This native plant is herbicide resistant and can be ten foot tall!!
By Carol Sitarski, Master Gardener Volunteer, Cornell Cooperative Extension Allegany County
Horseweed (Conyza canadensis, aka Erigeron canadensis), a member of the Asteraceae family (aster), sometimes called Canadian Horseweed, Canadian Fleabane, or Marestail is a North American native plant that has spread to South America and Europe. Why the two botanical names? Well, it was classified as Erigeron in 1753, then in 1943 it was reclassified to genius Conyza. Both names are still used today.
This winter-germinating annual can reach 6 to 10 feet in height. It begins as a basal rosette, then sends up a single bristly stem that branches into a many-topped crown. The plant produces clusters of tiny, daisy-like flowers, white-rimmed with yellow centers, that draw in even the smallest pollinators.
In 2001, horseweed became the first weed documented as resistant to glyphosate and other common herbicides. This resistance makes it especially challenging in agricultural fields, where tilling or cultivation can spread its seeds. Once in the soil, the seeds can remain viable for years, and the plant thrives in a wide range of habitats. Because of these factors, management is challenging, and control options are limited. The most effective strategies include applying a pre-emergent herbicide (or corn gluten meal, a natural alternative) in the fall to prevent germination, or hand-pulling plants before they go to seed. Fortunately, horseweed has shallow, fibrous roots that make it relatively easy to remove by hand.
It is noted that this plant has been used for centuries for its medicinal properties, and foragers find the taste appealing, so there is something worthwhile about it.
You may be wondering how horseweed got its name. If you look at the pictures, its tall, bristly stalks do resemble a horse’s tail. However, don’t confuse horseweed with horsetail. Horsetail is an entirely different plant: it never produces flowers, is equally difficult to control, and has also been valued for medicinal uses.
If you have questions about this or other weeds, or would like to learn more about gardening, reach out to your local Cooperative Extension Office. The group of Master Gardener Volunteers and educators is always happy to help.
