Issue No. 3, Article 5/April 22, 2011
This article was first published as an "Update" to issue 2 on April 14, 2011.
It seems as though each new growing season differs at least a bit from the last. This year it appears that several winter annual weed species have begun to flower a few days or weeks earlier than last spring. A timely harvest in 2010 may have helped promote earlier fall emergence of several species, which could have improved their overwintering survival. Add in a few days of warm temperatures in March and early April, and splashes of color are becoming evident across the otherwise drab Illinois landscape. Purple is currently most common, but white and yellow soon will follow.
Flowering henbit plants are common in fields where no fall tillage was done.
The two winter annual weed species producing the "purple patches" (as described by one inquisitive caller) are henbit (Lamium amplexicaule) and purple deadnettle (L. purpureum). Although close relatives taxonomically, henbit and purple deadnettle are distinctively different species. Both are winter annuals (although spring emergence has been known to occur), and both species have square stems characteristic of the mint (Lamiaceae) plant family. Henbit is more commonly found throughout Illinois, while purple deadnettle appears more often in the southern half of the state. The lower leaves of henbit are petiolate (attached to the stem with petioles), while the upper leaves grasp the stem (i.e., lack petioles). The upper leaves of purple deadnettle, however, are attached to the stem with petioles, are more triangular than those of henbit, are less deeply lobed, and tend to be reflexed, or pointed downward. As the name indicates, purple deadnettle has distinctive reddish to purple coloration of the foliage and stem. Table 1 summarizes similarities and differences between the two species.
Table 1. Similarities and differences between henbit (Lamium amplexicaule) and purple deadnettle (L. purpureum).
Winter annual life cycle
Henbit usually flowers earlier than purple deadnettle
The upper leaves of henbit lack petioles.
Purple deadnettle has a distinctive reddish to purple coloration.
The upper leaves of purple deadnettle are triangular and are attached to the stem with short petioles.
Flowering indicates that henbit is close to completing its life cycle and will likely be more difficult to control with burndown herbicides, but this does not mean that no attempt should be made to control existing plants before corn or soybean planting. Henbit and purple deadnettle are known to be hosts for a number of insect and disease pests, and mature seeds can survive in the soil seedbank for several years. Planting into dense patches of these species can be challenging and could result in poor seed placement. Preplant tillage or herbicides can provide good to excellent control of existing henbit and purple deadnettle.
In general, 2,4-D and dicamba are weak on henbit. Glyphosate can provide good control, but application rates should be at close to 1.1 lb ae/acre for these mature plants. Combining glyphosate and 2,4-D or adding these two herbicides to other residual herbicides is a popular broad-spectrum burndown. Atrazine (1.5-2 lb/acre) or atrazine-containing premixes have good activity on henbit, and adding crop oil concentrate often improves burndown activity. Control with paraquat is often improved when combined with atrazine or metribuzin. Saflufenacil alone can be weak on henbit, but control can be improved when it is combined with atrazine and/or glyphosate ± 2,4-D.
Cool temperatures can slow the activity of many burndown herbicides, and translocated herbicides sometimes act more slowly than contact herbicides under these conditions. Contact herbicides may not be as slow to act as translocated herbicides under cool conditions. When the forecast calls for several days or nights of cool air temperatures, don't be surprised if symptoms of activity on existing vegetation take several days to develop.--Aaron Hager