With planting hopefully right around the corner, many people are thinking about using Gramoxone (paraquat) as a last minute burndown for its effectiveness and lack of replant interval.  Gramoxone is a very good broad spectrum herbicide that is “money” on many winter annual broadleaf weeds and also quite good on poa.  On the main winter annual weed horseweed (marestail) though, control can range from 30 to 90%.  The question is why?  So we conducted research this spring to see if 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 Gramoxone, 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 a month, 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.  This is an important point to think about when preparing to make a last minute burndown application.

Weeds that have been injured and not killed from a previous herbicide application will only get more difficult and more expensive to remove, making effective burndown applications very critical.  While control from Liberty is best during the middle of the day, inversely, control from 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.

Getting the most out of your burndown herbicide application is especially important with weeds like horseweed that are difficult to control in the first place.  It is even more difficult this year as we are out of calendar with respect to burndown interval to planting.  Therefore the burndown chosen has to be effective as there will be no second chances. Please keep in mind that time of day can have a big effect on the performance of herbicides like Gramoxone and Liberty.