No. 23 Article 7/October 8, 2004

Green in the Fields at Harvest

At about this time every year, we receive many questions from agricultural professionals seeking to identify the various weed species they are encountering in corn and soybean fields at this late time of the season. These questions are often posed by those involved in taking a final assessment of weed-control practices prior to harvest, but sometimes the calls originate during the harvest operation itself. Indeed, inquiries via cellular telephone from the cabs of combines are much more common than in previous years.

This year, the weed species we have been asked about most frequently, and which appears to be more prevalent at harvest this year compared with previous years, is hophornbeam copperleaf (Acalypha ostryifolia). We have discussed this species in previous editions of the Bulletin, as well as at county agronomy days and the Corn and Soybean Classic, but with the renewed interest that seems to be prevalent this fall, we thought it would be beneficial to review what we know about the biology of this species.

Hophornbean copperleaf is a summer annual species in the Euphorbiaceae family. This plant family, also known as the Spurge family, includes several other problematic weed species, many of which have a milky sap that exudes from the stem or leaf petiole when it is cut. Hophornbeam copperleaf, however, does not contain the characteristic milky sap of other Euphorbiaceae family members. The plant is indigenous to Illinois and most commonly found in the southern third of the state. Over the past several years, however, we have identified populations in corn and soybean fields progressively farther north in Illinois. In 2000 we identified a population in Tazewell County, and in 2001 we received a sample from a population in Lee County. Several other copperleaf species can be found in Illinois, and while most of them are not generally considered problematic in agronomic production systems, Virginia copperleaf (Acalypha virginica) can be a troublesome weed species in southern Illinois.

Hophornbeam copperleaf has pubescent cotyledons and true leaves with short hairs and finely toothed (serrated) margins. The leaves are simple and alternate and somewhat heart-shaped at the base. Additionally, a reddish coloration is often observed where the main leaf vein intersects the petiole. Hophornbeam copperleaf may sometimes be misidentified, especially during early vegetative development, as prickly sida (Sida spinosa). The leaf margins of prickly sida are more coarsely serrated than those of hophornbeam copperleaf, and hophornbeam copperleaf does not have the small stipules (spines) in the leaf axils like prickly sida. Late in the year, many people tend to describe hophornbeam copperleaf as looking "like a hybrid between prickly sida and pigweed."

Hophornbeam copperleaf is monoecious (both male and female flowers on the same plant), with staminate (male) flowers produced on axillary spikes and pistillate (female) flowers produced on a long terminal spike. Seed pods of hophornbeam copperleaf are dehiscent (pods split open at maturity to release seed), and seeds appear to require warm temperatures for germination. Once emergence has begun, additional flushes of hophornbeam copperleaf frequently appear following precipitation. A recently published experiment reported that the average seed production of hophornbeam copperleaf plants growing alone (without competition) was about 12,518 seeds per plant, much greater than the average (980 seeds per plant) when grown with soybean.

Based on previous research we've conducted on this species, we know that the emergence characteristics of hophornbeam copperleaf are more atypical than most other summer annual weed species. Because seeds require warm temperatures for germination, it's rare to observe emergence of this species prior to late May or early June. Usually there are several emergence events that might persist throughout June and into July. A well-developed crop canopy is beneficial in suppressing growth and development of hophornbeam copperleaf, but as crop senescence begins in late summer and more sunlight can penetrate through to the soil, another flush of the species can emerge.

Several have asked what went wrong with their herbicide program this year that allowed hophornbeam copperleaf to be present at harvest. While we can't make an encompassing assessment of all possible herbicide programs, we can say that few (if any) herbicides are able to provide sufficient soil-residual activity to control the latest emergence event of hophornbeam copperleaf.

If you are interested in viewing pictures of hophornbeam copperleaf at various growth stages or in learning additional information on managing this weed species in corn and soybean, please visit the University of Illinois Weed Science Web site and click on the "Extension" button to the left of the screen.--Aaron Hager and Dawn Nordby

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