Understanding the environmental variables influencing the phenological development of weeds is essential for simulation model development. Temperature and photoperiod are important variables governing the phenological development of weeds. Growth cabinet studies were conducted to characterize the phenological development of Setaria viridis in response to variations in temperature and photoperiod and to determine the duration of the juvenile phase and the effect of temperature and photoperiod on reproductive development. Setaria viridis was adapted to a temperature range from 6.5 to 47 C. Phenological development of S. viridis was described accurately in terms of thermal days (cumulative day degrees above a base temperature) and biological days (Bd: chronological days at the optimum temperature and photoperiod). Four developmental phases of S. viridis were described: (1) a juvenile (photoperiod insensitive) phase of 2.6 Bd; (2) a photoperiod-sensitive inductive phase of 2.2 Bd; (3) a photoperiod-sensitive postinductive phase of 6.0 Bd; and (4) a photoperiod-insensitive inductive phase of 10.9 Bd. Photoperiod sensitivity of S. viridis did not differ with stage of development when expressed as a rate. Interpretation of constant sensitivity to photoperiod will simplify simulation of weed phenology in mechanistic models.

The goal of a mechanistic model is to determine the outcome of weed–crop interference. An understanding of weed phenology is essential for construction of such models because phenological development is a major factor determining the outcome of weed–crop competition. Growth cabinet studies were conducted to determine the influence of temperature and photoperiod on the phenological development of common lambsquarters. Common lambsquarters is a short-day species adapted to a temperature range of 6.5 to 44.5 C. Phenological development of common lambsquarters grown under a constant temperature of 20 C and an 8-h photoperiod was described in terms of biological days (Bd: chronological days at the optimum photoperiod and temperature). Three development phases of common lambsquarters were described as (1) a juvenile phase of 6.3 Bd, (2) a photoperiod-sensitive inductive phase of 8.2 Bd, and (3) a photoperiod-sensitive postinductive phase of 34.4 Bd. The photoperiod sensitivity of rate of development did not differ among phases of development across the life cycle. Interpretation of constant sensitivity to photoperiod will simplify simulation of weed phenology in mechanistic models.