Algae can smother rice seedlings or cause them to dislodge, resulting in yield loss. Algae are ubiquitous in the environment and grow well in the relatively shallow water conditions present in rice field. The species of algae present in a rice field shifts during the growing season from green algae and diatoms in early May to dominance by blue-green algae in late May-early June.
The amount of algae present in fields is associated with the concentration of phosphate in the water. In most cases, higher phosphorus levels result in a greater abundance of algae. Practices that reduce phosphorus inputs into the water, such as incorporating phosphorus fertilizer into the soil according to published UC recommendations, will lead to reduced algal growth. Copper compounds are available to control algae but must be applied before algal mats float to the surface. Some algae may be less susceptible to copper treatment than others, and the amount of residual rice straw present in a field from the previous growing season may reduce the efficacy of copper treatments.
American Pondweed (potamogeton nodosus), Cattail (typha spp.), River Bulrush (schoenoplectus fluviatilis), Gregg Arrowhead (sagittaria longiloba).
Use deep plowing and crop rotation to reduce infestations of perennial weeds. Plow 8 to 12 inches deep to expose underground stems of cattails and Gregg arrowhead, tubers of river bulrush, and winter buds of American pondweed; this will usually reduce populations of these perennial weeds if sufficient drying of the soil and reproductive plant parts is attained in spring. Combining deep plowing (8 to 12 inches) with nonirrigated crop rotation, such as safflower, to facilitate soil desiccation is an even more effective management practice. Avoid transferring stems, tubers, and buds to clean fields by tillage equipment. Rotate to irrigated crops where effective herbicides and mechanical cultivation can be used to reduce perennial weed problems.
Marshweed (Limnophila spp.)
Limnophila indica is a quarantined weed. All currently identified plants (15 from Butte County and 3 from Yuba County) in the Consortium of California Herbaria are identified as Limnophila X ludoviciana (a hybrid between L. indica and L. sessiliflora). The majority of samples have been taken from one general location in Butte County with the earliest collection being in 1977. Differences between these Limnophila are minute, making it difficult to positively identify them. It remains unclear whether there are any positive identifications of L. indica since there are currently no collections in the California herbaria. This plant is generally found in irrigation ditches and shallow water, like rice production fields. It is an emergent aquatic plant with small purple flowers. Technically, the hybrid is not L. indica and therefore not a quarantined weed.
Smallflower Umbrellasedge (Cyperus difformis)
Populations of smallflower umbrellasedge have been identified to be resistant to propanil and ALS-inhibitors. If resistance is suspected in a field, first determine that the application equipment was operating properly, the rate of herbicide was appropriate, sufficient weed exposure was contacted by the application and the growth stage timing was appropriate. Once all these factors can be dismissed, collect a sample for official testing. Foliarly applied carfentrazone has been successful in controlling the propanil- and ALS-inhibitor-resistant smallflower populations. Many populations of this weed are already resistant to several herbicides, particularly the herbicides in group 2.
In California, watergrass, also known as barnyardgrass, is the most serious weed in continuously flooded rice. It is variable in form and three distinct species occur in California rice fields—terrestrial barnyardgrass, early watergrass, and late watergrass.
Terrestrial barnyardgrass is the most widespread and easiest to control by flood water. In fact, the California system of water-seeding rice was established to control this species and maintaining water 4 inches (10 cm) deep still provides good control. The larger-seeded watergrasses will grow through 4-inch deep water and thus deep flooding (7 to 8 inches) is required to control them. Watergrass tolerant to deep water can be selected for if not controlled by follow-up herbicide treatments. This is more typically a problem in organic production.
Watergrass seedlings can be controlled by preflood cultivation if they are dislodged before secondary roots begin to grow. Herbicides are also important in the control of watergrass but must be properly timed to the growth stage of this weed in order to control it. Apply clomazone between day of seeding and one leaf stage of rice, penoxsulam at the two and a half leaf stage of rice, thiobencarb before the third leaf stage of watergrass, and propanil after the fourth leaf stage but before the stem elongates. Cyhalofop is applied when rice is between the 2-leaf stage to early tillering.
Very severe infestations of this weed may require rotation to another crop or alternative stand establishment techniques that reduce the population prior to seeding of rice.
Weedy Rice (Oryza sativa)
Weedy rice is a member of the same species as cultivated rice grown in California. It is also known as red rice, referring to the red bran covering the kernels. However, there are other weedy rice biotypes that have straw, gold, brown or black hull color. Weedy rice is a very troublesome weed for rice growers because it grows more vigorously than cultivated rice and competes better for resources, thus reducing rice yields. Seed heads of weedy rice mature over a long period of time and easily shatter when mature. Seed falls to the soil surface where it may germinate or remain dormant for several years. Certified seed is currently the best way to stop the spread of weedy rice.
Unfortunately, there are no selective herbicides to take weedy rice out of cultivated rice. Use hand rogueing (removing) of identifiable weedy rice plants prior to seed dispersal to control it. Additionally, do not disc the ground in the fall after rice is harvested to keep weedy rice seeds on the soil surface, allowing weather condition and predators to destroy them. Flooding the field in the fall without working out the ground may help cause seeds to rot.
Thoroughly clean harvesting equipment (combine, bank outs, trailers, etc.) in affected fields to make sure there is no carryover of weedy rice seed to other fields. In addition, make sure cleaning procedures include the removal of all plant material from the equipment, including mud from tires or tracks that may contain seeds.
This weed is currently found in 8 of the 9 major rice-growing counties in California. It has the potential to spread with increasing use of dry or drill-seeded rice. Eliminating this weed from infested fields is a multi-year effort because of the longevity of seed in the soil.
Domesticated colored bran varieties have been found in commercial white rice production and are often mistaken for weedy rice. Colored bran contaminants can reduce the grain grade of the intended rice crop.
To identify weedy rice and to learn more about weedy rice management, please visit the California Weedy Rice webpage.
Winged Primrose Willow (Ludwigia decurrens)
Initial dicovery of Ludwigia decurrens in Butte county was in August 2011. The agricultural commissioners and UC cooperative extension advisors determined the infestation covered several square miles south of Richvale. Most infestations are along borders of fields and irrigation canals. One field had an infestation throughout. It is likely that this weed went undetected for several years.
Ludwigia decurrens can grow to six feet or higher and produce many four petalled yellow flowers and eventually will produce seed capsules. The stem of the plant is winged or star-shaped in cross-section.
Seed capsules from this plant have thousands of seeds which are capable of floating on the water surface as a means of dispersal, especially along irrigation canals; this could be the reason for weed dispersal across the majority of the infestation area. Seeds sticking to tillage equipment and seeds remaining in combines between harvested fields are other potential means of spread. Additionally, it has been determined that plant fragments have the ability to grow roots within a day or two when in water. This suggests not mowing levees as a means of control because this practice may actually increase dispersal of this weed.
Testing in the greenhouse at the Rice Experiment Station at Biggs, California indicates that the plant germinates best when the soil is moist but not flooded. However, the seed can germinate under water and eventually grow above the water surface with the potential to survive in a rice field and set seed. This plant also has the ability to form roots that grow upwards through the water column in order to scavenge oxygen near the water surface.
Containment and erradication efforts continue where it has been identified. These efforts will need to continue for many years to achieve complete erradication. A strong, competitive rice crop combined with some of the registered herbicides—e.g., triclopyr (Grandstand)— will help keep this weed in check within conventionally grown rice fields.
To identify winged primrose willow and learn more about its management, please visit the Winged Primrose Willow webpage at University of California's Agronomy Research and Information Center: Rice.