In the fall of 2019, a comprehensive survey took place by soil sampling from fields in each major rice-growing county. Fields were randomly selected: 10 from Glenn, Butte, Sutter, and Colusa Counties, 5 from Yolo and Yuba Counties, and 2 from Sacramento and Placer Counties

The samples were collected after the rice harvest, from October to November. Each field was sampled four times, randomly throughout the field, approximately a gallon of soil for each sample, for a total of approximately 4 gallons of soil from the top 6 inches of soil in each field. The samples were wet-chilled in freezers (to break dormancy) until the beginning of January 2020. Samples were then placed in pots and grown out in the Department of Plant Biology greenhouses in Davis, CA for approximately 3-4 months. Each sample (four per location) were split equally into two 12-in by 12-in flats: one pot was flooded and one pot was kept at saturation (0 mPa). The total number of pots per location was 8 (4 flooded, 4 flushed). The two types of irrigation per sample were to ensure that weeds species that emerge under only one irrigation type versus the other were allowed to emerge.