Horseweed. Photo: Bill Curran, Penn State
The young plant produces a low, growing rosette of hairy leaves. The rosette is produced in late summer when horseweed grows as a winter annual. As the plant matures, it produces a single, hairy stem that can reach a height of approximately five feet. The small, white flowers with yellow centers are produced from mid to late summer and resemble small daisies. They are found near the uppermost growth of the plant on lateral branches.
This is a plant with amazing seed production. One plant can produce over 200,000 seeds that can be windblown up to a quarter of a mile. Horseweed is most common in landscapes, nurseries, and agricultural settings that are not well maintained. It can grow in turfgrass, but maintained (regularly mowed) turf does not have a problem with it. Don't confuse horseweed with horsetail, which is a different species and managed with different herbicides.
Horseweed has a fibrous root system and a shallow taproot. Depending on the soil conditions, you may be able to hand remove small plants. The proper use of mulch in ornamental plant areas can also effectively reduce this weed. Herbicide options include the pre-emergenents: dichlobenil (Casoran); dithiopyr (Dimension); flumioxazin (SureGuard); oxadiazon (Ronstar); oxyfluorfen (Goal), and oxyfluorfen+prodiamine (Biathlon), which is labeled for nursery and grounds maintenance and not lawn/landscape.
Post-emergenent herbicides include: glufosinate-ammonium (Finale); glyphosate (Roundup and others) and sodium salt of asulam (Asulox, very limited use in ornamental plant and turfgrass settings). Remember to always read the label for proper application sites and rates.