Common ragweed, also known as Roman wormwood or hogweed, is a summer annual weed native to North America that has become very common throughout the United States over the past 200 years. Common ragweed is perhaps best known for its abundant pollen production which is the number one cause of hay fever in North America. Common ragweed can be found in disturbed sites, cultivated fields, pastures, and roadsides.
Common ragweed seedlings have cotyledons that are round to oblong in outline, often purple beneath, and the first true leaves are arranged oppositely along the stem (Figure 1). Subsequent leaves become alternately arranged with age. Mature plants have stems that are green with purple spots and leaves that are uniquely divided (Figure 2). Leaves are covered with hairs along the upper leaf surface and margins. Common ragweed has small (2 to 4 mm wide), inconspicuous flower heads that occur in the upper portions of the plant. Common ragweed is monoecious, meaning that male and female flowers are produced in separate locations on the same plant. The male flowers usually occur at the top of the plant and are drooping, in order to deposit pollen to the female flowers, which occur below in the upper leaf axils.
Common ragweed seed germinate at soil temperatures of 55 to 60 degrees Fahrenheit, so this is one of the first species to emerge in agricultural fields and grass pastures in Missouri. Research conducted by Myers et al. (2004, 2005) has revealed that common ragweed is one of the first weed species to emerge in the spring and also that this species has a relatively short duration of emergence. In these studies, common ragweed emergence was 50 percent complete after 140 soil growing degree days were reached, and 95 percent complete by 420 soil growing degree days (about five weeks). Research has also shown that tillage will shift the emergence pattern of common ragweed by approximately two weeks when compared to no-till fields. Spring tillage also tends to reduce the total number of emerged common ragweed seedlings when compared to no-till fields.
Common ragweed commonly invades overgrazed pastures, reducing productivity. We have recently initiated a series of experiments to investigate the impacts of common ragweed on grass pastures and hay fields. Although this research is still ongoing, one of the things we have been able to document thus far at one research location is that the elimination of a common ragweed infestation from a grass pasture can increase total forage yields by 480 lbs/acre in a single growing season. Surprisingly however, even very high densities of common ragweed in the spring do not appear to deleteriously impact forage quality. We have found that 8- to 10-inch tall common ragweed that might typically occur at the time of the first spring hay harvest has a crude protein content of about 16% and is lower in acid detergent fiber (ADF) and neutral detergent fiber (NDF) than tall fescue harvested at the same time. Other research we have conducted also shows that common ragweed is highly digestible for cattle that may be forced to graze this weed during this time period.
For more information on common ragweed and other species, visit the MU Weed ID Guide Web site at http://weedid.missouri.edu.
REVISED: October 2, 2015