Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
We all want our herbicides to work NOW and to be 100% effective. As the old saying goes, you can have it done fast, effective, or cheap/convenient – pick two because all three are probably not achievable. This time of year I get calls about how fast specific herbicides take to work and when is the best time to spray.
You will not find a definitive answer on when to spray burndown applications. My rule of thumb is: 1) the wheat should be growing, and weeds are putting on growth; 2) temps are close to 60ish and 2) there is not an expected severe change in weather pattern the following 24hrs. Often in early spring, farmers are spraying to “beat the weather” and then temps drop and products do not work as well.
Another thought is how often does the treatment “not work” and how often we are not patient enough. Herbicides that rely on translocation work slow this time of year, don’t expect full results for at least 14 days for these types of herbicides (i.e. glyphosate, 2,4-D, dicamba, Harmony Extra, Canopy EX).
Contact herbicides require full sunlight for optimum control (i.e. paraquat, Sharpen, Liberty) and there is little we can do about this. Tankmixing paraquat with a triazine herbicide (atrazine, simazine, or metribuzin) can enhance its performance in the spring.
If weather is questionable, it is often better to wait for improved conditions than trying to get the field treated. With all that said, and working around windy conditions (or lack of wind in the case of temperature inversions) and rain, that gives us limited time in March and April to spray. So it is tough to expect a “one and done” approach for burndown treatments. Be sure to allow adequate time for the products to work as well as time to retreat or plan on treating in sequence with an application early for the most problem weeds and follow-up at planting with a second application.