Fall Weed Control in Grapes - 2006
Joseph Masabni, Department of Horticulture

Introduction
Fall-applied herbicides are an important component of a comprehensive weed control regimen, especially for control of perennials such as honeyvine milkweed, quackgrass, and johnsongrass. Growers are often busy in the fall with harvest and wine making and neglect weed control after harvest. In order to assist grape growers with their decision making, an experiment was conducted in the fall of 2005 to evaluate the residual control of various herbicides on weed pressure and other benefits in the spring of 2006.

Materials and Methods
Herbicides were applied using a CO₂-pressurized backpack sprayer with a two-nozzle shielded boom calibrated to spray a 3 ft band at 30 psi at a 3 mph walking speed. The 11002-nozzles were set at 7 inches above ground to obtain good spray overlap and complete weed coverage. The spray boom was moved in and out in the row to avoid spraying the vine trunks. Therefore, weeds at the base of vine trunks were taller throughout the season and did not reflect the effectiveness of the applied herbicides. Plots were 6 ft wide x 108 ft long. The experimental design was a randomized complete block with three replications.

The preemergence (PRE) treatments were applied on 15 April 2005. Grapes were in the 0.5-2 inch green shoot stage. As weeds had been growing since early March and were 3-4 inches tall, Roundup WeatherMax at 16 oz/A (0.68 lb ai/A) was included with all treatments. The postemergence (POST) treatments were applied on 15 June 2005. Roundup was also included with the POST treatments at same rate. All treatments were applied early in the morning when the average wind speed was 2.5 mph.

The fall treatments listed in the table below were applied on December 17, 2005 when soil temperatures were below 55°F but before soil freezing. Roundup was again included with all treatments for control of existing weeds.

Visual weed control ratings were made on 6 May and 15 June. The scale used in these ratings was 1-10, with 1 = no control and 10 = complete kill or no weeds present. A rating of 7 (70-75% control) or more is considered a commercially acceptable value.

Results and Discussion
Three weeks after PRE application, Karmex performed best, with about 90% of the weeds controlled (Table 1). Karmex was better than Princep on dandelion and clover and spring weeds such as chickweed and mustard, but was equal in marestail control. Both herbicides were better overall than Devrinol. Devrinol’s lack of control of dandelion and clover is because they are perennial weeds not generally controlled by preemergence herbicides. No weed regrowth was observed on this date in any treated plots.

Two months after PRE applications, annual grasses and broadleaves re-emerged together with clover which was already present (Table 2). Control of clover continued to improve with Princep and Karmex only. Karmex had the best overall weed control, except for redroot pigweed. In this field, clover is not considered a serious pest since it doesn’t get tall enough to interfere with the grape canopy.

On April 19, 2006 or 120 days after treatment, all three treatments were equally effective in weed suppression with about 6-14 weeds/sq.ft. This is different from the results observed in the similar studies conducted on apple and peach (see report in this issue). This could be attributed to the fact that the vineyard had much reduced weed pressure at the onset of the experiment in spring 2005 and continued to show reduced weed growth through summer and fall of 2005.

By 148 days after treatment or about 5 months (24 May 2006), Chateau had significantly fewer weeds in the sampled area compared to Casoron, but performed equally to Gallery.