No. 6 — April 29, 2019
Keep an Eye on the Weeds
Excessive rainfall amounts can reduce the effectiveness of pre-emergent herbicides. Most all pre-emergent herbicides are activated by 0.25 to 0.5-inches of rainfall. Then subsequent rainfall amounts in the range of 1 to 2 inches help distribute the pre-emergent herbicide as a “protective barrier” in the first couple of inches of the soil. When rainfall amounts are excessive the pre-emergent herbicide becomes less effective. Depending of the chemistry of the pre-emergent herbicide, the herbicide can be leached deeper into the soil profile thereby mov-ing out of the zone of germinating weed seeds. Some pre-emergent herbicides bind tightly to the soil and are washed away during excessive rainfall via erosion.
Keep an eye on annual weed growth and be ready to apply a post-emergent herbicide if needed. Use caution if considering another pre-emergent herbicide application as many pre-emergent herbicides have extended periods between reapplication. For example, Chateau SW has 30-days between applications, Zeus Prime has 60 days between applications.
With the current saturated soil conditions weed growth and vineyard floor grass should be allowed to grow to help remove excess soil moisture. As the soil dries down, be prepared for a flush of annual weeds.