Weeds have been a challenge this year because farmers were unable to apply herbicides optimally during the wet June and July. So, people who have hay fever may have more problems this fall.
Ragweed causes the most problems. Wet fields allowed weeds to get considerably larger before farmers were finally able to spray. Unfortunately, most herbicides do not work well on large ragweed.
There are two species of ragweed in our in area: common ragweed, Ambrosia artemisiifolia L., and giant ragweed, Ambrosia trifida L., which can be distinguished by plant height and leaf shape when mature.
Common ragweed is a shorter and bushier plant than giant. It generally reaches three to four feet high. Its leaves are dissected, featherlike, but not compounded.
Giant ragweed, as its name suggests, can reach 12 feet. It leaves are generally three-lobed, like a trident, and sometimes five-lobed. The leaves can be eight inches across and 12 inches long.
Giant ragweed is more prevalent than common ragweed. It also accounts for most of the hay fever pollen because of its prevalence and height. Wind can carry it for miles.
Both species are summer annuals native to North America. They can emerge as early as late April and continue to emerge until temperatures get too warm for new seed germination.
Ragweed produces separate male and female flowers on the same plant in late summer. The flowers are small and greenish in color.
The male flower stalks develop at the ends of the upper branches so pollen can easily be distributed by the wind. It has been estimated that one ragweed plant may produce a billion pollen grains.
The female flowers form in the axils of the upper leaves. A plant growing all season may produce 25,000 seeds.
Seeds may be distributed across the field by water erosion, birds, rodents, and human activity. They may remain viable in the soil for 25 years.
Seeds will require a cold period that occurs during winter before germinating the following spring. Germination will be triggered by optimum soil moisture and temperature. Seeds need to be near the surface.
Even without the wet June and July, ragweed has always been a difficult weed to control. Giant ragweed has consistently been the second problem weed identified in the annual fall soybean weed survey, only surpassed by marestail.
Ragweed tends to be more of a problem in soybeans than corn. Soybeans and ragweed are both broadleaf plants, so it is harder to separate with herbicides. Corn is a grass, so broadleaf herbicides do not affect it.
Common ragweed is easier to control than giant. Giant ragweed emerges at different times and requires a farmer to use products applied after the soybean plant has emerged.
The first post applied herbicides often were unable to completely control giant ragweed and application time was critical. The advent of Roundup Ready soybeans in 1996 finally gave farmers an effective tool to manage giant ragweed.
However, farmers overused Roundup, or glyphosate, and resistant weed populations began to appear. Today, farmers use herbicides with different modes of action to battle ragweed and to prevent weed resistance.
Soybean fields are not the only source of ragweed. It grows quite well along roadsides, ditches, and unmowed land.
If you are a hay fever sufferer, you can blame the weather for this year’s high pollen count. Relief will come when the first frost ends the life cycle of weeds such as ragweed.
Additional information may be found at: http://agcrops.osu.edu/specialists/weeds/giant-ragweed.
Lentz is extension educator for agriculture and natural resources for The Ohio State University Extension Service in Hancock County. He can be reached at 419-422-3851 or via email at lentz.38@osu.edu.
Lentz can be heard with Vaun Wickerham on weekdays at 6:35 a.m. on WFIN, at 5:43 a.m. on WKXA-FM, and at 5:28 a.m. at 106.3 The Fox.

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