Blog Post

Managing Tough-to-control Giant Ragweed

This agronomic image shows giant ragweed.

Giant ragweed

Giant ragweed is a large-seeded broadleaf weed that lives up to its name. It grows fast and can reach colossal heights of up to 20‘, pilfering water, nutrients and sunlight from surrounding corn and soybean plants as it grows. Its height and voracious appetite for resources aside, giant ragweed – like other large-seeded broadleaf weeds – is difficult to control because its seeds germinate deep within the soil profile where they are shielded from herbicide applications.

While herbicide active ingredients are easily applied to weed seeds on or near the soil surface, they may not always reach deeper seeds or reach them at reduced rates. To complicate matters, the deeper germination depth means giant ragweed emerges with more plant mass, making it harder to control with post-emergence herbicide applications.

Giant ragweed facts:

  • Can produce more than 5,000 seeds per plant
  • Is often 1 to 5 feet taller than the crop with which it is competing
  • Season-long competition from just 2 giant ragweed plants per 110 square feet can reduce corn yield by up to 13%

Learn how to identify and manage giant ragweed in this video with Joe Wuerffel, research and development scientist at Syngenta.

Syngenta recommends controlling giant ragweed with Acuron® or Acuron Flexi corn herbicides. Acuron has 4 active ingredients and 3 effective modes of action (Groups 5, 15 and 27), and Acuron Flexi has 3 active ingredients and 2 effective modes of action (Groups 15 and 27). Both herbicides contain the active ingredient bicyclopyrone (Group 27), which was developed to complement Callisto® herbicide (Group 27) and provide improved control of large-seeded broadleaf weeds like giant ragweed.

For soybeans, a pre-emergence herbicide applied prior to the weed reaching 4’’ in height is recommended. A post-emergence herbicide, such as Flexstar® GT 3.5 that delivers 2 different modes of action (Groups 9 and 14), can help control glyphosate- and ALS-resistant giant ragweed.

Backed by decades of research and development, Syngenta has been at the forefront of introducing herbicides with multiple effective modes of action to help fight resistance. The Syngenta Resistance Fighter® program provides education, local recommendations, and a comprehensive herbicide portfolio to help growers and retailers effectively manage resistant weeds in their area.

Visit to learn more about weed resistance management in corn, soybeans and wheat.

Sign up for the Know More, Grow More Digest to receive twice-monthly agronomic email updates pertinent to your area.

All photos are either the property of Syngenta or are used with permission.

Syngenta hereby disclaims liability for third-party websites.

Leave a comment

All fields marked (*) are required