A few calls and plant samples have come in during the past week on control of a broadleaf plant with roundish leaves and purple flowers. My first reaction is that this person has ground ivy, but I have seen a lot of henbit this spring which also looks like ground ivy.
Henbit is a winter annual while ground ivy or creeping charlie is a perennial. We need to tell them apart because ground ivy is a very invasive, hard to control weed, while henbit is considered a rather nonaggressive plant. These two plants are difficult to separate primarily because both produce round, toothed leaves, square stems and opposite leaf arrangement. Their flower shape (tubular) and flower color (lavender-blue) are also identical and both grow well in shady areas. Therefore, how do we tell them apart? There are a few differences. First of all, the upper leaves of henbit are attached to the stem, while ground ivy leaves have petioles. Secondly, henbit has hairy leaves, unlike the smooth leaves of ground ivy. Finally, ground ivy square stems usually root at each joint where they touch the ground; each rooted joint can become an independent plant if the main stem is severed. Henbit on the other hand, will have only a single tap-root. It does not produce roots at a joint that touches the ground.
Now that we can identify these two plants, the question of control often comes up. Henbit is usually pretty easy to control with a combination postemergence product containing dicamba. One can also control henbit by applying pendimethalin in the fall or very early spring. I generally have only a few of these plants growing in my lawn and flower beds and find that pulling them is the easiest control method. Ground ivy on the other hand, is a very hard to control weed no matter what kind of control strategy is used. Hand pulling is a never-ending event because the prostrate stem easily breaks at each rooted joint. One also has to be careful where you discard the plants that have been uprooted. The plants or plant segments can easily reroot in the new location that you have thrown them.
Preemergence herbicides will not control a perennial, which leaves us with postemergence control. The most effective postemergence application has contained a combination of at least two broadleaf herbicides. However, even this application will need to be repeated a couple of times at 10- 14 day intervals for complete control. Research at ISU has shown that a very late fall application was the most effective. However, if you missed that application, the next best time is when the plant is in its early flowering stage, which is about now. Of course, one needs to be extra careful with postemergence broadleaf herbicides in the spring due to the succulent growth of sensitive broadleaf plants in the landscape which are easily injured by spray drift or volatilization. Therefore, make sure the application is done during a rather calm period and when the air is moving away from the sensitive plants. Also avoid hot days when the air temperature is expected to be above 85oF the day of the application and a couple days following the application. This will help to eliminate any chances of volatilization.
If the ground ivy infestation is severe, consider a complete renovation of the area. In such instances Roundup, a non-selective herbicide should be used. For more information see Pm-1055, Turfgrass Renovation.
This article originally appeared in the April 29, 1992 issue, pp. , 1992 issue, pp. 68-69.