Giant Ragweed

Early season scouting in western Pennsylvania has resulted in identification of at least one isolated population of Giant ragweed within crop production fields this spring. Giant ragweed is naturalized within Pennsylvania, where it can be found within floodplain and roadside habitats. However, this species has also become adapted to cropland acres in the Eastern Corn Belt over the past few decades. ALS-resistant (Group 2), glyphosate-resistant (Group 9), and multiple-resistant (Group 2/9) biotypes have become more widespread in this region. In production regions within Pennsylvania that source seed, feed, and equipment from the Eastern Corn Belt, we suggest taking an aggressive approach to controlling within-field populations of Giant ragweed given the potential that an herbicide-resistant population was the source of a new introduction.

Giant ragweed is a large-seeded summer annual that can emerge early in the spring (early-April), prior to corn or soybean planting. An emerging giant ragweed plant will produce disc-shaped cotyledons. True leaves are opposite, slightly hairy, and have distinct serrations with 3-5 lobes (Image 1).

Image 1. Emerging giant ragweed plant (Photo credit: John Wallace, Penn State)

Giant ragweed grows rapidly and can significantly reduce crop yields at even low densities due to its competitive ability in both corn and soybean (Image 2). It is capable of emerging from several inches deep within soil due to its large seed size. By distributing seeds throughout the soil profile, tillage can promote more variable emergence patterns and reduce the efficacy of soil-applied herbicides.

Image 2. Mature giant ragweed plant in a soybean field (Photo credit: John Wallace, Penn State)

We suggest remaining on high-alert during early-season scouting for new introductions of giant ragweed populations. Take note of weed control escapes when using ALS-inhibitor products (i.e., chlorimuron, cloransulam) and glyphosate. Weed control recommendations for managing herbicide-resistant giant ragweed populations in soybean can be found at "Management of herbicide-resistant giant ragweed."

Eradication is a feasible management goal when new giant ragweed populations are identified. It is not a prolific seed producer, so new introductions may be limited to only a few established individuals within an isolated area. Walking fields and rogueing out small populations to prevent seed production will prevent a more persistent weed management problem in future years. As they say, an ounce of prevention is worth a pound of cure.