Exotic annual weeds on western U.S. rangelands owe at least part of their invasion success to prolific seed production. Reproductive output is determined largely by timing of flowering in relation to soil moisture, light, and competing vegetation. This study investigated the influence of temperature (including vernalization), photoperiod, and intraspecific competition on reproductive phenology in three populations (two varieties, typica and bracbypappa) of common crupina. Monthly plantings at four densities were grown in field plots through two consecutive cycles, and the date each plant reached bud and flowering stage was recorded. Development of greenhouse-grown plants from the same populations was compared with those grown outside for 4, 6, 8, and 10 wk of cold induction. Reproductive development in common crupina was influenced by all factors investigated: temperature, photoperiod, density, and vernalization. Conditions fostering the shortest thermal time from emergence to floral induction were a period of cold temperatures/short photoperiod as seedlings followed by warming temperatures and lengthening photoperiod. Both varieties of common crupina responded quantitatively to vernalization, but lack of vernalization caused a greater delay of flowering in var. brachypappa than in var. typica. Vernalization contributed to convergence of spring flowering among the August to December plantings, but earlier emergence and lower plant density greatly increased final plant size and fecundity. Lack of vernalization, high temperatures, and shortening day length delayed flowering of spring/summer emergents, increasing the probability of succumbing to drought before producing seed.

Thermal time models for predicting phenological development require an estimate of base temperature, an attribute not previously defined for common crupina, a Mediterranean winter annual introduced in western North America. The stage of seed germination was selected for estimating base temperature, because facilities were available for experiments over a range of constant temperatures and base temperature is relatively constant throughout the life cycle in other species. Achenes from three populations of common crupina, including two varieties, typica and brachypappa, were produced under uniform conditions. Cumulative germination was recorded at 12 h intervals for achenes in darkness and optimum moisture at 23 constant temperatures from 4 to 17 C. The time course of germination was best described by a logistic growth curve from which time to 50% germination was estimated. A parabolic model provided the best fit in a regression of germination rate (reciprocal of time to 50%) against the temperature gradient, yielding base and optimum temperatures of 1 and 10.5 C, respectively. Bootstrap confidence intervals indicated no significant difference in base and optimum temperatures in germination between the two varieties nor between two populations of var. typica of common crupina introduced in the northwestern United States.