Images 1. Common ragweed. DiTomaso, J. UC-Davis, Bugwood.org.
We are hearing a lot about Palmer amaranth, but don't forget about some other resistant weed species that have historically been a problem in the region. Common and giant ragweed are closely related members of the Aster family sharing the same genus (Ambrosia). Ambrosia is Greek and implies immortality and food for the Gods. It is more commonly used today to refer to herbs with a fragrant, but not necessarily attractive smell. Both species have a history of herbicide resistance; common ragweed claims resistance to four different herbicide classes (2, 5, 9, and 14) and giant to two different groups (2 and 9). Resistance to the group 2 (ALS-inhibitors) and group 9 (glyphosate) herbicides are most wide spread and present the biggest challenge for management. Luckily, herbicide resistant biotypes are not yet wide spread in the Commonwealth.
Common ragweed (Ambrosia artemisiifolia) is very familiar throughout parts of the Northeast and has historically been a particular problem in soybean. Common ragweed is an erect, branching, summer annual that can be 3-4 feet tall. Seedlings have cotyledons that are thick, dark green, and spoon-shaped. Youngest leaves are opposite, becoming alternate by forth node, and are rounded or slightly pointed lobes. Mature plant leaves are hairy to nearly smooth. Most leaves are alternate; sometimes the lower leaves are opposite. Flowers produce large amounts of wind-dispersed pollen. Seed heads are small and inconspicuous in clusters on terminal branches. Pollen from common ragweed is a primary cause of hay fever.
Giant ragweed (Ambrosia trifida) is more scattered with hot spots on some farms or in in some fields, but it is not widespread in our region. Interestingly, giant ragweed is in the top five weed problems in the state of Ohio and is problematic in some other areas of the Midwest Giant ragweed is an erect summer annual that can be 10-15 feet tall. Seedling cotyledons are spoon-shaped and thick. Young leaves are opposite and have rough hairs. First pair of leaves are unlobed and subsequent leaves usually have three lobes. Mature plants have stems and leaves that are rough and hairy. Leaves are opposite and palmately compound. Lower leaves are more deeply lobed; upper leaves are often simple. Individual flowers are small and inconspicuous.
In corn, the POST PGR herbicides (Group 4) are quite effective on both species as well as some Group 2 herbicides, atrazine, and glyphosate depending on herbicide resistance. In soybean, the Group 2 herbicide FirstRate (cloransulam) can be fairly effective on smaller plants followed by Classic (chloriumuron). Some POST Group 14 herbicides (PPO inhibitors) have good activity, but must be applied while plants are still small. Giant ragweed is generally the more difficult of the two to manage with most herbicides being slightly less effective compared to common ragweed. Visit the Penn State Agronomy Guide for specific control suggestions.