Redstem is an important weed in California water-seeded rice fields because of its aquatic habit, wide distribution, interference with harvest, and resistance to the herbicide bensulfuron. Our objective was to understand the mechanisms of competition for light between rice and redstem, with the goal of improving redstem control. A replicated greenhouse experiment was done in 1993 and 1994. Rice was water-seeded at a rate of 400 seeds m−2, and redstem was seeded simultaneously at approximate densities of 0, 50, and 100 seeds m−2 in continuously flooded 0.77 m2 basins. Plants were harvested once at final harvest in 1993 and twice in 1994, with an additional nondestructive sampling 34 days after seeding (DAS). Despite slower early growth, redstem height exceeded rice height about 45 DAS. At the midseason harvest in 1994 (56 DAS), no effects of redstem competition on any rice response variables were detected. However, at final harvest (110 and 118 DAS, 1993 and 1994, respectively) redstem competition at both treatment densities reduced rice tiller density, panicle density, shoot drymass, and grain drymass. Redstem competition reduced rice growth only after penetrating the canopy. Shade cast by redstem through rice maturity decreased shoot and grain production and increased tiller mortality. Lodging caused by redstem further affected rice growth. Season-long competition from redstem at mean densities of 67 and 110 plants m−2 reduced rough rice yields 31 and 39%, respectively, making redstem the most competitive broadleaved rice weed yet studied. Improved understanding of rice-redstem interactions indicates that using alternative herbicides to bensulfuron is unlikely to increase yield losses to redstem, and that control may be improved by increasing rice plant densities or slightly delaying early season chemical control. Because these strategies are mechanistic, they may also be useful for controlling other rice weeds with growth patterns similar to redstem.

Because of the large number of potentially invasive species, and the time required to complete weed risk assessments (WRAs) with the use of the current, mandated system in the United States, species need to be prioritized for assessment and possible listing as Federal Noxious Weeds. Our objective was to rank the potential invasiveness of weedy or pest plant species not yet naturalized in the United States. We created a new model of invasiveness (hereafter the U.S. weed-ranking model) based on scoring factors within four elements: (1) invasiveness potential, or likelihood to exhibit invasive behavior; (2) geographic potential, or habitat suitability; (3) damage potential, or likely impact; and (4) entry potential, or likelihood to be introduced. The ranking score was the product of the four elements. We scored 250 species satisfactorily, from a list of 700 +. We analyzed model sensitivity to scoring factors, and compared results to those from a WRA model for Hawaii. For species not in cultivation in the United States, the top 25 species included a mix of annuals, perennials, sedges, shrubs, and trees. Most had exhibited invasive behavior in at least several other countries. Because of greater entry potential scores, the highest-scoring species were weeds in cultivation. Twenty-nine such species, out of 44 total, had scores greater than the highest scoring species not in cultivation. In comparison to the Hawaii WRA model, correlation and regression analyses indicated that the U.S. weed-ranking model produced similar, but not exact, results. The ranking model differs from other WRAs in the inclusion of entry potential and the use of a multiplicative approach, which better suited our objectives and United States regulations. Two highly ranked species have recently been listed as Federal Noxious Weeds, and we expect most top-tier species to be similarly assessed.

California arrowhead is a broadleaf weed widespread in water-seeded rice. Bensulfuron is the only herbicide currently available for use throughout the California rice growing region that provides complete control of California arrowhead; however, resistance to bensulfuron has been detected in California arrowhead and in several other weed species. Growers have herbicide alternatives for weed species other than California arrowhead but continue to use bensulfuron year after year for control because they believe California arrowhead reduces rice yields. However, damage thresholds have not been determined for this weed, and the crop may be able to tolerate relatively high California arrowhead densities. In this work, the damage thresholds for California arrowhead were determined in field and greenhouse experiments. Water-seeded rice was grown in mixture with California arrowhead in a 1992 greenhouse experiment and in field experiments in 1992 and 1998. Rice tiller density and grain yields were not affected by California arrowhead densities up to 200 plants m−2 in any year. Rice was taller than California arrowhead throughout the growing season in all experiments, and the weed senesced well before rice maturity. The ability of the crop to overtop the weed and grow weed-free during the latter part of the season may explain why California arrowhead is such a weak competitor with water-seeded rice. The results suggest that growers may be able to tolerate California arrowhead densities up to 200 plants m−2 without detectable yield losses. Implications for weed management are discussed.