Results of using Tenacity herbicide on St Augustine for violets and nimblewill

From Gary P.: Last year I spoke with you during your Saturday morning Lawn & Garden Show about using a systemic herbicide, brand name Tenacity, for a grassy weed, common name nimblewill.

The scary thing to me about Tenacity was the application rate of one half teaspoon per 1,000 square feet, so we talked about testing it on a portion of the lawn. Tenacity provides a plastic syringe with teaspoon gradients that greatly helped with the dosing.

The test results last year were consistent with what the product literature described would happen as you will see in the attached photos of the first application made this year on May 25th. On the eighth day after application the nimblewill blades begin to turn white (loss of photosynthesis) and a small amount of yellowing appeared on the St. Augustine grass. The photos were taken on the twelfth day after application. The Nimble Will “white bloom” has maxed out and the yellowing is clearing up with new St. Augustine growth.

I believe I am getting better results this year by making the applications earlier after green up. Last year the first application was made on June 15th. This year June 15th is the target date for the recommended second application (three weeks after the first).

Tenacity is labeled as a pre-emergent and post-emergent herbicide for many grassy and broadleaf weeds, including wild violets. The control for the violets in my yard has been less effective than for the nimblewill.