Parthenium (Parthenium hysterophorus L.) is an invasive alien species of global significance which is rapidly expanding in Nepal and other Asian countries with negative impacts on species diversity, health of human and livestock, and productivity of pasture and agriculture. To understand the invasive success of this weed, we analysed soil of its invaded sites, morphological traits, biomass allocation, leaf attributes, and phenology in the Kathmandu valley. The roadside soil, which is loamy sand in the valley is highly suitable for the growth and proliferation of the parthenium weed where its density has become double in less than a decade. An average size plant was 112 cm tall with 12 cm long tap root and produced 2637 achene per plant (max. 3865 per plant). A combination of different leaf traits enables the parthenium to grow under diverse habitats such as resouces poor condition (due to low specific leaf area, SLA), disturbed habitats (due to low leaf dry matter content, LDMC) as well as productive sites (due to high leaf nitrogen content). During rainy season it completes lifecycle in 16-18 weeks. Relatively long (12-16 weeks) reproductive period with high output of small seeds and their capacity to germinate and flower anytime in the year make parthenium a successful invasive weed in the Kathmandu valley.