Canadian horseweed (Conyza canadensis, family Asteraceae) has become notorious in recent years for failing to respond to glyphosate (e.g. Roundup) in agricultural fields and landscapes. This annual weed, which is also known as just horseweed, Canadian fleabane, coltstail, and marestail, has moved in recent years from being a plague in field crops to become a scourge in landscapes and nurseries. Indeed, this native North America plant has become such a problem in Ohio it has been added to the state's noxious weeds list.
Challenges with managing this weed centers on three issues. First, horseweed can flourish under a wide range of growing conditions. It tolerates a wide range of cultivation conditions from field crops to nurseries to landscapes and will endure drought conditions as well as water-logged soils such as in drainage ditches. Plants will produce viable seeds in poor, low nutrient soils as well as highly fertile soils. Growth appears unaffected by soil pH with plants enduring both alkaline and acidic soils.
The second challenge is this annual weed's opportunistic life-cycle with the ability to behave as a summer annual and a winter annual. Seeds may germinate in late-summer to early fall (winter annual cycle) or in the spring (summer annual cycle). Once seeds germinate, the plant forms a ground-hugging rosette that can be easily mistaken for other weeds. As a winter annual, horseweed remains in the rosette stage through the winter, and then it bolts in the spring. As a summer annual, the weed remains in the rosette stage for only a very short time, and then it bolts in early to mid-summer. Thus, seed production is asynchronous with seed heads appearing at different times of the year. Indeed, the unexpected appearance of the seed heads can present an identification challenge.
Once horseweed bolts, it quickly forms a single, unbranched hairy stem that is densely covered in alternating oblanceolate leaves measuring 3 - 4 inches in length. Leaves near the base of the stem are longer and somewhat toothed compared to leaves near the top of the stem. As flowers are produced, old leaves on the lower stem wilt and turn brown. Numerous small flowers are borne on multi-stemmed panicles at the top of the stems. The common names "marestail," and "coltstail" are based on the broom-like flower structures. Horseweed is a prolific seed producer and membership in the Asteraceae family is clearly demonstrated by the tiny, puff-ball-like seed heads which resemble miniature dandelion seed heads. Seeds can be wafted considerable distances by the wind.
Finally, the most serious issue with managing horseweed is herbicide resistance, including resistance to the many forms of glyphosate (e.g. Roundup, Glyphomax, etc.). Roundup Ready soybeans were released in 1996; horseweed resistance to glyphosate was first reported in 2000 with glyphosate resistant biotypes now found in 13 states. Adding to the challenge, horseweed biotypes have also been identified that are resistant to other common agricultural herbicides including paraquat, diquat, atrazine, simazine, chlorimuron, diquat, linuron, and diuron. Indeed, it would appear the only thing keeping horseweed from clearly becoming a true "super-weed" is its annual lifestyle.
Horseweed can be culturally managed by maintaining a 2 - 3" mulch layer to bury seed and limit seed germination. Physically moving plants through cultivation or hand-pulling before seeds are produced is also effective. A note of caution: while the single, stout stem makes a nice "handle" for pulling this annual weed, hand pulling works best in wet, loose soils. The stem often breaks away from root systems anchored in dry and/or compacted soils. Decapitated plants behave like the Lernaean Hydra of Greek mythology by producing multiple stems to replace the single lost stem. Continual mowing or string trimming prevents seed head development and will eventually exhaust the resources of this annual plant.
Although herbicide resistance has become a major issue with managing Canadian horseweed, there remain a number of pre- and post-emergent herbicides that will suppress this stubborn weed. Effective preemergent herbicides include flumioxazin (e.g. SureGuard, BroadStar); oryzalin (e.g. Surflan); and isoxaben (e.g. Gallery).
Effective post-emergent herbicides include carfentrazone-ethyl (e.g. QuickSilver) if mixed with a phenoxy-type postemergent herbicide such as 2,4-D, and 2,4-D is effective if mixed with dicamba, MCPP, or MCPA. The mantra "read and follow label directions" is particularly important with these high-risk applications; pay very close attention to recommended distances to desired plants including warnings about plant root zones. However, no horseweed management strategy should depend entirely on herbicides. That's how we got into trouble with this weed in the first place!