Phenology of Parthenium hysterophorus - a key factor for the success of its invasion.
A phenological study of Parthenium hysterophorus was carried out on three 2 m2 plots demarcated at selected sites in Islamabad over a period of three years. Monthly observations regarding the following stages were made for individual plants in each plot: Sprouting, Vegetative growth, Budding, Flowering, Fruiting, Dispersing, Senescence and Dead. The results revealed that the aggressiveness of the weed owes to its twin sprouting seasons in addition to its capability to withstand wide climatic ranges. The seeds of Parthenium germinate in the hottest month of June (mean maximum 38.7°C) and flourish by producing flowers, fruits and shedding seeds in the coldest months of December and January, mean minimum 3.4 and 2.6 respectively.