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 winter radish, fiddleneck, wild mustard, black mustard, London rocket, shepherd's-purse, and spiny sowthistle. Many of the winter annual weeds, especially coast fiddleneck and the mustards and their allies, compete keenly with grain through the jointing stage. If such weeds are not controlled before the boot stage of the crop, they ultimately tower above it, creating severe competition 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 lambsquarter, kochia, Russian thistle, and common sunflower can be economically important because they can reduce yields and cause harvest problems. Smartweed (kelp) is a major problem in Delta areas with high water tables.
In cooler areas, where grain is only irrigated 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 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 tables for winter weeds and spring and summer weeds. Recommended rates for individual herbicides are listed in the HERBICIDE TREATMENT TABLE.
Annual Grass Weeds
Grassy weeds in California small grains are generally winter annuals. Wild oats, Italian ryegrass, ripgut brome, foxtail, wild barley, and hood canarygrass are the major grass weeds in small grains grown in California.
Wild oat is a major weed throughout the 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.
Ripgut brome, wild barley, and foxtail barley are mainly problems in new fields that were previously rangeland or pasture. The seeds of ripgut brome have a major impact on marketability of grain. Two to six ripgut brome seeds per quart of grain make it unsaleable for livestock feed.
Other problem grass weeds include littleseed canarygrass, rabbitfoot polypogon, and jointed goatgrass. Littleseed canarygrass is the major grass weed in southern deserts. Rabbitfoot polypogon can be a problem in areas of high water tables. These grasses are generally very competitive with grain, particularly during the seedling stage.
An annual grass weed found only in Siskiyou County, jointed goatgrass, is similar to wheat in all respects except for the head. There are no chemicals registered for its control in wheat; management of this weed is either when the field is fallow or planted to a rotation crop.
Crop rotation is of utmost importance for control of grassy weeds. Important rotational crops include cotton, corn, alfalfa, sugarbeet, and tomato. Consequently, grassy weeds are normally 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. Some specific grassy weeds may be controlled in grains with difenzoquat (see the susceptibility tables for winter weeds and spring and summer weeds, as well as the HERBICIDE TREATMENT TABLE).
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 (Banvel) after harvest. Observe plantback restrictions for dicamba.
Johnsongrass creates problems in many areas, especially in the Sacramento/San Joaquin 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 by glyphosate (Roundup). Repeated summer tillage also 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 using selective grass herbicides in corn, cotton, and alfalfa during crop rotation.