Weed of the week: Horseweed

Written by Blake Edwards, Tom Eubank and Jason Bond

Horseweed

Family: Asteraceae

Scientific name: Conyza canadensis (L.) Cronq.

Synonyms: Canadian fleabane, mare’s tail, fireweed

            Horseweed is a winter annual which typically grows in a rosette pattern before growing more erect prior to flowering. Horseweed reproduces and spreads via a wind-blown seed with the aid of a papus. The leaves are arranged in an alternate pattern with the leaves being sessile in which the leaf blade partially or wholly surrounds the stem. Cotyledons are extremely small, smooth and appear to lack veins. The first leaves in the rosette are entire with subsequent leaves being toothed and the tip being somewhat pointed. Mature plants have numerous inflorescence or hairs covering the stem and sometimes on the leaves. The flowers of horseweed are often white to lavender in color and sometimes yellow. Horseweed can often be confused with tall goldenrod (Soldago canadensis). Both of these plants have similar growth habits; however, tall goldenrod only has yellow flowers. Leaves of tall goldenrod are smooth on top and pubescent below contrary to that on horseweed leaves. Also, tall goldenrod has serrated leaves as opposed to toothed. Horseweed has bristly trichomes on the stems and tall goldenrod has pubescence.

Horseweed is native to North America and can be found throughout the United States.  It is common in crop fields, roadsides, and pastures throughout Mississippi. Horseweed is an early transitional weed that prefers undisturbed soils such as in no-till or conservation-tillage systems. Horseweed typically emerges in early-fall but has been shown to emerge throughout the year. Fall-emerged horseweed will typically grow through the rosette stage in winter before bolting and flowering in late spring to early summer; however, spring-emerged horseweed may bypass the rosette stage and begin growing erect immediately after emergence. Horseweed is problematic in rice, soybean, cotton and corn and has also evolved herbicide resistance to both glyphosate and paraquat in Mississippi. Postemergence herbicide recommendations typically include the addition of 2,4-D, dicamba or sharpen to various burndown programs. Fall-applied residual herbicides have been shown to offer an effective means of preventing horseweed emergence.

Bryson, C.T. and M.S. DeFelice. 2009. Weeds of the South. Athens, GA: University of Georgia Press. pp. 218