Erigeron bonariensis aka Argentine Fleabane

Erigeron bonariensis is in the daisy family, Asteraceae. People call it hairy fleabane, flaxleaf fleabane, or small horseweed. It is native from Mexico through tropical America. It has spread to Europe, Africa, and Asia. It has also reached parts of the United States. US records include Oregon and the Gulf states. It is still rare in North Carolina. The RHS uses another name for it, Conyza bonariensis. This name is a synonym.

This weed grows fast and has a dense, upright form. It has a coarse texture. It grows as a summer annual or a biennial. These plants live for one or two years. Its straight stems are gray-silver, with grooves along their length. Stiff hairs cover the stems. Hairs also grow on the leaves and flowers. The simple leaves grow at the base. They also grow in turn along the stems. They look gray-green and feel soft. Some are narrow and linear, shaped like lines. Others are elliptical, with an oval shape. Leaf edges may be smooth or have small teeth. Leaves reach 0.25–0.5 feet [0.08–0.15 metres] long.

Groups of flower heads grow at the stem tips. They bloom in summer but can bloom all year. Green bracts overlap to form a cup below each head. Small yellow disk flowers grow among cream-colored bristles. Each fruit is an achene, a dry, one-seeded fruit. The tiny, narrow fruits form pale, fuzzy balls. These look like dandelion seed heads. Fruits appear in summer and fall. Wind blows the fluffy seeds away. Rain and human acts also spread them. Seeds can stay alive for up to three years.

Hairy fleabane grows in full sun and withstands dry ground. Records list clay, sand, and soil rich in organic matter. It can grow in poor soil too. It grows by roads, in fields, and on disturbed ground. It also grows in towns and in cracks in paving. Its native climate is subtropical. The evidence gives no mature height or cold-hardiness rating. The listed planting space does not show its mature spread.

Its main garden problem is its invasive, weedy growth. NC State says not to plant it in gardens or natural areas. It disrupts crops in orchards. Some weedkillers do not work against it, making control harder. In South America, it can host lettuce mosaic virus. Its name does not prove that it works against fleas. The evidence does not support that use. Touching any part can cause dermatitis, a form of skin rash. NC State rates its poison risk as low. It names sesquiterpene lactones as the cause. The evidence gives no edible use. It does not assess safety for pets.