We tested Gramoxone and other common burndown herbicides applied at sunrise, noon or sunset for horseweed control.  Control from Gramoxone will be better when applied earlier in the morning or later in the evening. 

For Gramoxone, differences were hard to see initially, or at seven days after the application. All horseweed plants suffered the initial burn that is so characteristic of Gramoxone. After a month, application time of day effects were very apparent.  The application time of day did not have a significant impact on Sharpen, Clarity, or 2,4-D. 

At this interval, horseweed control from Gramoxone was 70 percent from sunrise applications, 20 percent from noon applications, and 90 percent from sunset applications.  This is an important point to think about when preparing to make a last-minute burndown application.

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 percent to 90 percent.

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.  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.