STATE COLLEGE, Pa. — Giant ragweed is a North American native that seems to be on the rise in Pennsylvania.
This is a plant that is commonly seen in some areas of the state generally along field edges and often on the margins of woodlots and other less managed areas.
Increase noticed
Bill Curran, Penn State weed specialist, said he has noticed a marked increase in the prevalence of this species around his house in Centre County over the last five years and although mostly contained along the edges of some woods, it does have his attention.
This weed is one of the most difficult to manage if it enters tilled fields and is a common problem in Ohio corn and soybean fields. Historically, giant ragweed has been less of a problem as you move west.
Research in the Midwest has documented differences in the emergence behavior for giant ragweed potentially making the weed better adapted to survival in tilled fields. In this research, Curran said, giant ragweed seed was collected from different habitats in Iowa, Illinois and Ohio and emergence behavior was documented.
Results
The results showed only small differences in the initial emergence date for the 12 populations sampled. Giant ragweed populations collected in floodplains and other non-cultivated areas in the three states all exhibited a large initial emergence flush that accounted for the majority of that year’s emergence.
Most of the seedlings emerged before May 1 in these populations. Three populations collected in Iowa agricultural fields had similar emergence patterns to those collected in non-cultivated areas, whereas ragweed collected in Ohio farm fields possessed a prolonged emergence pattern.
Like the Iowa populations, the Ohio collections started emerging in early April, but they continued to emerge into late June.
Emergence patterns
Giant ragweed seeds collected in Illinois fields were split between the short and long emergence patterns, Curran said. The emergence pattern observed in the Ohio provides a mechanism for giant ragweed to escape herbicides and probably is largely responsible for the greater problem this weed causes in Ohio and in some Pennsylvania fields.
To make matters worse, giant ragweed biotypes resistant to the ALS inhibiting herbicides and to glyphosate are also a problem in Ohio.
When thinking about managing this weed in corn and soybean fields, an integrated approach that relies on multiple tactics will be necessary, Curran said.
Several effective post-emergence herbicides are available in corn, while options in soybean are fewer and resistance to glyphosate would further reduce options in soybean.
(Here’s a .pdf from Purdue on giant ragweed biology and management.)
