With the planting season started, many people are thinking about using Gramoxone (paraquat) as a burndown for many weeds present at planting.  Gramoxone is a very good broad-spectrum herbicide that has been the go to for burndown of many winter and summer annual broadleaf weeds. With Palmer amaranth starting to emerge, Gramoxone is the only sure thing that will control it. However, it is less consistent controlling some weeds like horseweed (marestail).

Horseweed control with paraquat can range from 30 to 90%.  Why?  In order to address this question, we conducted research sometime back to see if the time of day Gramoxone is applied has an effect on horseweed control.

In our research, we tested Gramoxone as well as other common burndown herbicides applied at sunrise, noon, or sunset for horseweed control.  The application time of day did not have a significant impact on Sharpen, Clarity, or 2,4-D.  For paraquat, differences were hard to see initially (7 days after the application), as all horseweed plants still suffered the initial burn that is so characteristic of Gramoxone. However, after 30 days, application time of day effects were very apparent.  At this interval, horseweed control from Gramoxone was 70% from sunrise applications, 20% from noon applications, and 90% from sunset applications.  It should be noted applications were made on sunny days. The application time of day effect is somewhat mitigated with overcast, cloudy conditions.  This research was conducted 7 and 8 years ago. Since that time, observations we have made with respect to consistency of control seen from paraquat would suggest that some grass weeds, though not to this extreme, react similarly to the application time of day effect.

In short, Gramoxone will be better when applied earlier in the morning or later in the evening.  Although making all applications at these times of the day may not be practical all the time, it is definitely important to keep this in mind when making a burndown application with Gramoxone or other paraquat-based products. One tactic to improve the consistency of control across the application time of day effect with Gramoxone is to tankmix a photosystem II inhibitor herbicide like atrazine, metribuzin or diuron with it.