Application Time of Day Effects Gramoxone

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.

Gramoxone - Noon Application
Gramoxone – Noon Application
Gramoxone - Sunset Application
Gramoxone – Sunset Application

 

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4 thoughts on “Application Time of Day Effects Gramoxone

  1. Interesting, but not entirely new. 35 years ago when Dr. Larry Jeffries was teaching us about weeds at UTK, he mentioned that his experience using paraquat (The “Gramoxone” brand didn’t exist yet.) in his home garden was that it was more effective and appeared to be very slightly systemic if sprayed at dusk or after dark. The flip side, which is what he was really warning us about, was that one had to be more careful about a little overspray on sweet corn plants when spraying at dusk, because the damage was worse than from a mid-day application. As far as I know, Dr. Jeffries never did any research on this issue.

    Thanks for following up on your observations. This information could be very useful.

    1. Thanks for comment. Yes some of this time of day work has been done… but not on these glyphosate resistant weeds

  2. Would be interesting to see work repeated with and without a PSI herbicide with the Gramoxone. On sunny days, or perhaps mid day, Gramoxone works too quickly with cells are killed too quickly. A PSI, such as Aatrex, Caparol…. Slows down burn, but results in better, final kill of weeds.

    1. Thanks….I agree… Thanks thought of it after we put it out…

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