Weed Spotlight: Giant Ragweed (Ambrosia Trifida)

Identification

Giant ragweed is one of the earliest annual weeds to germinate and emerge in the spring. Seedlings are identified by large, spoon shaped cotyledons. Giant ragweed averages 5 feet in height, but can grow as large as 10 feet or more, making it one of the largest annuals to plague farmers. The weed has a relatively short taproot, hairy stem and large lobed leaves.

Habitat

Giant ragweed is native to North America. It likes moist soils and is commonly found in river valleys, along roadsides and in fencerows but also in cultivated fields. Glyphosate resistant giant ragweed populations have been confirmed in the U.S. and in Southwest Ontario. Giant ragweed resistance to ALS herbicides (Group 2) is widespread in the U.S. and has been found in Southwest Ontario.

Reproduction

Giant ragweed is an extremely competitive annual weed. It is one of the first weeds to germinate and emerge in the spring. Seedlings can continue to emerge until the middle of July increasing the chance of escaping early herbicide applications. Giant ragweed seeds can be very persistent in the soil and a single giant ragweed can produce more than 5,000 seeds.¹

Yield Loss

Timing of herbicide applications is very important. If giant ragweed gets too large prior to application, control will be reduced. To prevent yield loss in soybeans, a field must be kept free of giant ragweed for 8-10 weeks after emergence.¹ In soybeans, 0.1 plants/m2 resulted in yield loss by as much as 50%.2 In corn, 0.2 plants/m2 reduced yields by 13%.²

How to Outsmart Giant Ragweed

• ​Scout early and often.
• Start clean and control giant ragweed when it is small.
• Always use two effective modes of action on the target weeds to help reduce resistance selection pressure.
• Control weed escapes throughout the season and reduce the weed seed bank.