Usually if rain occurs too soon, weeds that the herbicide is poorest against are those most likely missed. Partial control and regrowth can occur for normally susceptible weeds. Surfactants, such as crop oils or nitrogen-based fertilizers, are critical for postemergent herbicide absorption into plants. They increase the rate of herbicide absorption into the plant and the faster the rate of absorption the sooner the herbicide is rainfast. Surfactants also reduce the amount of herbicide needed for activity, which can impact on the amount of time needed for rainfastness.
Herbicides differ in the amount of time they require before becoming rainfast.
Some herbicide labels provide a clear guide on the amount of time needed, others are less helpful. In the remainder of the article I will discuss rainfast periods for some common postemergent herbicides. The Roundup Ultra label is less clear on the amount of time needed before Roundup is rainfast. Older formulations of Roundup required 24 hours before the herbicide was rainfast. The adjuvants used in the newer Roundup Ultra formulation have improved the herbicide's rainfastness.