By Dr. Henry Wu, technical sales support representative, Chemtura AgroSolutions

As the California grape harvest winds down, vineyard owners are turning their attention to late-season vineyard maintenance and clean-up. Herbicide-resistant weeds on the vineyard floor are one of the major challenges growers face in maintaining a clean, healthy vineyard through the winter months.

Glyphosate-resistant horseweed (marestail), fleabane and ryegrass have taken root in many California vineyards, endangering the long-term efficacy of glyphosate as a weed control tool. Completely controlling these weeds through herbicide rotation is the only way to prevent them from producing seeds with the resistant trait that will grow next season.

Glyphosate-resistant weeds are easiest to control when they are small (<3” – 4”) and actively growing—typically from January through March. A tank-mix combination of a nonglyphosate burndown herbicide and a residual product can control the first flush of emerging weeds as well as succeeding flushes.

A good option for controlling both grass and broadleaf weeds is Firestorm® herbicide from Chemtura AgroSolutions. Designed as a tank-mix partner for residual herbicides, Firestorm knocks down glyphosate-resistant weeds to help prevent continuing glyphosate resistance.

Ensuring thorough spray coverage is just as important as selecting the right materials. Burndown products like Firestorm are contact materials, meaning they will only work if they make contact with the target weed.

To get good spray coverage, use a minimum spray rate of 20 gallons per acre and include a nonionic surfactant in the tank. In addition, clear away any vine debris and prune back canes that may get in the way of herbicide applications.

If you are unsure whether you have glyphosate-resistant weed species in your vineyard, the University of California Extension’s Weed Research and Information Center can help. Simply enter the physical characteristics of the weed into the Weed Identification Tool, and it will provide the weed’s name with photos to confirm identification.

The Center also includes a list of UC weed scientists and farm advisor weed specialists throughout California, a database of weed susceptibility to herbicides, and links to pages with information about specific weeds and control options.