Issue No. 14, Article 7/June 30, 2006
A Growing Interest in (and Prevalence of) Copperleaf
Walking fields this past week, we reached the tentative conclusion that 2006 appears to be a "good year" for hophornbeam copperleaf (Acalypha ostryifolia), especially in soybean fields. We have described this weed species in years past but felt it might be beneficial to review what is known about its growth characteristics and the options available for controlling it in soybean fields.
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 cut. Hophornbeam copperleaf, however, does not contain the characteristic milky sap. It is indigenous to Illinois and, at least historically speaking, is most commonly found in the southern third of Illinois. Over the past several years, however, populations in cornfields and soybean fields have been identified progressively farther north in the state, including a population from Lee County. Several other copperleaf species can be found in Illinois, and while most are not generally considered problematic in agronomic crops, Virginia copperleaf (Acalypha virginica) can be troublesome 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 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. People tend to describe hophornbeam copperleaf late in the year as something that looks 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 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 approximately 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 hophornbeam copperleaf before late May or early June. Usually there are several emergence events each season 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 is able to penetrate through to the soil, another flush of the weed can emerge.
The list of postemergence soybean herbicides for control of hophornbeam copperleaf is relatively short and is identical to the list of postemergence herbicides that control waterhemp. Three diphenylether herbicides (acifluorfen, fomesafen, lactofen) and glyphosate offer good to excellent postemergence control; most other postemergence soybean herbicides provide much less control. Plant size is important with diphenylether herbicides, with about 2- to 3-inch copperleaf an "ideal" size for optimal control; expect control to decrease when these products are applied to larger plants.
Sometimes folks ask what might have gone wrong with their herbicide program to allow hophornbeam copperleaf to be present at soybean harvest. While we can't make an all-encompassing assessment of every possible herbicide program, we can say that few herbicides, if any, can provide sufficient soil residual activity to control the latest emergence event of the species.--Aaron Hager and Dawn Nordby