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Giant Ragweed: Return of a Native
Bob Hartzler

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November 4, 2004 The dominant weeds faced by Iowa farmers in the 1970’s and 80’s were species brought intentionally or accidentally to the United States by European settlers.  Although it is unclear how velvetleaf arrived in North America, it was widely grown as a fiber crop in the mid-1700’s.  It is believed cocklebur entered North America tangled in the coats of the first shipments of sheep and goats brought to the New World.  Giant foxtail is a more recent invader, and was first introduced to the U.S. from China in the 1920’s.  While these exotic invaders continue to plague Iowa farmers, in the past two decades we have seen two native Iowa plants rise in importance as weeds of agronomic crops.  Waterhemp is now the leading weed problem for a majority of Iowa farmers and is found throughout the state.  The second, giant ragweed, is present throughout Iowa but is more of a problem in eastern Iowa.  This article will provide information regarding the biology and management of giant ragweed.

The scientific name for giant ragweed is Ambrosia trifida.  Ambrosia is Greek for ‘food of the gods’, but it is unclear why the genus was given this name.  While the name suggests a species held in high regard, giant ragweed is a probably looked at with disdain as much as any plant found in Iowa.  The reason for this contempt is that the ragweeds (giant and common) are the most important cause of hay fever in the eastern U.S.  A single ragweed plant can produce up to a billion pollen grains, and it is estimated that more than 10 million pounds of ragweed pollen are produced annually in the U.S.  Some have suggested that if giant ragweed is a food of the gods, it would be for the gods of horses rather than humans.  The plant is favorite forage of horses, and this affection is the reason why giant ragweed is called horseweed throughout many parts of Iowa and the Midwest.

Giant ragweed is found throughout North America, but is most common in the eastern U.S.  Prior to European settlement the plant was limited to areas of natural disturbance such as flood plains.  Conversion of forests and prairies to farmland created abundant habitat for the species.  The plant is considered the most difficult weed to manage in Ohio corn and soybean fields.  The extent of problems caused by giant ragweed diminishes moving west through the Corn belt; however, in recent years the weed has become more of a problem in Illinois and Iowa.

Biology of giant ragweed  The weedy potential of giant ragweed, and any other annual weed, all begins with the seed.  Giant ragweed has one of the largest seeds of important agronomic weeds.  Giant ragweed seeds range from 27 to 45 mg in weight, whereas waterhemp and velvetleaf seed typically weigh 0.5 and 8 mg, respectively.  The large seed provide emerging seedlings with relatively high energy reserves, making seedlings more tolerant to stress factors and allowing them to emerge from greater depths than smaller seeds.  The seed contain large photosynthetic cotyledons that provide seedlings with a quick start following emergence.  One disadvantage to large seeds is that an individual plant is able to produce fewer seeds.  Seed production estimates for giant ragweed range from < 500 to 5000 seeds per plant.  Waterhemp, for comparison, can produce over a million seeds per plant.  The enhanced vigor provided by the increased energy reserves of the large seed allows giant ragweed to overcome the low seed output.

Giant ragweed typically begins to emerge in late March or early April, and is the earliest emerging summer annual weed found in Iowa corn and soybean fields (Table 1).  At four locations in Iowa, giant ragweed initiated emergence approximately one week earlier than velvetleaf and three weeks earlier than giant foxtail and waterhemp.  Giant ragweed emergence occurs over a narrower time frame than most weed species.  The emergence pattern of giant ragweed can be a disadvantage to the species in ag fields since the majority of plants may emerge prior to planting.  This allows the bulk of the population to be killed by seedbed preparation or burndown herbicides.  Earlier planting dates for both corn and soybeans have contributed to the increasing problems associated with giant ragweed.

 Table 1.  Emergence characteristics of several annual weeds at four Iowa locations. 


Initial Date of Emergence
(Days after Giant Ragweed)

Mean Time of Emergence

Giant ragweed






Giant foxtail






Hartzler and Buhler.  1998.  ISU and USDA-ARS.

Rapid emergence and growth allows giant ragweed to dominate annual plant communities early in the season since it is the first species to capture resources.   Illinois researchers found that the presence of giant ragweed suppressed the establishment of other weeds by more than 90% (Table 2).  Soybean stands in areas of a field with giant ragweed were reduced by more than 60% due to early-season competition in a no-till field where a burndown herbicide was not applied.  The rapid growth and early-season competitiveness of giant ragweed requires that farmers go into the season with a well thought out management plan to prevent giant ragweed from getting an upper hand for the growing season.

Table 2.  Influence of giant ragweed (GIRW) on the establishment of annual weeds.


GIRW Present


GIRW Removed


Giant ragweed






Giant foxtail



PA smartweed



Common ragweed






Abul-Fatih and Bazzaz.  1979.  New Phytol.




Giant ragweed can cause significant yield losses due to its rapid growth and ability to shade crops.  In Illinois, giant ragweed is more than three times as competitive as waterhemp and ten times as competitive as giant foxtail.  In Ohio, a single giant ragweed plant every 3 ft of row caused approximately a 65% yield loss in corn and an 80% yield loss in soybean (Figure 1).  When delayed emergence of giant ragweed provided corn with a four week head start, yield losses due to competition were greatly reduced.

The large stems of giant ragweed are preferred hosts of numerous insects, including the European corn borer, stalk borer, and cocklebur weevil.  Presence of the insects reduces the competitiveness of giant ragweed by disrupting the plant’s vascular system.  Illinois researchers found that over 60% of six inch giant ragweed plants in a corn field were infested with stem boring insects.  The reduction in competitiveness due to vascular damage could be viewed as beneficial and a form of biological control.  However, the damage to the vascular system caused by insects has been found to reduce the effectiveness of glyphosate and other systemic herbicides.

 Adaptation in giant ragweed  Successful weeds adapt to production practices, ensuring their survival in spite of repeated control efforts.  Much of the focus on weedy adaptation has been on the selection of herbicide resistant biotypes due to the importance of herbicides in current weed management systems.  Giant ragweed biotypes resistant to ALS inhibiting herbicides were first reported in Illinois, Indiana, and Ohio in 1998.  The resistant biotypes were found in fields treated with cloransulam (FirstRate/Amplify), a product especially strong on giant ragweed and other members of the Composite family.  ALS resistant giant ragweed was first reported in Iowa in 2000.  The resistance mechanism provides resistance to the three classes of ALS herbicides:  sulfonylureas, imidazolinones and sulfonanilides.

Research has documented that emergence behavior of giant ragweed have  evolved, making the weed better adapted to survival in agronomic fields.   Giant ragweed biotypes were collected from different habitats in Iowa, Illinois and Ohio.  Relatively small differences were observed in the initial emergence date of twelve biotypes.  Giant ragweed biotypes collected in floodplains and other non-cultivated areas in the three states all exhibited a large initial emergence flush that accounted for the majority of that year’s emergence.  Three biotypes collected in Iowa agricultural fields had similar emergence patterns to the biotypes collected in non-cultivated areas, whereas ragweed collected in Ohio ag fields possessed a prolonged emergence pattern similar to waterhemp.  Giant ragweed biotypes collected in Illinois fields were split between the short and long emergence patterns.  The emergence patterns of three of the biotypes are illustrated in Figure 2.

            Giant ragweed is considered the number one weed problem for Ohio corn and soybean growers, whereas in Iowa it remains a somewhat localized problem.  The short emergence pattern observed in biotypes found in floodplains and non-cultivated fields is advantageous in these habitats since it allows ragweed to smother out other annual plants (Table 2).  However, this trait is a disadvantage for survival in ag fields since the majority of the population emerges prior to planting and may be killed by tillage or burndown herbicides.  The emergence pattern observed in Ohio biotypes provides a mechanism for giant ragweed to escape herbicides and probably is largely responsible for the greater problem this weed causes in the eastern Corn belt.  It is likely that this characteristic is beginning to appear in Iowa ragweed populations because of the large advantage it provides for survival in agronomic fields.


Management of giant ragweed  Like waterhemp, giant ragweed is a species that requires an integrated management program to achieve consistent control.  Giant ragweed frequently dominates fence lines, creeks and other areas that border ag fields. These areas serve as a continuous source of seed to move into agronomic fields.  Controlling giant ragweed in these areas and establishing a permanent vegetative cover should be a priority in battling giant ragweed.  Postemergence applications of 2,4-D will provide effective control of giant ragweed in these areas.  Minimize overspraying of glyphosate and other herbicides in these areas to allow the perennial sod to compete more effectively with giant ragweed.

 If possible, delay planting fields with heavy infestations of giant ragweed until other fields are planted.  This may allow a significant portion of the ragweed population to be killed with tillage or the burndown treatment.  Due to the rapid growth of giant ragweed, it is important to plant into a weed-free seedbed.  The large seed of giant ragweed provides tolerance to most preemergence herbicides, thus it is unlikely that  a preemergence herbicide by itself will provide consistent control in fields with heavy giant ragweed pressure.  However, a preemergence program will provide a good foundation for giant ragweed management.  Premix products that provide at least a pound of atrazine, Callisto/Lumax, and Hornet all have activity on giant ragweed, but performance will vary widely with soils, weed pressure and environment.

Several postemergence herbicides provide good to excellent control of giant ragweed.  Timely application is critical for consistent control.   The early emergence and large energy reserves in the seed allow giant ragweed to reach the appropriate size for treatment relatively early in the season.  The temptation to delay application until later emerging weeds (foxtail, velvetleaf, waterhemp) are ‘fully emerged’ probably is the number one reason for giant ragweed control failures.  Postemergence products that can provide acceptable giant ragweed control include dicamba, Beacon, Permit, Spirit, NorthStar, glyphosate and Liberty ATZ.  Products in italics rely on ALS herbicides and will not control ALS-resistant biotypes.

Soybean  The same principles described for corn are as important, if not more important, to achieve acceptable control in soybean (plant into a weed-free seed bed, delay planting if possible, timely postemergence herbicide applications).  Limited preemergence options are available to help manage giant ragweed.  Canopy and FirstRate/Amplify will provide partial control of ALS sensitive populations when applied preemergence.  Glyphosate in RR soybean is the standard postemergence treatment for managing giant ragweed.  The majority of control failures are due to waiting until the giant ragweed is too large for consistent control.  Fields with heavy giant ragweed pressure generally require two postemergence applications for consistent control.  The diphenyl ethers (Cobra, Flexstar, UltraBlazer) and Raptor have activity on giant ragweed.  Early application while weeds are small (four inches or smaller) is essential for consistent control with these product.  FirstRate/Amplify will provide control of larger giant ragweed if the populations are sensitive to ALS herbicides.  The presence of giant ragweed in a field will force changes in timing of postemergence herbicides due to its rapid growth rate.

Summary  Unlike many of the weeds we face in corn and soybean fields, giant ragweed is native to North America.  Fossil records document that it was a dominant plant following retreat of the glaciers in Iowa and neighboring states.  Establishment of the tall grass prairie across the Midwest landscape relegated giant ragweed to floodplains and other areas where the perennial vegetation was destroyed by natural forces.  Conversion of the prairies to cropland created millions of acres of suitable habitat for giant ragweed and other annuals with weedy traits.  Understanding the biology of giant ragweed will aid in the development of integrated weed management programs that can provide consistent, economic control of this weed.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
Submit questions or comments here.  

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