Description: Perennial 1 to 6 feet. Leaves lanceolate, bright green to gray-green, smooth to toothed margin. Basal leaves larger than upper leaves. White flowers develop in dense clusters near the ends of branches. Fruit a two-seeded capsule. Reddish-brown seeds round, flat, slightly hairy, and about 1/16 inch long.
Habitat: Native to southern Europe and western Asia. Grows in waste areas, wet areas, ditches, roadsides, cropland, along waterways, and dry habitats such as road cuts and fills. Robust, deep-seated spreading roots and numerous seeds make this weed very difficult to control. Out competes native vegetation and crops, forming its own monoculture. Attempts at mechanical removal spreads the plant and increases its numbers.
Distribution: Large infestations of tall whitetop are found along the Susan River (Lassen County), the Lower Klamath Wildlife Refuge (Siskiyou County) and the Truckee River (Washoe County, Nevada) Scattered infestations occur along the Sacramento River north of Lake Shasta (Shasta County), as well as along roadways in Lassen County and Washoe County, Nevada.
Flowering Period: June to August.