Cyperus difformis L. (smallflower umbrella sedge) is the most common weed found in rice fields, capable of causing yield losses of up to 50% in dense infestations. Due to the increased infestation of herbicide-resistant accessions, innovative management tools are needed to develop adequate options for C. difformis control. In California, alternative rice establishment systems such as the stale-seedbed method have been developed to address the increasing issue of herbicide resistance. This technique consists of herbicide applications anterior to rice sowing. Delays due to time needed for weeds to germinate and emerge prior to herbicide applications are a main concern in such systems. Knowledge of the temporal pattern of germination will contribute to optimize the timing of control measures, thus maximizing efficacy.

The effects of constant temperature and water stress on germination (coleoptile extension) were investigated in seeds of two Cyperus difformis accessions resistant (R) to ALS-inhibiting herbicides, and on three susceptible (S) ones in order to develop thermal- and hydro-time germination models. No germination was recorded at 13.3 C, and the rate of germination increased linearly from 16.4 to 33.7 C, indicating that the optimum temperature for germination is above the latter. Estimated base temperatures ranged from 16.4 to 19.8 C, with no clear pattern between R and S accessions. R seeds tended to have less germination synchronicity across water potentials, and ability to germinate faster than S accessions, especially in cold temperatures (i.e. temperatures near to the base temperature for germination of that accession). Base water potential for seed germination varied significantly across different temperatures and among accessions, and ranged from −0.24 to −1.13 MPa. R accessions, moreover, showed lower (more negative) base water potential values, indicating greater capacity to germinate in dryer conditions.

Thermal times to emergence following germination in field soil varied across four water regimes (e.g. intermittent moisture, daily irrigation, saturated soil and flooding), with emergence occurring faster under daily irrigation. But, total seedling density under daily irrigation was lower than other water regimes, and was inconsistent across two soils tested. The intermittent moisture treatment, in which irrigation performed every 3 days, did not produce moisture conditions suitable for seedling emergence.

Our research suggests that in order to optimize springtime seedling emergence for this species, soil moisture should be kept around field capacity, since germination rates are significantly reduced at lower water potentials. Results also enable for improvements in the stale-seedbed method by allowing for earlier irrigation due to the ability of R accessions in germination faster at colder temperatures, as well as decreasing the time necessary for smallflower umbrella sedge emergence due to the more rapidly germination of R in comparison to S accessions. Moreover, R accessions had significantly lower base water potential values and are thus able to germinate at dryer conditions, which could contribute to intermittent irrigation systems in which fluctuations of the soil water potential values are expected.

By predicting when most of the seed population germinates, the thermal time model can address issues on timing for herbicide application and improve Cyperus difformis control by a better understanding of the parameters involved in its germination.