Flower:

Flowers are in a large open cluster at the top of the plant. Individual flowers are about 1/8 inch across with 20 to 40 minute white or pinkish petals (ray flowers) and a yellow center. The flowers aren't very noticeable until you get relatively close to the plant, and look like they are just starting to open up even when in full bloom.

Leaves and stem:

Leaves are narrow and blade-like, to 4 inches long and less than ½ inch wide, becoming progressively smaller as they ascend the stem. The leaf edges on the lower part of the plant have small widely spaced teeth, those on the upper part are toothless. There is little or no leaf stem. Leaves are alternately attached and crowded along the main stem. The leaf edges and the main stem are covered in stiff hairs.

Fruit:

Seeds have a tuft of light brown hair attached to carry them off in the wind.

Notes:

Canadian Horseweed is a weedy plant that grows almost anywhere. It seems to prefer dry soil, but grows in moist habitats as well. I have a bunch of it growing in my back yard, not by choice.