Keep in mind, though, that the newer pre-emerge products come with a higher price tag than diuron and simazine. That’s another reason to use both pre- and post emergent products, he notes. “It’s more economical in the long run to follow up your pre-emergence treatments with a timely burn down rather than relying only on the post emergent sprays to control your weeds,” he says. “You’ll make fewer trips across the field. That savings can be worth a lot.”

As he points out, once a pre-emergent herbicide has been applied to the soil during the dormant period, rainfall is required to activate it. “About one-eighth to a quarter-inch of rain within about a month after application should be enough,” he says. “Once activated, residual activity of most pre-emergent herbicides lasts about 120 to 180 days. So, if you put it on between January and March, it should remain effective through the summer.”

Post emergent materials, like paraquat, 2, 4-D, or saflufenacil, should be added to the spray tank if weeds are up when the pre-emergents are applied, Hembree adds. This treatment can be followed by application of a post emergent product as weeds emerge later. Timing of such a burn down application is critical, he notes.

“The biggest downfall in use of a post emergence herbicide is waiting too long to spray,” Hembree adds. “You have to treat when the weeds are really small.  Waiting until the weeds have grown 1 to 2 feet tall is asking a lot for a post emergent product.”