Giant Ragweed

Ambrosia trifida

Giant ragweed
Giant ragweed plant. Photo courtesy of Howard F. Schwartz, Colorado State University. Licensed under CC BY 3.0.
Giant ragweed is a major pest known for its rapid growth rate and large size (6-20 ft in height) which allow it to tower over crops, blocking sunlight.
Giant ragweed seedlings can initially be identified by their large cotyledons. The second pair of "true leaves" will have the characteristic 3 lobe appearance.
Small giant ragweed plant.
Small giant ragweed plant. Photo courtesy of the Agricultural Research Service.
Giant ragweed leaves typically have 3 lobes but can have up to 5. They grow in opposite pairs on the stem.
Giant ragweed leaf
Giant ragweed leaf. Photo courtesy of Bruce Ackley, The Ohio State University. Licensed under CC BY 3.0.
Giant ragweed plants produce large staminate spikes with small flowers near the base when mature. The seeds have hard spikes giving them a crown-like appearance.
Diagram of a giant ragweed plant
Diagram of the major parts of the giant ragweed plant.
Giant ragweed plants can quickly grow taller than most crops, blocking sunlight and hence reducing yield. Early control in short crops (e.g. soybeans) is particularly important.
Giant ragweed can be controlled by many herbicides, although resistance is common in some areas. Timing is important because this fast growing plant can quickly begin to compete with surrounding crops, reducing yield. Always read and follow all label instructions.