This annual rank, weedy plant has numerous, linear leaves on a ridged and bristly erect stem. These leaves are 1-4 inches long, dark green, hairy and often slightly toothed. Many tiny flowers spring from in clusters from the upper leaf axils. If these small (about 1/8 inch) flowers are examined closely they look like miniature daisies with a yellow central disk and many short white ray florets. These rays, however are compressed and often do not spread out. The flower head has many smooth green bracts. When mature the flower forms fruit containing one seed and many bristles for wind distribution.

The plant grows 1-7 feet high and is found on roadsides, fields and waste places throughout North America, except parts of the far north. There are many branches that arise from the main stem. Horseweed blooms from July to November. It often is considered a weed in agricultural areas where it can reduce crop yields. It is resistant to many herbicides. Horseweed is native to North America but has spread to Europe where it easily colonizes disturbed sites. It is usually crowded out once perennial plants become established on the site. Horseweed blooms from July to November.

Native Americans used a preparation from the leaves as a treatment for dysentery and sore throat. It is also called Canadian horseweed, colts tail, mares tail, and butterweed.  A single plant may contain hundreds of flowers. By this time of year the plant usually has a mixture of newly emerged flowers and those that have gone to seed.