It seems that lately I've gotten a lot of calls about the same weeds — all of which are hard-to-control and none of which are easily eliminated. Most of these are not widespread problems throughout the state, but in the areas where they occur, I would definitely watch out for them and try to keep them from developing into more of a problem. Unfortunately, there hasn't been a lot of research conducted on these weeds and there isn't much information available in the weed science literature, but I have attempted to provide a summary of what we do know about these weeds in this article.

Field Horsetail (Equisetum arvense)

Figure 1. The reproductive stems of field horsetail. Notice "cones" on top and leafless stalks

Field horsetail is a perennial with a spreading rhizome system from which new plants arise. Field horsetail is known by a variety of different common names, usually dependent on where you are in the state. Some call it scouring rush, some call it ribbon grass, and it is even called "poor-man's-tinker toys" in some areas. As with johnsongrass or any other perennial plant that has rhizomes, the underground root structures are what make these plants difficult to control. Field horsetail is also unique in that two types of stems, and two different types of growth habits, occur within the same plant. The first type of stems to appear in the early spring are the unbranched, reproductive stems (Figure 1). These stems have "cones" at their ends, are often whitish to pink in color, round in cross-section, without leaves, and often grow to one or two feet in height. The second type of stems or growth habit that appears later in the season are the vegetative stems. These are highly branched and resemble a horse's tail, thus the name (Figure 2).

Figure 2. The vegetative stems of field horsetail. Notice high degree of branching and similarity to a horse’s tail.

In Missouri, field horsetail is probably most common in wet areas such as ditchbanks around corn and soybean fields. It has a tendency to encroach into the low-lying or wet areas of these fields from the ditchbanks and it will thrive in these areas once it is established. Since it has rhizomes, any kind of shallow tillage such as chisel plowing or disking only serves to spread the rhizomes and magnify the problem.

Few herbicides provide effective control of field horsetail in either corn or soybean. Many studies have shown that standard and even high application rates of glyphosate are virtually ineffective on this species. Some research conducted by weed scientists in Canada has shown that in corn, sulfonylurea herbicides like Beacon and Permit in combination with dicamba (Banvel, Clarity, Distinct) should provide about 80% field horsetail control during the season of treatment as long as the applications are made to the vegetative stems. Given the nature of its extensive root system, I'm not sure how much long-term field horsetail control is actually provided by these treatments, but at least the competitive effects of this weed are being reduced during the season of treatment. This research has also revealed that Python and Hornet are some of the only preemergence herbicides that will provide some degree of residual control of field horsetail, so inclusion of this product in with a burndown herbicide is another option to consider.

In soybeans, glyphosate in combination with Python as a preemergence burndown treatment is one of the only options available for field horsetail. As mentioned previously, Python is one of the only products that offers any residual activity of field horsetail. And, unfortunately, no effective postemergence treatments have been identified in soybean. Follow-up applications of glyphosate will only provide minor suppression at best.

Figure 3. Asiatic dayflower

In noncrop areas (which on some labels includes non-irrigation ditchbanks), Telar, Oust, and Surmount are a few of the herbicides that are labeled for application on field horsetail. Each of these herbicides is very persistent and can cause severe injury to crops so extreme care should be taken when applying these herbicides near or in areas that will be planted to corn and soybean.

Asiatic Dayflower (Commelina communis)

Asiatic dayflower is an annual weed in the monocot family that is becoming more of a problem particularly in no-till soybean fields throughout the Midwest. Asiatic dayflower has blue flowers and roots at the ground where it comes in contact with the soil. It forms dense colonies that can cause severe yield losses in both corn and soybean (Figures 3-5).

Figure 4. Asiatic dayflower

Few herbicides provide acceptable control of Asiatic dayflower in soybeans. Recently, weed scientists at Iowa State have conducted a number of trials to identify treatments for the management of this species. This research has revealed that in soybeans, Spartan, Firstrate, and Sencor are some of the only active ingredients that will provide acceptable Asiatic dayflower control when applied preemergence. Similarly, Firstrate is one of the only conventional herbicides that will provide acceptable control of this species when applied as a postemergence treatment in soybeans, but applications must be made before this species reaches six inches in height. Many other preemergence and postemergence herbicide treatments that have

Figure 5. Asiatic dayflower forms attractive blue flowers when mature

been evaluated, including Command, Dual, Define, Valor, Resource, Cobra, and Aim, provide little to no control of Asiatic dayflower. Glyphosate at standard rates in Roundup Ready soybeans will usually only provide some degree of suppression, and the calls I've received lately regarding this weed suggests to me that our common glyphosate burndown programs are providing little to no control of this species. Other research has shown that increasing the rates and making at least two applications of glyphosate can provide higher levels of Asiatic dayflower suppression and also that three applications of glyphosate at 0.75 lb a.e. per acre can provide about 80% control. Based on all of the available research, it seems clear that preemergence programs that include Sencor or Firstrate are the most effective and economic way to manage this species in soybean.

Field Pansy (Viola rafinesquii)

Figure 6. Field pansy seedling

Field pansy is a winter annual that can germinate in either the fall or spring, and is sometimes also called "Johnny-jump-up". Field pansy has attractive pale yellow to purple flowers and will only reach 5 or 6 inches in height when fully mature (Firgures 6 and 7). Field pansy is another weed that is not controlled easily by glyphosate in the spring, or even by standard glyphosate plus 2,4-D burndown treatments. Recently, weed scientists at Kansas State have conducted research on this species, where it is also becoming more and more troublesome. This, and other research has revealed that fall applications, especially fall applications of herbicides with residual activity, will provide good control of this species. Research has also shown that even a single glyphosate application in the fall will provide good control of this species, while this same treatment in the spring is virtually ineffective on field pansy. If fall applications are not made and this weed is present in the spring, the addition of dicamba (Banvel, Clarity, Distinct) to a glyphosate burndown will

Figure 7. Mature field pansy in the early spring

provide good control of field pansy. Some researchers have found that even as little as 2 fluid ounces of Clarity per acre plus glyphosate plus 2,4-D will provide essentially complete control of field pansy. Another option is to increase the rate of dicamba and drop the 2,4-D from the mix altogether.

If you have significant infestations of these weeds or have found any other successful strategies for their control, I'd be interesting in hearing about them: bradleyke@missouri.edu.

Kevin Bradley
bradleyke@missouri.edu