Horseweed

Images above: Upper left: Horseweed plant (Antonio DiTommaso, Cornell University). Upper right: Horseweed mature plant (Joseph Neal, North Carolina State University). Bottom: Horseweed seedling (Antonio DiTommaso, Cornell University).

Identification

Other common names:  blood-stanch, butterweed, Canada fleabane, colt's tail, fireweed, hogweed, mare's tail, pride weed, flea wort, mule tail, fleabane, bitterweed, mares-tail, Canadian fleabane, stickweed

Family:  aster family, Asteraceae

Habit:  Erect, winter or summer annual herb with densely leafy, essentially unbranched stalks.

Description:  Short hairs give seedlings a fuzzy appearance.  Cotyledons are small, 0.1” (0.3 cm) long, and egg-shaped.  Young leaves are oval, toothed, hairy, and arranged in a basal rosette.  Leaves at the rosette base have distinct stalks, and upper leaves taper to the rosette base.  Toothed leaf edges become deeper with age.  A short taproot is supplemented by a fibrous root system.  Seedlings emerging in the spring form basal rosettes, yet produce only a few leaves, and soon after emergence these plants begin to bolt.  Mature plants bolt on a single stem that can reach 1-6 ft (0.3-1.8 m) in height.  The basal rosette frequently disintegrates when plants mature.  Stems are covered with bristly, short hairs and many closely spaced, hairy, alternately arranged, downward angled, baseball bat shaped leaves measuring 4” (10 cm) long by 0.4” (1 cm) wide.  Toothing, when present, is widely spaced and shallow.  The leaf tip has two small, uneven indentations located across from one another.  The upper stem branches produce a panicle of small, 0.2” (0.5 cm) diameter, white, daisy-like flowerheads.  These consist of 25-50 white to pink-tinged one-petaled flowers along the edge surrounding 7-12 small, yellow, knobby disk flowers.  At the base of each flowerhead are 1-2 rows of small, green bracts.  The outer coat of the seed is a thin dry layer of fruit tissue.  Seeds are brown, torpedo shaped, and attached to a pappus of bristly hairs.  Seed length, not including the pappus, is less than 0.1” (0.3 cm). 

Similar species:  Hairy fleabane [Conyza bonariensis (L.) Cronquist] is shorter, branched from the base and has densely hairy gray-green leaves.  Its ray flowers are greenish-yellow rather than white and its disc flowers are inconspicuous and white rather than yellow.  Annual fleabane [Erigeron annuus (L.) Pers.] and rough fleabane (Erigeron strigosus Muhl. ex Willd.) are similar to horseweed at all stages.  Annual fleabane flowerheads are 2-3 times larger in diameter than those of horseweed.  The stem has ridges and branches, and the leaves are more distinctly toothed.  Rough fleabane leaves are paddle-shaped and have hairs that lie parallel to the leaf surface. 

Two species in the mustard family, Virginia pepperweed (Lepidium virginicum L.) and shepherd’s-purse [Capsella bursa-pastoris (L.) Medik.], have similar seedlings.  Virginia pepperweed can be distinguished by its hairless, 0.4” (1 cm) long cotyledons and deeply lobed young leaves that are hairy under the midvein; it also emits an odor when crushed, while horseweed seedlings do not.  Shepherd’s-purse seedlings have round or egg-shaped cotyledons, irregularly edged young leaves with lobed, toothed or smooth edges, and star-shaped hairs on the upper surface.

Management

The keys to controlling horseweed are tillage, crop competition and management of the species in areas around fields.  Horseweed rosettes are easily destroyed by spring tillage (Brown and Whitwell 1988), which is why the species is most frequently found in overwintering crops like strawberry and winter or no-till grain crops (Buhler and Owen 1997, Weaver 2001).  Moreover, plow tillage that buries the previous year's seeds will permanently eliminate nearly all of them since most will die before subsequent tillage returns them to the soil surface environment that they need for establishment.  Since new seedlings arise at the soil surface and are slow to establish, they are easily killed by tine weeding in field crops and by precision weeding tools and hoeing in vegetable crops.  However, plants become difficult to control with tine weeding in late spring because of rapid growth at this time.

            Horseweed is sensitive to crop competition, so steps that increase crop competitiveness can greatly reduce horseweed problems (see Ch. 3 for methods).  Spring germinating horseweed can be a problem in weakly competitive crops like carrot and onion.  To avoid problems in these crops, precede them with two years of highly competitive or short season crops in which horseweed is suppressed and prevented from going to seed (Weaver 2001).  Rye, legume, or forage radish cover crops that establish a leaf canopy quickly in fall can effectively suppress this species prior to spring planted crops (Pittman et al. 2019, Wallace et al. 2019).  Sufficiently high rye biomass is required in spring to suppress horseweed that emerges after termination of the cover crop (Sherman et al. 2020).  The combination of a rye cover crop and narrow soybean row spacing can effectively suppress horseweed density and biomass (Fisher and Sprague 2022, 2023). 

            Since horseweed reproduces prolifically and the seeds blow easily into surrounding fields, cleaning up non-cropland to prevent seed production is critical (De Cauwer et al. 2008, Sosnoskie et al. 2007).  Plant stream sides, banks of irrigation ditches, edges of equipment yards and other potentially weedy habitats with competitive species that suppress horseweed.  Mowing will usually not completely eliminate seed production because new sprouts will grow from dormant buds.  Mowing greatly reduces seed production, however, and lowers the launching point so that seeds cannot blow as far (Weaver 2001).  If horseweed has not yet gone to seed at harvest, be sure to clean up the field afterward with disking or other appropriate tillage.  Electrocution with a Weed ZapperTM controlled most plants that were reproductive and growing above a crop canopy (Schreier et al. 2022).

            Even low rates of organic mulch effectively suppress horseweed if placed before seedlings begin to bolt (Main et al. 2006, Weaver 2001), probably because energy reserves in the seed are negligible and the seedlings are tiny and weak.  Horseweed is especially sensitive to rye straw, which releases toxins that poison the seedlings in addition to casting shade (Weaver 2001).  Consequently, straw mulch can be very effective for suppressing horseweed in crops like strawberry and garlic where spring tillage is not an option.

Ecology

Origin and distribution:  Horseweed is native to North America, but is much more widespread now than prior to European settlement.  It occurs throughout southern Canada, the entire U.S.A., Mexico and much of Latin America.  It is widely naturalized in Europe, temperate and tropical Asia, Australia, New Zealand and southern Africa (Holm et al. 1997).

Seed weight:   0.072 mg (Weaver 2001), 0.032 mg (Milberg et al. 2000).

Dormancy and germination:  Most seeds are not dormant when dispersed from the parent plant (Karlssen and Milberg 2007, Weaver 2001).  Chilling is not required for germination (Zinzolker et al. 1985).  Light is the primary requirement for germination of most populations (Karlsson and Milberg 2007, Milberg et al. 2000, Zinzolker et al. 1985).   Seeds germinate best at 68/50 to 86/68 °F (20/10 to 30/20 °C) day/night temperatures (Karlsson and Milberg 2007).  No seeds germinated at 54/43 °F (12/6 °C) (Nandula et al. 2006).  The lowest continuous temperature giving maximum germination was 63 °F (17 °C) (Tozzi et al. 2014).  The base temperature for germination ranged from 49 °F (9.5 °C) for a population from Ontario to 57 °F (14 °C) for a population from Spain (Steinmaus et al. 2000, Tozzi et al. 2014).

Seed longevity:  Horseweed seeds are generally short-lived in the soil.  Horseweed soil seed populations declined by 76% in 10 months from October to August, but 6% of the seeds remained after two years of good weed control (Davis et al. 2007).  Horseweed is present in seed banks of pastures where the plant has not been present above ground for many years (Weaver 2001) and it was the most abundant species in the seedbank after 12 years of a tillage experiment (Bàrbari and Lo Cascio 2001), suggesting that a portion of seeds may last for several years in the soil.  However, these observations could be complicated by seeds that blew in from elsewhere (Bàrbari and Lo Cascio 2001).  Long-distant seed dispersal is probably more important for survival of this species than seed longevity (see Dispersal section below).   

Season of emergence: Many plants emerge in the fall and overwinter as a rosette of leaves (Buhler and Owen 1997, Regehr and Bazzaz 1979, Tozzi and Van Acker 2014, Weaver 2001).  Other plants emerge in the spring, and a few seedlings may emerge throughout the summer (Buhler and Owen 1997, Davis and Johnson 2008, Karlsson and Milberg 2007, Main et al. 2006, Schramski et al. 2021b).  Seed vernalization at 39 °F (4 °C) for 4 weeks probably accounts for emergence in an upright rather than rosette growth form in spring (Schramski et al. 2021a).  Emergence was reported as greatest in fall in cooler climates such as Canada and the upper midwestern U.S.A. (Buhler and Owen 1997, Mobli et al. 2022, Regehr and Bazzaz 1979, Tozzi and Van Acker 2014, Weaver 2001) and was greatest in spring in milder climates such as Indiana and southern Sweden (Davis and Johnson 2008, Karlsson and Milberg 2007).  The lack of seed dormancy means that microsite conditions play an important role in determining whether horseweed functions as a winter or summer annual species (Tozzi and Van Acker 2014).  In Tennessee emergence occurred during months when daytime temperatures were between 50 and 60 °F (10 and 16 °C) leading to peak emergence in the fall at some sites and in the spring at other sites (Main et al 2006).

Emergence depth:  Seedling emergence declines very rapidly with depth of burial.  The majority of seedlings emerge from seeds at the soil surface.  In one experiment, only 4% as many seeds emerged from 0.1” (0.25 cm) as emerged from seeds placed on the soil surface, and no seedlings emerged from greater depths (Nandula et al. 2006).  In another experiment, 10% emerged from 0.4” (1 cm) (Weaver 2001).

Photosynthetic pathway:  C3.

Sensitivity to frost:  Winter annual horseweed plants tolerate frost well and overwinter as a rosette of leaves, even in very cold climates.  During sunny winter weather, leaf temperature may be as much as 18 °F (10 °C) above air temperature, and leaves photosynthesize actively (Regehr and Bazzaz 1976).  Plant survival in Iowa and Minnesota ranged from 59 to 91%, with higher survival rates associated with larger rosettes before winter (Buhler and Owen 1997).  However, in southern Indiana, only plants with small rosettes survived winter, and their survival rate was only 24% (Davis and Johnson 2008).  Likewise, in Sweden few rosettes were found to survive winter (Karlsson and Milberg 2007).  The principle cause of winter mortality is frost heaving (Regehr and Bazzaz 1979), but the specific interactions between soil conditions and rosette size remain unresolved (Davis and Johnson 2008).  Possibly, rosettes survive better in regions with continuously cold conditions and a snow pack than in regions with alternating freezing and thawing temperatures.  

Drought tolerance:  Horseweed tolerates drought well (Weaver 2001).

Mycorrhiza:  Horseweed is mycorrhizal (Harley and Harley 1987, Pendleton and Smith 1983, Weaver 2001).

Response to fertility:  Horseweed commonly colonizes badly eroded, abandoned farm land (Weaver 2001) which indicates a tolerance of low soil fertility and limited response to applied nitrogen.

Soil physical requirements:  Horseweed occurs most commonly on sandy, stony or loam soils with good drainage.  It does not tolerate flooding.  Seeds will germinate in salty environments.  (Weaver 2001)

Response to shade: Horseweed growth is stunted by shading from crops or other weeds (Alcorta et al. 2011).  Plant weight declines approximately linearly with increasing shade (Fisher and Sprague 2023).

Sensitivity to disturbance:  Overwintering rosettes of horseweed are easily killed by spring tillage and the species is generally rare or absent from spring tilled fields (Weaver 2001).  Slow development of the seedling makes the species easily controlled by tine weeding, raking or hoeing.  Mowing prevents or delays seed production (Weaver 2001). 

Time from emergence to reproduction:  Flower stalks of overwintering plants begin elongating in mid spring and flower about two to three months later (Shrestha et al. 2010).  Seeds mature 10 days (Shrestha et al. 2010) to 3 weeks (Weaver 2001) after pollination.  Seed production continues for several weeks.  Spring-emerging plants rapidly pass through a rosette phase resulting in shorter time to bolting and flowering than fall-emerging plants (Tozzi and Van Acker 2014).

Pollination:  Pollen is released before the flowers fully open which promotes self-pollination.  Insects regularly visit the flowers, however, and the outcrossing rate for an Ontario population was estimated at 4%.  (Weaver 2001)

Reproduction:  Flower heads typically produce 35-70 seeds each (Dauer et al. 2007, Tozzi and Van Acker 2014).  The number of flowers produced by a plant depends on plant height.  Thus, fall-emerging plants 16” (40 cm) tall produced about 2,000 seeds whereas plants 5 ft (150 cm) tall produced about 230,000 seeds (Regehr and Bazzaz 1979).  Un-crowded plants on fallow ground produce about 200,000 to 400,000 seeds each (Shrestha et al. 2010, Weaver 2001).  On the other hand, spring-emerging glyphosate-resistant plants growing within a crop canopy had a maximum production of only 10,000 to 60,000 seed per plant (Davis and Johnson 2008, Steckel and Gwathmey 2009).  Plants emerging earliest, either in the fall or spring, had the highest survival rate and produced the most seed (Tozzi and Van Acker 2014).

Dispersal:  The tiny, plumed seeds stayed aloft longer than those of nineteen species tested in a controlled seed drop experiment (Anderson 1993).  Although the majority of seeds remain within 330 ft (100 m) of the source plant, dispersal distances of up to 1,640 ft (500 m) were documented (Dauer et al. 2007).  Some seeds that ascend into the atmosphere by turbulence could be carried 75 miles (120 km) (Dauer et al. 2009) or more (Shields et al. 2006).  Seeds float well on water and probably disperse long distances in streams and irrigation canals (Weaver 2001).  The plumed seeds are also caught and carried on vehicles.

Common natural enemies:  Aster yellows mycoplasma can decrease seed production by over 50% (Regehr and Bazzaz 1979).

Palatability:  The species is not eaten by people.  It is little used by livestock and is irritating to horses (Weaver 2001).

Notes:  Horseweed can produce allelopathic compounds that inhibit forage species including orchardgrass, white clover and alfalfa under controlled environments (Djurdjević et al. 2011).  However, numerous crops have shown no adverse influences when seeded into terminated stands of horseweed under field conditions (M. VanGessel, U. of Delaware, personal communication).

References:

• Alcorta, M., M. W. Fidelibus, K. L. Steenwerth, and A. Shrestha.  2011.  Effect of vineyard row orientation on growth and phenology of glyphosate-resistant and glyphosate-susceptible horseweed (Conyza canadensis).  Weed Science 59:55-60.
• Anderson, M. C.  1993.  Diaspore morphology and seed dispersal in several wind-dispersed Asteraceae.  American Journal of Botany 80:487-492.
• Bàrbari, P., and B. Lo Cascio.  2001.  Long-term tillage and crop rotation effects on weed seedbank size and composition.  Weed Research 41:325-340.
• Brown, S. M., and T. Whitwell.  1988.  Influence of tillage on horseweed, Conyza canadensis.  Weed Technology 2:269-270.
• Buhler, D. D., and M. D. K. Owen.  1997.  Emergence and survival of horseweed (Conyza canadensis).  Weed Science 45:98-101.
• Dauer, J. T., D. A. Mortensen, and M. J. VanGessel.  2007.  Temporal and spatial dynamics of long-distance Conyza canadensis seed dispersal.  Journal of Applied Ecology 44:105-114.
• Dauer, J. T., D. A. Mortensen, E. C. Luschei, S. A. Isard, E. Shields, and M. J. VanGessel.  2009.  Conyza canadensis seed ascent in the lower atmosphere.  Agricultural and Forest Meteorology 149:526-534.
• Davis, V. M., K. D. Gibson, T. T. Bauman, S. C. Weller, and W. G. Johnson.  2007.  Influence of weed management practices and crop rotation on glyphosate-resistant horseweed population dynamics and crop yield.  Weed Science 55:508–516.
• Davis, V. M., and W. G. Johnson.  2008.  Glyphosate-resistant horseweed (Conyza canadensis) emergence, survival, and fecundity in no-till soybean.  Weed Science 56:231-236.
• De Cauwer, B., D. Reheul, I. Nijs, and A. Milbau.  2008.  Management of newly established field margins on nutrient-rich soil to reduce weed spread and seed rain into adjacent crops.  Weed Research 48:102-112.
• Djurdjević, L., M. Mitrović, G. Gajić, S. Jarić, O. Kostić, L. Obedran, and P. Pavlović.  2011.  An allelopathic investigation of the domination of the introduced invasive Conyza canadensis L.  Flora 206:921-927.
• Fisher, J. L., and C. L. Sprague.  2022.  Narrow-row soybean and a cereal rye cover crop suppress glyphosate-resistant horseweed (Conyza canadensis).  Weed Technology 36:781–788. 
• Fisher, J. L., and C. L. Sprague.  2023.  Contributions of shading, soybean (Glycine max) row width, and planting green on horseweed (Conyza canadensis) management compared with soil-applied residual herbicides.  Weed Technology 37:383–393.
• Harley, J. L., and E. L. Harley.  1987.  A check-list of mycorrhiza in the British flora.  New Phytologist 105:1-102.
• Holm, L., J. Doll, E. Holm, J. Pancho, and J. Herberger.  1997.  World Weeds: Natural Histories and Distribution.  John Wiley: New York.
• Karlsson, L. M., and P. Milberg.  2007.  Comparing after-ripening response and germination requirements of Conyza canadensis and C. bonariensis (Asteraceae) through logistic functions.  Weed Research 47:433-441.
• Main, C. L., L. E. Teckel, R. M. Hayes and T. C. Mueller.  2006.  Biotic and abiotic factors influence horseweed emergence.  Weed Science 54:1101-1105.
• Milberg, P., L. Andersson, and K. Thompson.  2000.  Large-seeded species are less dependent on light for germination than small-seeded ones.  Seed Science Research 10:99-104.

• Mobli, A., M. Oliveira, L. Butts, C. Proctor, N. Lawrence, and R. Werle.  2022.  Emergence pattern of horseweed (Erigeron canadensis L.) accessions across Nebraska.  Weed Technology 36:655–662.

• Nandula, V. K., T. W. Eubank,  D. H. Poston, C. H. Koger, and K. N. Reddy.  2006.  Factors affecting germination of horseweed (Conyza canadensis).  Weed Science 54:898-902.
• Pendleton, R. L., and B. N. Smith.  1983.  Vesicular-arbuscular mycorrhizae of weedy and colonizer plant species at disturbed sites in Utah.  Oecologia 59:296-301.
• Pittman, K. B., J. N. Barney, and M. L. Flessner.  2019.  Horseweed (Conyza canadensis) suppression from cover crop mixtures and fall-applied residual herbicides.  Weed Technology 33:303-311.
• Regehr, D. L., and F. A. Bazzaz.  1976.  Low temperature photosynthesis in successional winter annuals.  Ecology 57:1297-1303.
• Regehr, D. L., and F. A. Bazzaz.  1979.  The population dynamics of Erigeron canadensis, a successional winter annual.  Journal of Ecology 67:923-933.
• Schramski, J. A., C. L. Sprague, and E. L. Patterson.  2021a.  Environmental cues affecting horseweed (Conyza canadensis) growth types and their sensitivity to glyphosate.  Weed Science 69:412–421.
• Schramski, J.A., C. L. Sprague, and K. A. Renner.  2021b.  Effects of fall-planted cereal cover-crop termination time on glyphosate resistant horseweed (Conyza canadensis) suppression.  Weed Technology 35:223–233.
• Schreier, H., M. Bish, and K. W. Bradley. 2022. The impact of electrocution treatments on weed control and weed seed viability in soybean. Weed Technology 36:481–489.
• Sherman, A. D., E. R. Haramoto, and J. D. Green.  2020.  Integrating fall and spring herbicides with a cereal rye cover crop for horseweed (Conyza canadensis) management prior to soybean.  Weed Technology 34:64–72.
• Shields, E. J., J. T. Dauer, M. J. VanGessel, and G. Neumann.  2006.  Horseweed (Conyza canadensis) seed collected in the planetary boundary layer.  Weed Science 54:1063-1067.
• Shrestha, A., B. D. Hanson, M. W. Fidelibus, and M. Alcorta.  2010.  Growth, phenology, and intraspecific competition between glyphosate-resistant and glyphosate-susceptible horseweeds (Conyza canadensis) in the San Joaquin Valley of California.  Weed Science 58:147-153.
• Sosnoskie, L. M., E. C. Luschei, and M. A. Fanning.  2007.  Field margin weed-species diversity in relation to landscape attributes and adjacent land use.  Weed Science 55:129-136.
• Steckel, L. E., and C. O. Gwathmey.  2009.  Glyphosate-resistant horseweed (Conyza canadensis) growth, seed production, and interference in cotton.  Weed Science 57:346-350.
• Steinmaus, S. J., T. S. Prather, and J. S. Holt.  2000.  Estimation of base temperatures for nine species.  Journal of Experimental Botany 51:275-286.
• Tozzi, E., and R. C. Van Acker.  2014.  Effects of seedling emergence timing on the population dynamics of horseweed (Conyza canadensis var. canadensis).  Weed Science 62:451-456.
• Tozzi, E., H. Beckie, R. Weiss, J. L. Gonzalez-Andujar, J. Storkey, S. Z. H. Cici, and R. C. Van Acker.  2014.  Seed germination response to temperature for a range of international populations of Conyza canadensis.  Weed Research 54:178-185.
• Wallace, J. M., W. S. Curran, and D. A. Mortensen.  2019.  Cover crop effects on horseweed (Erigeron canadensis) density and size inequality at the time of herbicide exposure.  Weed Science 67:327-338.
• Weaver, S. E.  2001.  The biology of Canadian weeds. 115. Conyza canadensis L. Cronquist.  Canadian Journal of Plant Science 81:867-875.
• Zinzolker, A., J. Kigel, and B. Rubin.  1985.  Effects of environmental factors on the germination and flowering of Conyza albida, C. bonariensis, and C. canadensis.  Phytoparasitica 13:229-230.