According to the University of Florida IFAS Extension, there are 2 main reasons why herbicides fail in drought situations:

1. Less herbicide absorption through the leaf surface
2. Slow/stopped translocation of the herbicide within the plant

This is normal, as a plant’s defense mechanism during drought is to close the stomata to reduce water evaporation and conserve resources. The weed may also protect itself even further by increasing the waxy layer covering the leaf surface to decrease water loss.

Dr. Ramon Leon, University of Florida IFAS Weed Science Specialist, said ironically healthy weeds are needed for herbicides to be effective. When weeds are stressed by drought, herbicide uptake by leaves and roots, and herbicide movement within the weed are reduced, so it’s more difficult for the herbicide move within the plant to kill it.

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