Soon after I arrived in Napa last year, I issued a warning about herbicide-resistant weeds. If they were not already in vineyards, I reasoned, they would be soon.

The weeds most closely associated with herbicide resistance in California, Rigid Ryegrass in the Sacramento Valley and Horseweed in the southern San Joaquin, are still relatively minor weeds in most of the North Coast region.

The evolution of these weeds can be traced to over-reliance on a small number of herbicides. Because of this I continue to discourage the use of a single weed control method or single herbicide, but I have realized that there are factors other than herbicide resistance that are contributing to less than satisfactory weed control in many vineyards.

I have observed that application timing, rate and coverage are more important than herbicide resistance in reduced herbicide efficacy at this point in time.

A recent trend in vineyard herbicide use has been to go to a “Roundup only” program consisting of a single application just prior to bud break and sometimes a second application during the growing season. The second application is often a spot spray with a backpack or spray gun. This tactic has been successful in the control of many annual weeds but less successful with perennials and some harder to control annuals: Tall Annual Willowherb, Fillaree, Sharppoint Fluvellin, and Little Mallow or Cheeseweed. Many of these hard to control annuals germinate several times during the year or over a long period of time making post-emergent control difficult.

Application timing is critical for both pre- and post-emergent herbicides. Pre-emergent herbicides must be applied before weeds germinate and in most cases followed within a month or less by a substantial rain event (more than .50 inches). Many growers wait until the rainy season begins, increasing the odds of this rain event. By delaying the application of pre-emergent herbicides many weeds will germinate and a post-emergent or ‘burn down’ herbicide must be added to control these weeds. Annual weeds are best controlled when small, so post-emergent herbicides are often applied late enough to control as many weeds as possible, but early enough to still be effective. Factors that contribute to less than desirable control such as waxy leaves, woody stems and a large tap root all increase with the age of the weed.

Applying the herbicide at the full label rate and application number will usually control most or all of the weeds on the label.

Herbicide applications are often made at less than the full label rates. Herbicide cost, application fuel cost, or a desire to reduce pesticide inputs are all valid reasons to use less than the allowable maximums. However, if a grower chooses to reduce the rate or number of applications in your vineyard, it is critical to properly identify the weeds and know their biology (annual, perennial, and critical growth period).

If a grower has had several years of good control and has the weeds “under control,” a slight rate reduction may be acceptable. On the other hand, if a grower continues to have the same weed problems year after year he or she should consider increasing the rate, increasing the number of applications, using an alternative herbicide, or choosing a different method of control.

Application timing and herbicide rate should not be considered as independent factors. For post-emergent herbicide applications later in season, a higher rate may be needed to get acceptable control. For pre-emergent herbicides the duration of control needed will often dictate the rate. Applications made early in the season, in many cases, will need to be at a higher rate than those later in the season.

The third factor to consider is coverage. The amount of water used in the application of herbicides can be a factor in efficacy. Many growers use a 20 to 30 gallon per acre application rate. This appears to be effective in most situations.

Coverage can have another, and in particular cases, a much more important meaning. How much are the grape leaves that are left on the ground at the end of the season affecting the amount of herbicide that hits its target?

Willowherb and Fillaree especially appear to “hide” under the leaves, escaping control of both pre- and post-emergent materials.

Preliminary data from a grower-applied field trial shows that the amount of leaves present can dramatically influence the success of herbicide applications.

This is nothing new to many growers who wait until the leaves have blown away before applying herbicides. Several factors including total rainfall, and cover crop height and density may keep the leaves in place well into the winter.

Several growers in the Central Valley are using blowers to remove the leaves mechanically. Equipment that sweeps debris from under the vine after pruning may also remove leaves.