Horticulture/Conyza canadensis

Conyza canadensis (formerly Erigeron canadensis L.) is an annual plant native throughout most of North America and Central America. Common names include Horseweed, Canadian Horseweed, Canadian Fleabane, Coltstail, and Butterweed. Horseweed is a common weed of fields, meadows, wastes, and gardens throughout its native range, colonizing areas where perennials are not established. It often grows in cracks of hardscape surfaces as well, and can cause problems with heaving in thin materials such as flagstone.

It is an invasive species in Britain, found from northern Scotland to Cornwall. It is the only one of the British Conyza species that grows as a weed of arable land: the others are casuals of waste and disturbed ground in towns and by roads and railways. It is not invasive of any natural or semi-natural habitats.

Description
Horseweed is an annual plant growing to 1.5 m (5 ft) tall, with sparsely hairy stems. The leaves are slender, 2-10 (0.5" to 4") cm long and up to 1 cm (0.5") broad, with a coarsely toothed margin. The flowers are produced in dense inflorescences 1 cm (0.5") in diameter, with a ring of white or pale purple ray florets and a centre of yellow disc florets.

Control
In summer, patrol upwind meadows and remove plants if possible.