Determination of Cardinal Temperatures and Phenological Stages of Two Species of Spiny Cocklebur (Xanthium spinosum) and Common Cocklebur (X. strumarium) Based on Growing Degree-Day

Two species of common cocklebur (Xanthium strumarium L.) and spiny cocklebur (X. spinosum) are common annual summer weeds in soybean fields and other summer crops. The current research was carried out with the aim of determining the cardinal temperatures of germination and determining the phenology stages of these two species in Sari University of Agricultural Sciences and Natural Resources. In order to determine the cardinal temperatures of germination of seeds of two weed species, the seeds were examined at constant temperatures of 5 to 45 °C in a completely randomized design in 4 replications in a laboratory experiment. Also, the phenology stages of two weed species were investigated in a field experiment. The highest germination% was observed for the spiny cocklebur (51.3%) and the common cocklebur (58.1%) at temperatures of 25 and 35 °C, respectively. According to the different parameters of segmented model, the base temperature, lower optimum temperature and ceiling temperature for spiny cocklebur are 1.4, 23.2 and 54.7 °C, and for common cocklebur are 4.8, 34.6 and 48.4 °C, respectively. Time from emergence to physiological maturity for the spiny cocklebur and common cocklebur were 136 and 128 days, respectively. The results suggested that seed germination of the two weed species occurs in a wide range of temperatures, i.e. of 5 to 45 °C, and the best time to control this weed is late summer (before the beginning of the flowering stage).