this species is a known allergen.

Worldwide Distribution: Native to eastern North America.  Giant ragweed has been introduced to, and is now established in western North America and much of Asia and Europe.

Official Control: Giant ragweed is listed as a noxious weed in at least three states (California, Delaware, and Illinois).

California Distribution: Giant ragweed has been reported in 10 California counties (Contra Costa, Glenn, Orange, Los Angeles, Madera, Monterey, San Diego, San Joaquin, San Luis Obispo, and Siskiyou).  Most of these records are from residential gardens.  According to one source, it is not naturalized in California, but occurs as a waif and/or garden escape.

California Interceptions: Recent collections from 2011 through 2014 originated from gardens or as seed contaminants intercepted at the CA border.  It is commonly intercepted in feed seed shipments entering the state.

The risk giant ragweed would pose to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction: Giant ragweed thrives in temperate climates, and it is already present in ten counties in California. Therefore, it receives a High (3) in this category.

-Low (1) Not likely to establish in California; or likely to establish in very limited areas.

-Medium (2) may be able to establish in a larger but limited part of California.

-High (3) likely to establish a widespread distribution in California.

2) Known Pest Host Range: Giant ragweed does not require any one host, but grows wherever ecological conditions are favorable. It receives a High (3) in this category.

-Low (1) has a very limited host range.

-Medium (2) has a moderate host range.

-High (3) has a wide host range.

3) Pest Dispersal Potential: Most seeds of giant ragweed fall near the parent plant and they are apparently not eaten in large number by wildlife. However the seeds float and are apparently able to be transported readily via water.  Therefore, it receives a Medium (2) in this category.

-Low (1) does not have high reproductive or dispersal potential.

-Medium (2) has either high reproductive or dispersal potential.

-High (3) has both high reproduction and dispersal potential.

4) Economic Impact: Giant ragweed is an important weed in crop systems, especially corn and soybean. Infestations could lead to a loss of markets.  Giant ragweed has already developed resistance to herbicides, so it could increase production costs.   Therefore, it receives a High (3) in this category.

Economic Impact: A, B, C

A. The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

G. The organism can interfere with the delivery or supply of water for agricultural uses.

Economic impact Score: 3

-Low (1) causes 0 or 1 of these impacts.

-Medium (2) causes 2 of these impacts.

-High (3) causes 3 or more of these impacts.

5) Environmental Impact: Giant ragweed is able to out-compete other plants in open areas, including crop systems as well as grasslands. It has been scored as a plant with a high risk of invasiveness by the California Invasive Plant Council.  Giant ragweed is often controlled with herbicides, and it has developed resistance to certain pesticides.  It could trigger new treatment programs.  Therefore, it receives a High (3) in this category.

Environmental Impact: A, D

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered species.

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment programs.

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Environmental Impact Score: High (3)

-Low (1) causes none of the above to occur.

-Medium (2) causes one of the above to occur.

-High (3) causes two or more of the above to occur.

Consequences of Introduction to California for giant ragweed: High (14)

-Low = 5-8 points

-Medium = 9-12 points

-High = 13-15 points

6) Post Entry Distribution and Survey Information: Giant ragweed has been reported in ten California counties, but it seems likely that its distribution within the state could expand further. Therefore, it receives a Medium (-2) in this category.

-Not established (0) Pest never detected in California, or known only from incursions.

-Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

-Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

-High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

The final score is the consequences of introduction score minus the post entry distribution and survey information score: Medium (12).


As stated before, giant ragweed has been present in California for at least 80 years.  Many of the available records are from residential gardens.  It has not, so far, been shown to be a serious problem either agriculturally or environmentally in this state; nevertheless, there is a chance that if it made its way to a new, favorable locality in the state it could behave differently.

Conclusion and Rating Justification:

Based on the potential economic and environmental impact of this weed, and the fact that it is commonly found on feed mill seed entering the state (and is likely to have further opportunities for establishment without regulation), a “B” rating is justified.


Bassett, I.J. and C.W. Crompton.  1982.  The biology of Canadian weeds. 55. Ambrosia trifida L.  Canadian Journal of Plant Science.  62: 1003-1010.

Bullock, J.M., Chapman, D., Schafer, S., Roy, D., Girardello, M., Haynes, T., Beal, S., Wheeler, B., Dickie, I., Phang, Z., Tinch, R., Čivić, K., Delbaere, B., Jones-Walters, L., Hilbert, A., Schrauwen, A., Prank, M., Sofiev, M., Niemelä, S., Räisänen, P., Lees, B., Skinner, M., Finch, S., and C. Brough.  2010.  Assessing and controlling the spread and the effects of common ragweed in Europe.  Contractor: Natural Environment Research Council, UK.  456 pp.

California Invasive Plant Council.

Consortium of California Herbaria.


Goplen, J.J., Sheaffer, C.C., Becker, R.L., Coulter, J.A., Breitenbach, F.R., Behnken, L.M., Johnson, G.A., and J.L. Gunsolus.  2016.  Giant ragweed (Ambrosia trifida) seed production and retention in soybean and field margins.  Weed Technology.  30: 246-253.

Megyeri, K.  2011.  The impact of Ambrosia trifida (giant ragweed) on native prairie species in an early prairie restoration project.  Thesis.  University of New Orleans.  Accessed from:

Webster, T.M., Loux, M.M., Regnier, E.E., and S.K. Harrison.  1994.  Giant ragweed (Ambrosia trifida) canopy architecture and interference studies in soybean (Glycine max).  Weed Technology.  8: 559-564


Kyle Beucke, 1220 N Street, Room 221, Sacramento, CA, 95814, 916-403-6741,[@]

Responsible Party:

Dean G. Kelch, Primary Botanist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 403-6650;[@]


You must be registered and logged in to post a comment.  If you have registered and have not received the registration confirmation, please contact us at[@]

Comment Format:

♦  Comments should refer to the appropriate California Pest Rating Proposal Form subsection(s) being commented on, as shown below.

Example Comment:
Consequences of Introduction:  1. Climate/Host Interaction: [Your comment that relates to “Climate/Host Interaction” here.]

♦  Posted comments will not be able to be viewed immediately.

♦  Comments may not be posted if they:

Contain inappropriate language which is not germane to the pest rating proposal;

Contains defamatory, false, inaccurate, abusive, obscene, pornographic, sexually oriented, threatening, racially offensive, discriminatory or illegal material;

Violates agency regulations prohibiting sexual harassment or other forms of discrimination;

Violates agency regulations prohibiting workplace violence, including threats.

♦  Comments may be edited prior to posting to ensure they are entirely germane.

♦  Posted comments shall be those which have been approved in content and posted to the website to be viewed, not just submitted.

Pest Rating: B

Posted by ls