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Waterhemp - The Perfect Weed?
Bob Hartzler

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December 18, 2003   (Article prepared for 2004 Weed Science Society of America - North America Weed Management Association Invasive Plant Species Workshop, Kansas City, Feb. 12-13.)

Waterhemp currently is one of the most troublesome weeds for farmers in the western Cornbelt.  While the plant is native to the Midwest, many of the characteristics that allow it to proliferate in agronomic fields enable it to invade other disturbed habitats.  This paper will provide a brief review of some of the biological traits that have let to the success of this species.

 Classification and distribution.  Waterhemp is a member of the Amaranthus genus which includes approximately 75 species of worldwide distribution, including the pigweeds, waterhemps and grain amaranths.  (Pratt and Clark, 2001).  This genus has been subdivided into two subgenera, Amaranthus which contains the monecious amaranths and the Acnida which contains the dioecious amaranths (Robertson, 1981).   A dioecious species has individual plants that have either male or female flowers, wheras monecious species have both male and female flowers on the same plant.   Mosyakin and Robertson recently proposed a third subgenera for the genus, the Albersia.

Traditionally waterhemp has been divided into two species, common waterhemp (Amaranthus rudis Sauer) and tall waterhemp (Amaranthus tuberculatus (Moq.) Sauer) (Sauer 1955).  Common waterhemp was described as occurring from Nebraska south to Texas, whereas tall waterhemp prevailed from Indiana east to Ohio.  Both species were common in the area encompassed by Iowa, Illinois and Missouri.   Pratt and Clark (2001) investigated the regional variations in morphology and isozymes among waterhemp populations in the Midwest.  They suggested that waterhemp is a single, highly polymorphic species (Amaranthus tuberculatus (Moq.) Sauer).

While waterhemp is native to the region, until the 1980’s it was considered a relatively obscure species (Pratt and Clark, 2001).  The primary pigweed species reported as a weed in agronomic fields in the 1940’s -1980’s was redroot pigweed (Amaranthus retroflexus).  It is unclear whether the lack of reports of waterhemp as a weed is due to it not being adapted to production practices of the time or due to the lackadaisical taxonomic practices common among farmers and agronomists (i.e. I don’t care what it is, I just want to kill it).  Regardless, there is no doubt that waterhemp populations in corn and soybean fields exploded in the late 1980’s and 1990’s.  Weed shifts are common in agriculture, but the speed in which waterhemp rose to prominence is somewhat unique.

Weedy traits  Baker (1974) developed a list of  twelve characteristics that he felt would be found in the ideal weed.    Baker stated that no single species possesses all of these characteristics, but that the weediness of a species is directly related to the number of weedy traits it possessed.  The following are traits described by Baker  possessed by waterhemp:

1)      Germination under many environments – Like most weeds, waterhemp seed can germinate under a wide range of environmental conditions.  Research has shown that waterhemp seed have a light requirement, as do many small-seeded species, but that the relative importance of light is diminished at high temperatures (Leon and Owen 2003).  Waterhemp seed germinate under a wide range of temperature and soil conditions.

      Discontinuous germination – Prolonged emergence is a favorable trait for survival in habitats with frequent mortality events.  Waterhemp emerges throughout the growing season, and typically does not produce large flushes representing a majority of the cumulative emergence for the season.  The emergence of patterns of waterhemp and giant ragweed are depicted in Figure 1.  Giant ragweed reached 80% of cumulative emergence in the second week after emergence initiated, whereas waterhemp did not reach 80% emergence until ten weeks after the first seedling emerged.  The prolonged emergence prevents mortality events occurring during the spring and early summer from eliminating a waterhemp infestation in a field.

Figure 1.  Weekly emergence of waterhemp and giant ragweed in Ames, IA.  2000.  Sandell, Buhler and Hartzler, Iowa State University.

3)  Long-lived seeds – While there have been relatively few studies investigating the longevity of waterhemp seed, evidence suggests the seeds are persistent.   In a Nebraska study, one to three percent of seed buried at an 8” depth for 17 years germinated (% viable seed was not determined) (Burnside et al. 1996).  Twelve percent of waterhemp seed remained viable after 4 years of burial in the upper 5 cm of soil in an Iowa study, compared to 5, 0 and 0% for velvetleaf, giant foxtail and woolly cupgrass (Buhler and Hartzler, 2001).

4)  Cross-pollination not requiring specialized vectors -  Since waterhemp is a dioecious species, it can only reproduce via cross-pollination (pollen moving from a male plant to the flowers on a female plant).  Waterhemp is wind pollinated, thus not requiring an insect or other pollinating agent for fertilization.  The pollen is very light and is carried long distances by wind currents.  Cross-pollination results in high variability within the species, resulting in a myriad of biotypes suited for a range of environments.

5)  Very high seed output in favorable conditions – Most weedy species have the ability to produce abundant seed, but this is a feature in which waterhemp truly excels.  Waterhemp plants emerging at the same time as soybean typically produce more than 300,000 seeds per plant (Hartzler et al. 2004).  Under ideal conditions a plant competing with soybean can produce up to 5 million seeds.  In contrast, velvetleaf typically produces less than 4000 seeds when competing with soybean.

5)  Produces some seed during unfavorable conditions – Plasticity, the ability to adjust to environmental conditions, is another characteristic of waterhemp.  The prolonged emergence of waterhemp allows plants to escape mortality events that occur early in the growing season.  While an effective avoidance mechanism, late-emergence often places waterhemp at a competitive disadvantage with plants that emerge earlier in the growing season.  However, waterhemp growing under these unfavorable conditions retains the ability to produce abundant seed.  Waterhemp emerging seven weeks after soybean emergence was still able to produce 3000 seed per plant (Hartzler et al. 2004).

The dioecious trait is often considered a disadvantage for colonizing plants since it requires two plants (a male and female) to reproduce.  In the highly disturbed habitats that waterhemp and other annual weeds exist it is not uncommon for single plants to survive.  The dioecious pigweeds have a few adaptations that reduce the risk of failure of isolated plants to reproduce.  First, the pollen is carried long-distances by wind, increasing the likelihood that a solitary female plant will be contacted by pollen.  Second, female plants may alter their growth habit when deprived of pollen.  Female plants of  Amaranthus cannabinus,  another dioecious pigweed, were found to delay flowering and senesce later in the season when pollen was withheld (Quinn, Bram and Taylor 2000).  These traits minimize the risk of isolated plants failing to produce seed.

6)  Rapid growth – Although the seedlings of waterhemp are rather frail due to the small seed size of this species, once waterhemp seedlings become established they are able to successfully compete with other plants due to a rapid growth rate.   The relative growth rate of waterhemp is 50-70% greater than that of other annual weedy species (Horak and Loughin, 2000; Seibert and Pearce, 1993;).  This rapid growth allows waterhemp to overcome the initial advantage possessed by plants developing from reproductive structures with larger energy reserves.

7)  Adaptations for short and long-distance dispersal - Seed of waterhemp do not have any specialized structures that favor dispersal by wind or animals.  However, the small seed are readily transported when equipment is moved from field to field.  The prolific seed production also increases the likelihood of seed being dispersed over long distances.

8)  Propensity for herbicide resistance – Although not one of Baker’s weedy traits, herbicide resistance is a characteristic that has contributed to the weediness of waterhemp in agronomic systems.  A survey of waterhemp populations in Illinois found that 90% of the populations contained individuals resistant to ALS-inhibiting herbicides and 24% of the populations possessed resistance to the triazine herbicides (Patzoldt et al. 2002).  Fifteen percent of the populations included individuals with resistance to both of theses herbicide classes.   More recently, waterhemp populations resistant to diphenyl ether herbicides have been identified in Illinois and Kansas (Shoup et al.  2003).   Populations with resistance to all three classes of herbicides also have been identified.  No other species in the Midwest has developed such widespread herbicide resistance.  Plateau (imazapic), a herbicide commonly used to aid the establishment of native vegetation, is an ALS-inhibiting herbicide, and therefore ineffective against most waterhemp populations. 

Summary   Waterhemp possesses numerous biological traits that allow it to rapidly invade any disturbed habitat where permanent vegetation has been damaged or eliminated.  The weak point in the species’ life cycle is the transition from seed to seedling.  Due to the small seed size, waterhemp is poorly adapted to establishment in habitats with dense, established vegetation.  Cultural practices that promote growth of the desired vegetation are the best tactic for managing waterhemp.  Although many herbicides are effective against waterhemp, prolonged emergence, prolific seed production, and persistent seed make it likely that waterhemp will reinfest areas where herbicides are used as the lone tactic to control the weed.

Literature Cited

Baker, H.G.  1974.  The evolution of weeds.  Ann. Rev. Ecol. Syst. 5:1-24.

Buhler, D.D. and R.G. Hartzler.  2001.  Emergence and persistence of seed of velvetleaf, common waterhemp, woolly cupgrass, and giant foxtail.  Weed Sci. 49:230-235.

Burnside, O.C., R.G. Wilson, S.Weisberg and K.G. Hubbard.  Seed longevity of 41 weed species buried 17 years in eastern and western Nebraska.  Weed Sci.  44:74-86.

Hartzler, R.G., B.A. Battles and D. Refsell.  2004.  Effect of common waterhemp emergence date on growth and fecundity in Glycine max.  Weed Sci.  52: In press.

Horak, M.J. and T.M. Loughin.  2000.  Growth analysis of four Amaranthus species.  Weed Sci.  48:347-355.

Leon, R.G. and M.D.K. Owen.  2003.  Regulation of weed seed dormancy through light and temperature interactions.  Weed Sci.  51:752-758.

Mosyakin, S. L. and K. R. Roberson.  New infrageneric taxa and combinations in Amaranthus (Amaranthaceae).  Ann. Bot. Fenn. 33:275-281.

Patzoldt, W.L., P.J. Tranel and A.G. Hager.  2002. Variable herbicide responses among Illinois waterhemp (Amaranthus rudis and A. tuberculatus) populations.  Crop Protect. 21:707-712.

Pratt, D.B. and L.G. Clark. 2001.  Amaranthus rudis and A. tuberculatus – one species or two?  J. Torrey Bot. Soc. 128:282-296.

Quinn, J.A., M.R. Bram and T.E. Taylor.  2000.  Female resource allocation in response to pollen availability in plants from freshwater and salt marsh populations of Amaranthus cannabinus.  J. Torrey Bot. Soc. 127:83-86.

Robertson, K.R. 1981.  The genera of Amaranthaceae in the southeastern United States.  J. Arnold Arbor. 62:267-314.

Sauer, J.D.  1955.  Revision of the dioecious amaranths.  Madrono 21:426-434.

Seibert, A.C. and R.B. Pearce.  1993.  Growth analysis of weed and crop species with reference to seed weight.  Weed Sci.  41:52-56.

Shoup, D.E., K. Al-Khatib and D.E. Peterson.  2003.  Common waterhemp resistance to protoporphrinogen oxidase-inhibiting herbicides.  Weed Sci. 51:145-150.


Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
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Fax: (515) 294-9985
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