Special Weed Problems

ANNUAL BROADLEAF WEEDS

The spectrum of broadleaf weeds in California grain fields differs considerably in fall- and spring-planted regions and varies from the northern Sacramento Valley to the southern Imperial Valley. In fall-planted grain, the most troublesome broadleaf weeds are wild radish, fiddleneck, wild mustard, black mustard, London rocket, shepherd's-purse, common chickweed, burning nettle, malva, and spiny sowthistle. Many of the winter annual weeds, especially coast fiddleneck and the mustards and other associated weeds, compete with grain through the jointing stage. If these weeds are not controlled before the boot stage of the crop, they ultimately tower above it, creating severe competition and yield reduction for the grain plants.

Spring-germinating weeds become serious problems in fall-planted grain because they emerge after the cutoff date for applying phenoxy herbicides. In spring-planted grain, summer annuals such as redroot pigweed, common lambsquarters, kochia, Russian thistle, and common sunflower can be economically important because they can reduce yields and cause harvest problems. Pale smartweed (kelp) is a major problem in the Sacramento–San Joaquin Delta (Delta) areas with high water tables.  In the intermountain region where phenoxy herbicides can be used later in the year, summer annual weeds are effectively controlled with applications.

In cooler areas, where grain is irrigated only a few times during the growing season, weeds are primarily a problem in the summer and late season where they interfere with harvest unless controlled.

Wild beet is a problem in small grains only in the Imperial Valley; even a small amount of its purple seed can stain wheat flour and make it unusable. Dicamba (Clarity) or 2,4-D can control wild beet and prevent quality problems.

To determine the susceptibility of annual broadleaf weeds to various herbicides, check the herbicide susceptibility table for winter weeds and the Herbicide Treatment Table.

ANNUAL GRASS WEEDS

Grassy weeds in California small grains are generally winter annuals. The major grass weeds in small grains grown in California are wild oat, Italian ryegrass, ripgut brome, foxtails, wild barley, downy brome, and hood and littleseed canarygrass.

Wild oat is a major weed throughout many grain-growing areas in California because it emerges throughout the cool season from autumn through spring. It causes lodging, slows harvest, clogs harvester screens, and lowers yields dramatically. If wild oat populations are extreme, consider cutting small grains for hay or silage to greatly reduce the weed population in subsequent crops. Wild oat exhibits primary and secondary seed dormancy, allowing for later emergence under favorable conditions.

Ripgut brome, wild barley, and foxtail barley are mainly problems in new fields that were previously rangeland or pasture. Grain marketability is significantly affected by the seeds of ripgut brome. Two to six ripgut brome seeds per quart of grain make the grain unsalable for livestock feed.

Littleseed canarygrass is the major grass weed in southern deserts. Rabbitfoot polypogon can be a problem in areas with high water tables. These grasses are generally very competitive with grain, particularly during the seedling stage.

Jointed goatgrass is an annual grass weed found in parts of the intermountain region. It is similar to wheat in all respects except the head. Few herbicides are registered for its control in wheat, and this weed is managed either when the field is fallow or planted to a rotation crop. Wheat and jointed goatgrass will occasionally hybridize, but seeds produced by the hybrids are typically sterile. Jointed goatgrass seeds can be nearly impossible to separate from grain and are a common seed contaminant.

Crop rotation is crucial for controlling grassy weeds. Important rotational crops include cotton, corn, beans, sunflower, safflower, alfalfa, sugarbeet, and tomato. Grassy weeds are typically a greater problem in dryland production than on irrigated land because grains are not grown in rotation with broadleaf crops in these areas. In irrigated systems, broadleaf crops provide an opportunity to control grasses because fields can be cultivated or mowed, and herbicides effective against grasses may be used.

FIELD BINDWEED

Field bindweed is the most widespread and destructive perennial broadleaf weed in California, infesting grains and numerous other crops from the Oregon border to Baja California. Field bindweed can be suppressed with 2,4-D during the growing season or with glyphosate (Roundup) or dicamba (Clarity) after harvest. Observe plantback restrictions for dicamba.

JOHNSONGRASS

Johnsongrass creates problems in many areas, especially in the Delta and the Central and Imperial valleys. Johnsongrass germinates in spring and may interfere with spring harvest. Growth just before and after harvest can be controlled with glyphosate (Roundup). Repeated summer tillage helps reduce johnsongrass competition in the next crop. Increased seeding rates help control johnsongrass, especially in the Delta. Johnsongrass can be more easily controlled in glyphosate-tolerant corn or cotton or by using selective grass herbicides in corn, cotton, and alfalfa during crop rotation. Johnsongrass is an important host for overwintering sugarcane aphid, a serious pest of sorghum, and should be controlled to minimize infestation in sorghum planted after small grains in the same field or nearby fields.