Iowa State University

Giant Ragweed: Return of a Native
by Bob Hartzler

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. 

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.

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.