Common ragweed is a monoecious weedy species that is native to eastern North America. It is now found throughout most of North America (USDA 2010), and has invaded Europe, where it has spread from the southeast to several northern European countries. This is a wind-pollinated species that is a major source of serious allergy in eastern North America where pollen levels are high. It is a species of disturbed ground where it can grow in abundance in some regions. It is a late spring emergent species; flowering generally begins in mid-July, and continues until the first frost, often around mid-October in the east. Plants can go unnoticed because of their overall green colour and lack of showy flowers. Seeds are set in late summer and early fall. Greenhouse experiments show that increasing levels of CO2 will result in an increase in the pollen output of this species (Wayne et al. 2002). Hybrids between this species and the western ragweed Ambrosia psilostachya) have been observed. The two species can be separated by leaf morphology. The earliest collection of common ragweed in the UBC Herbarium is from 1926, when it was collected by John Davidson on Fraser Avenue in Vancouver. Check Flora North America for more information on morphological characteristics and distribution in the United States.