Waterhemp: biology and control

A ten-leaf hairless seedling plant with narrower, wavy margined leaves compared to other pigweed species.


INDEX

  1. Waterhemp identification
  2. Ontario counties with confirmed resistant populations (May 2022)
  3. Testing services to identify waterhemp and if herbicide resistant
  4. Waterhemp control in corn
  5. Waterhemp control in soybean
  6. Waterhemp control in winter wheat
  7. Influence of cover crops on waterhemp
  8. Influence of tillage on waterhemp
  9. Acknowledgments


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SPECIES INFORMATION
Lifecycle: Annual
Propagation: Reproduces by seed, dioecious, has separate male and female plants.
Emergence: Spring, typically after lamb’s-quarters, emergence continues through late summer or early autumn depending on weather conditions.
Habitat: Not native to Canada. Now found in several counties throughout Ontario in agricultural fields. Historically it was found mainly along waterways and beaches. Prefers full or partial sun, moist or wet conditions.
Competitiveness: In University of Guelph research trials, poor control of waterhemp has resulted in up to 99 percent yield loss in both corn and soybeans.

IDENTIFICATION CLUES
Seedlings
Cotyledons: Linear, narrow, purple-red underneath.
Mature leaves: Lance-shaped (tapered and typically with a pointed tip), with a wavy margin.
Mature Plant
Stem: Alternate leaf orientation; smooth with no hairs; variable colouring (green to red/purple); up to 3.5 m in height.
Seed heads: Long, spikey clusters (somewhat similar to ragweed) that can vary in colour from green to red to purple. Only the female plants produce seed.
Roots: Taproot

OFTEN MISTAKEN FOR
I know it’s NOT … because…

Redroot pigweed … The stem of redroot pigweed is covered in dense short hairs, whereas waterhemp is smooth and hairless. The tips of waterhemp leaves are typically not notched.

Green pigweed … The upper stem of green pigweed has a cluster of dense hairs whereas the stem of waterhemp is smooth and hairless. Waterhemp will typically have narrower leaves with wavy margins and without a notched tip.

Eastern black nightshade … The leaf margins of young eastern black nightshade leaves are hairy and the older leaves have wavy toothed margins whereas waterhemp leaves are hairless with wavy margins.

A ten-leaf hairless seedling plant with narrower, wavy margined leaves compared to other pigweed species.
A ten-leaf hairless seedling plant with narrower, wavy margined leaves compared to other pigweed species.
A close-up of a mature and narrow waterhemp leaf
A close-up of a mature and narrow waterhemp leaf
The completely hairless and smooth stem of waterhemp
The completely hairless and smooth stem of waterhemp
The very long, fingerlike spikes of the female seed head
The very long, finger-like spikes of the female seed head
Male (left) and female (right) plants with reddish-purple flowers in a Chatham-Kent county soybean field
Male (left) and female (right) plants with reddish-purple flowers in a Chatham-Kent county soybean field
Waterhemp plants found in a soybean field in Norfolk county during the 2019 growing season. They were eventually confirmed to be resistant to group 2, group 9 and group 14 herbicides.
Waterhemp plants found in a soybean field in Norfolk county during the 2019 growing season. They were eventually confirmed to be resistant to group 2, group 9 and group 14 herbicides.
Waterhemp in a soybean field weeks after glyphosate had been applied. All other weeds died.
Waterhemp in a soybean field weeks after glyphosate had been applied. All other weeds died.


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Ontario Counties with herbicide resistant waterhemp

Waterhemp that is resistant to multiple herbicide groups is common in Ontario.


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Testing services to identify waterhemp and determine if herbicide resistant

There are two testing solutions available in Ontario. The plant bioassay service offered by the Tardif lab at the University of Guelph requires that mature seed is collected in the fall and submitted. Plants are then grown from that seed and sprayed at various doses of the herbicide(s) suspected. Results are provided typically by March, the service is provided at no cost to Ontario farmers. The molecular testing service, requires plant tissue that is roughly “loonie sized”, so a single leaf is often adequate. Results can be obtained in a matter of days, but it is a fee based service.

  1. Free plant bioassay testing by the University of Guelph
  2. Fee based molecular testing by Harvest Genomics (private lab)


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CONTROL IN CORN

Control of waterhemp is best achieved with a two-pass herbicide program where the first pass is with a pre-emergence (PRE) soil applied herbicide and the second with a postemergent (POST) herbicide. Consistency of waterhemp control with soil applied herbicides has varied and one should not expect perfect season long control. However, a soil applied herbicide will eliminate the majority of early seedlings and will reduce the risk of having large, less susceptible seedling plants emerged at the time of the post-emergence herbicide pass.

CORN: PREEMERGENCE HERBICIDES

HerbicideWSSA GroupControl (%)Corn Hybrid Trait
ACURON5,15,27,2792All
INTEGRITY14,1591All
CONVERGE XT5,2789All
CALLISTO + AATREX5,2786All
PRIMEXTRA II MAGNUM5,1585All
ACURON FLEXI15,27,2782All
HARNESS (acetochlor – not in Canada)1582All
MARKSMAN4,579All
ZIDUA SC1576All
ENGARDE2,2773All
FRONTIER MAX1572All
DUAL II MAGNUM1569All
AATREX 480561All
ENGENIA (dicamba)449All
BROADSTRIKE RC244All
Efficacy ratings (percent control) were taken 8 weeks after application. Source: Dr. P.H. Sikkema, University of Guelph (Ridgetown)

Waterhemp control several weeks after a preemergence application of Acuron herbicide. (Source: Dr. Peter Sikkema, University of Guelph - Ridgetown)
Waterhemp control several weeks after a preemergence application of Acuron herbicide. (Source: Dr. Peter Sikkema, University of Guelph – Ridgetown)
Waterhemp control several weeks after a preemergence application of Integrity herbicide. (Source: Dr. Peter Sikkema, University of Guelph - Ridgetown)
Waterhemp control several weeks after a preemergence application of Integrity herbicide. (Source: Dr. Peter Sikkema, University of Guelph – Ridgetown)

CORN: POSTEMERGENCE HERBICIDES

HerbicideWSSA GroupControl (%)Corn Hybrid Trait
ACURON5,15,27,2796All
LAUDIS + AATREX 4805,2794All
ACURON FLEXI15,27,2792All
SHIELDEX + AATREX 4805,2792All
CONVERGE XT5,2792All
CALLISTO + AATREX 4805,2791All
HALEX GT + AATREX 4805,9,15,2789All
ENLIST DUO2,989E3 (Enlist)
ENGENIA (dicamba – high rate)484All
PRIMEXTRA II MAGNUM5,1583All
ARMEZON + AATREX 4805,2782All
MARKSMAN4,581All
PARDNER + AATREX 4805,680All
DESTRA IS2,2778All
2,4-D477All
ENGENIA (dicamba – low rate)471All
DISTINCT4,1969All
LIBERTY 200 SN1064E3 (Enlist), Liberty Link
VIOS G32,2762All
AATREX 480562All
PERMIT227All
Efficacy ratings (percent control) were taken 8 weeks after application. Source: Dr. P.H. Sikkema, University of Guelph (Ridgetown)


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CONTROL IN SOYBEAN

Control of waterhemp is best achieved with a two-pass herbicide program where the first pass is with a pre-emergence (PRE) soil applied herbicide and the second with a postemergent (POST) herbicide.

SOYBEANS: PREEMERGENCE HERBICIDES

HerbicideWSSA GroupControl (%)Soybean Variety Trait
FIERCE14,1590All
VALTERA14,1583All
TRIACTOR2,5,1482All
BIFECTA5,1481All
AUTHORITY SUPREME14,1580All
TAVIUM4,1579RR 2Y Xtend
BOUNDARY LQD5,1578All
CANOPY PRO2,576All
ZIDUA1574All
DUAL II MAGNUM1572All
INTEGRITY14,1566All
FRONTIER1565All
SENCOR560All
OPTILL2,1456All
AUTHORITY1455All
ERAGON LQ1447All
ENGENIA (dicamba)443RR 2Y Xtend
FREESTYLE2,225All
PROWL H2O324All
PURSUIT217All
CLASSIC210All
Efficacy ratings (percent control) were taken 8 weeks after application. Source: Dr. P.H. Sikkema, University of Guelph (Ridgetown)
Waterhemp control several weeks after a preemergence application of Fierce (left) compared to an un-sprayed strip (right). (Source: Dr. Peter Sikkema, University of Guelph - Ridgetown)
Waterhemp control several weeks after a preemergence application of Fierce (left) compared to an un-sprayed strip (right). (Source: Dr. Peter Sikkema, University of Guelph – Ridgetown)

SOYBEANS: Postemergence herbicides

HerbicideWSSA GroupControl (%)Soybean Variety Trait
TAVIUM4,1580RR 2Y Xtend
ENLIST DUO4,975E3 (Enlist)
ENGENIA (dicamba)474RR 2Y Xtend
HURRICANE6,1462All
BLAZER1459All
REFLEX1456All
LIBERTY 200 SN1045E3 (Enlist), Liberty Link
PINNACLE233All
CLASSIC226All
FIRSTRATE222All
CLEANSWEEP2,621All
BASAGRAN FORTE620All
PURSUIT25All
Efficacy ratings (percent control) were taken 8 weeks after application. Source: Dr. P.H. Sikkema, University of Guelph (Ridgetown)

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CONTROL IN CEREALS

Overall, the experience in Ontario has been that winter cereals are a good rotational crop for reducing waterhemp populations. A limited amount of research has been completed in Ontario primarily because waterhemp has not been a problem in winter wheat thus far. This could be, in part, because the fall seeded winter wheat crop tends to out-compete spring emerging summer annuals like pigweed species which include waterhemp.

During a plot tour of Dr. Sikkema's waterhemp control studies, a participant, Joe Vink, observed how there was little waterhemp in the neighbouring winter wheat crop compared with the soybean plots.
During a plot tour of Dr. Sikkema’s waterhemp control studies, a participant, Joe Vink, observed how there was little waterhemp in the neighbouring winter wheat crop compared with the soybean plots.

However, if waterhemp recruitment is a concern, Eragon LQ has been cited as being effective on waterhemp (Knezevic et al., 2010) and can be applied pre-plant to winter wheat. Otherwise, it would be logical to assume that cereal herbicides that are the most effective at controlling pigweed, would also be effective at controlling waterhemp should it show up in your winter wheat crop.



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INFLUENCE OF COVER CROPS
An Illinois study found that fall seeded cereal rye that was crimped prior to planting soybean consistently reduced biomass of waterhemp over a 3 year period compared to vetch or no cover crop (Davis, 2010). The main challenge with using cereal rye as a weed suppressing cover crop is that its thick residue can negatively affect soybean stand establishment which can result in variability of soybean yield as observed by Davis (2010) and Liebel et al., (1992). This should not be viewed as a deal breaker but rather the importance of residue management at the time of planting which has become easier with improvements in equipment units like strip tillers and row cleaners.



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INFLUENCE OF TILLAGE
Multiple studies have shown that waterhemp emergence is greater in no-till cropping systems compared to ones that include tillage (Refsell and Hartzler, 2009). An Iowa study demonstrated that duration of waterhemp emergence was 26 days longer in a no-till cropping system compared to one that used a chisel-plough (Refsell and Hartzler, 2009).



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Acknowledgments: Dr. François Tardif co-wrote  the identification section and Dr. Peter Sikkema provided the herbicide efficacy data.

Further reading: Waterhemp was first identified in an Ontario field in 2003, when Dan Foster came upon some odd looking pigweed in a soybean field. You can read more about the “early days” of waterhemp in Ontario in this article.