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Iowa State University Capstones, Theses and
Creative Components Dissertations
Spring 2021
Waterhemp Flowering Guide-Paper and Guide
Brent Larson
Follow this and additional works at: https://lib.dr.iastate.edu/creativecomponents
Part of the Other Education Commons
Recommended Citation
Larson, Brent, "Waterhemp Flowering Guide-Paper and Guide" (2021). Creative Components. 759.
https://lib.dr.iastate.edu/creativecomponents/759
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0
Waterhemp Flowering Guide
By
Brent Larson
A creative component submitted to the graduate faculty
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
Major: Agronomy
Program of Study Committee:
Robert Hartzler, Major Professor
Allen Knapp
Iowa State University
Ames, Iowa
2021
1
Project origin
The idea for my creative component project goes back to my childhood on a family grain farm in
southwest Minnesota. Like many children who grew up before Roundup Ready technology, I had the
privilege of hand weeding soybean fields. Even after the advent of Roundup Ready technology in 1996,
my family continued hand weeding, growing conventional soybeans to capture premiums until about
2012. The hand weeding continues today on the Larson Farm in Roundup Ready soybeans as a tool to
manage late escapes. It is through this hand weeding experience that I learned weed identification.
Identifying weeds became relatively easy, but there was still a lot of confusion about when it was safe to
leave an uprooted weed in the field to dry out or when to remove it because viable seed might be
attached to the plant.
One of the main weeds that caused confusion was waterhemp {Amaranthus tuberculatus (Moq.)
Sauer}. We did not know that waterhemp is a dioecious plant (Waselkov and Olsen, 2014). Thus, plants
are either male or female. Since male plants only produce pollen, they need not be removed from the
field because they will not add to the seed bank. Female plants can be left in the field after pulling if
viable seeds have not yet been formed. Determining if a female waterhemp plant has viable seeds can
be a challenge. This causes a lot of confusion as to whether uprooted plants should be removed from
the field or if they can be left uprooted to dry out.
My goal is to create a guide that farmers can use to increase the efficiency of hand weeding.
Waterhemp seed becomes viable relatively soon after pollination. Bell and Tranel (2010) conducted
germination tests on waterhemp seed harvested in one day increments after pollination. They
discovered that the waterhemp seed starts becoming viable nine days after pollination. Based on this
knowledge, I decided to take pictures daily of waterhemp plants as they went through the reproductive
process. My goal with these pictures was to make a guide using waterhemp flowers for growers to
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reference and determine the presence of viable seed. In hopes of making the guide clearer and more
concise, I also decided to take pictures of male vs female flowers to help growers make that distinction.
In 2015, a majority of farmers surveyed in the Midwest stated that waterhemp was their worst
weed (AgWeb 2015). Because of its dioecious reproduction, waterhemp adapts quickly to control
measures. It emerges late, grows rapidly, and has evolved resistance to most herbicides used in the
corn-soybean rotation. Since I work in Ag retail, I have the privilege of helping farmers manage their
pest problems. In my experiences throughout the northwestern Cornbelt, no other weed or pest takes
up more time and resources for control than waterhemp.
Waterhemp Biology and Adaptation
Waterhemp is native to the Midwest. In the early part of the 20 th century, it was generally
found in wetland and stream areas. Redroot pigweed and smooth pigweed were the dominant
Amaranthus species infesting crop fields at that time. In the 1990s, waterhemp rose to preeminence as
an agricultural weed (Hartzler, 2019). Waterhemp was considered two separate species, common
waterhemp (Amaranthus rudis) and tall waterhemp (Amaranthus tuberculatus). The difference
between the two species was based on minor flowering characteristic differences (Hagar and Sprague,
2001). However, waterhemp was recently reclassified as a single species (Amaranthus tuberculatus
(Moq.) Sauer) (Pratt and Clark, 2001).
Waterhemp has thrived through the transition from traditional moldboard plowing to reduced-
till, strip-till, or no-till (Waselkov and Olsen, 2014). Waterhemp is adapted to reduced tillage since the
discontinued use of moldboard plowing leaves most waterhemp seeds near the soil surface where the
seeds readily sprout. Before the mid-1980s, in-row cultivation helped reduce waterhemp pressure.
Most producers in conventional agriculture today do not cultivate anymore, further exacerbating the
problem.
3
Waterhemp also has developed resistance to many of the herbicides used for its control. It is
resistant to seven different herbicide sites of action, and biotypes rapidly accumulate multiple
resistances. These sites of action include photosystem II inhibitors, ALS inhibitors, PPO inhibitors, EPSP
synthase inhibitors, HPPD inhibitors, growth regulators, and long-chain fatty acid inhibitors (Heap,
2021).
Unless new herbicides are developed, farmers will need to consider alternative methods of
weed control. An alternative method worth considering is hand pulling weeds. Hand pulling has been
and always will be a valuable cultural practice for weed control. As long as the plant is not massive,
waterhemp is relatively easy to pull out of the ground. This guide will provide more information about
how to avoid unnecessary work when hand pulling. Plants that do not contain viable seed can simply be
pulled and left in the field, rather than removed. Sometimes the philosophy farmers use is “better safe
than sorry” when hand-pulling, but the problem with this is it greatly increases the time requirement if
the removal in unnecessary.
Flowering Biology
Waterhemp is a dioecious species; plants have either male flowers or female flowers.
Waterhemp flowering starts with a spike on the terminal bud at the top of the plant. Both male and
female waterhemp florets are small, less than 0.3 cm long. Both are surrounded by narrow bracts that
end in a point. The unscented flowers vary in color from green to red to pink (Hilty, 2020). As flowering
continues, new spikes develop in the leaf axils. As the spikes get longer, individual florets are grouped in
a cluster called glomerules, and the glomerules separate from each other. The glomerules are more
condensed and densely arranged on the female spikes than on the male spikes.
Stigma on female plants become receptive to pollen 10-11 days after the first flowers appear
(Owen and Wu, 2014). Approximately nine days after pollination, the seeds become viable (Bell and
4
Tranel, 2010). On male plants, pollen sheds about ten days after the flowers appear. Waterhemp pollen
can remain viable for up to five days, and it can pollinate stigma on female plants up to 800 meters away
(Davis et al., 2012). The female flowers are supplanted by capsules that break and discharge the seeds.
Every capsule has a single small seed shaped like a flat disc. These seeds are shiny and black to dark
brown in color (Hilty, 2020). The total number of seeds produced by a waterhemp plant is highly
correlated to the biomass of that plant. Seeds are generally retained in the florets when the weed and
the crop reach maturity. In Arkansas soybean fields, nearly 100% of waterhemp seeds are retained in
the florets of waterhemp plants at soybean maturity (Norsworthy et al, 2016). This characteristic of
waterhemp allows for the removal of late maturing or fully mature waterhemp plants at the end of the
growing season without the risk of leaving a high percentage of seed in the field.
Materials and Methods
A site was selected in an agricultural field near Dawson, Minn. that had heavy waterhemp
pressure. A 93 m2 area was left fallow during the spring and summer of 2020. Waterhemp seedlings
were allowed to germinate and grow. After they grew to a height of 13 cm, they were thinned to one
seedling per 1.1 m2. Weekly weeding was performed to keep other waterhemp and weeds from
encroaching in the 1.1 m2 area. At the initiation of flowering, pictures were taken daily of twelve plants’
terminal inflorescences against a white backdrop. Multiple angles of each plant were taken. The
pictures were taken around 6-7 PM each day. Pictures were taken from July 6 through August 16, 2020.
During the flowering process, each plant was categorized as male and female.
Two plants of each sex were selected for use in the guide. One picture from each day was
selected and cropped and compiled for use in the guide. The guide has a section showing the
progression of a female beginning with the appearance of flowering. There is a picture every day from
seven days before pollination leading up to the start of pollination. Pollination is indicated by the
emergence of stigma. The pictures then progress from one day after pollination (DAP) to 16 DAP. The
5
same process was used with a male plant. The first picture captured is seven DBP and the last one is 12
DAP. Finally, the last section of the guide compares male to female plants. Male and female plants are
compared at pollination start, five DAP, and nine DAP.
Use of Guide/Conclusion
The goal has been to make a useful guide for farmers who are dealing with inadequate
waterhemp control later in the season. The goal of this project is to help farmers determine when they
need to remove waterhemp plants from the field based on flowering progression by use of concise and
easy to understand guide. In addition, the guide will help farmers distinguish between male and female
waterhemp plants. This will help prevent unnecessary removal of male plants from the field. Farmers
may still want to uproot the male plant to stop the weed competition, but they would not need to worry
about the male plant producing seed if it is left in the field. My hope is that after this guide is vetted by
industry experts, it could be printed for farmers as a free resource to them. A laminated copy would
make it more durable so it could stay in a farmer’s pickup or tractor. This guide will be another tool in
the toolbox for farmers to use in the battle against waterhemp.
Acknowledgments
I would like to thank Bob Hartzler for his help in guiding me through this project.
6 References AgWeb. 2015. “The Worst Weed, According to Farmers.” August 20. Accessed November 6, 2019. https://www.agweb.com/article/the-worst-weed-according-to-farmers-NAA-ben-potter. Bell, Michael S., and Patrick J. Tranel. "Time Requirement from Pollination to Seed Maturity in Waterhemp ( Amaranthus Tuberculatus )." Weed Science 58, no. 2 (2010): 167-73. Hager, Aaron, And Sprague, Christy. 2001. “Waterhemp--Biology, Identification, and Management Considerations.” April 27. Accesed December 29, 2020. http://bulletin.ipm.illinois.edu/pastpest/articles/200105l.html#:~:text=Specifically%2C%20the%20only% 20way%20to,(utricle)%20fractures%20when%20separated. Hartzler, Bob. “Waterhemp: A 'Friendly' Native Evolves into the Cornbelt’s Worst Weed Problem.” Integrated Crop Managment. Iowa State University, November 11, 2019. https://crops.extension.iastate.edu/blog/bob-hartzler/waterhemp-friendly-native-evolves-cornbelts- worst-weed- problem#:~:text=Waterhemp%20(Amaranthus%20tuberculatus)%20is%20native,commonly%20found% 20in%20crop%20fields. Heap, Ian. “Herbicide Resistant Tall Waterhemp Globally (Amaranthus Tuberculatus (=A. Rudis)).” International Herbicide-Resistant Weed Database. WeedScience.org, January 1, 2021. http://weedscience.org/Pages/Species.aspx. Hilty, John. Water Hemp (Amaranthus rudis). Illinois Wildflowers. Accessed February 1, 2021. http://www.illinoiswildflowers.info/weeds/plants/water_hemp.htm. Liu, Jianyang, Adam S. Davis, and Patrick J. Tranel. "Pollen Biology and Dispersal Dynamics in Waterhemp (Amaranthus Tuberculatus)." Weed Science 60, no. 3 (2012): 416-22. Norsthworthy et al. "Tall Waterhemp (Amaranthus Tuberculatus) and Palmer Amaranth (Amaranthus Palmeri) Seed Production and Retention at Soybean Maturity." Weed Technology 30, no. 1 (2016): 284- 90. Pratt, Donald B., and Lynn G. Clark. “Amaranthus Rudis and A. Tuberculatus, One Species or Two?” Journal of the Torrey Botanical Society 128, no. 3 (2001): 282–96. https://doi.org/10.2307/3088718. Sosnoskie, Lynn. “Pigweed Identification (a Quick Guide).” ANR Blogs. Univiersity of California Ag and Natural Resources, June 13, 2018. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=27466. Waselkov, Katherine E, and Kenneth M Olsen. "Population Genetics and Origin of the Native North American Agricultural Weed Waterhemp (Amaranthus Tuberculatus; Amaranthaceae)." American Journal of Botany 101, no. 10 (2014): 1726-1736. Wu, Chenxi, and Owen, Micheal D. K. "When Is the Best Time to Emerge: Reproductive Phenology and Success of Natural Common Waterhemp (Amaranthus Rudis) Cohorts in the Midwest United States?" Weed Science 62, no. 1 (2014): 107-17.
Farmer Guide
to
Waterhemp
Flowering
Brent Larson
Waterhemp Flowering Key Points
• Waterhemp is termed the “worst weed’” by a majority of farmers in the Midwest (AgWeb 2015)
• A diverse set of tactics is needed to control waterhemp. Chemical only control can leave escapes
• Waterhemp plants retain >95% of their seed in the florets at crop maturity (Norsthworthy et al, 2016)
• Waterhemp is a diecious species, meaning that some plants are male and some plants are female. Male
plants do not need to be controlled for purposes of weed seed bank management
• No waterhemp seeds in the florets are viable until at least 9 days after the start of pollination. Initiation of
pollination is indicated by the appearance of stigma on the female plants (Bell et al, 2010)
• Hand weeding can be a very effective weed control tactic for managing the waterhemp seed bank if:
-The plants being removed are female
-Female plants are uprooted and left in field before viable seed is produced
-Female plants are removed from the field if they already have viable seed
Goals of this guide
• Provide farmers and agronomists with a practical guide to the waterhemp flowering process
• Education on the timing of female waterhemp flowering and seed production to limit unnecessary trips out
of the fields with removed weeds
• Demonstrate differences between male and female waterhemp plants to save time and prevent unneces-
sary weeding of male plants. This is a difficult distinction to make, but it can be learned with time.Visual Seed Viability Chart
Based on time between pollination and removal of flowers
Start of Seed Viability
Plants uprooted need not be removed Plants uprooted need to be removed
Top: Images of seeds based on harvest DAP. Bottom Chart: Seed Immature seeds with no color and seeds with some color
germeination chart based on DAP. (Bell et al, 2010)Female Waterhemp Flowering Description
1. Flowering begins in terminal (top of plant) bud. A flower- 2. Flowering spikes elongate and new ones continue to grow
ing spike is developed. Then flowering continues in the leaf out of leaf axils.
axils as new flowering spikes are developed there.
New flowering spikes orig-
Terminal Bud inating in the leaf axils New flowering spikes
4. Flowering spikes elongate throughout the flowering process.
3. As the spike grows, flowers group together in clusters
The Glomerules are generally more condensed on the female
called glomerules. Stigma will appear when the flower
flower spike than on the male flower.
opens up and is receptive to pollen.
Stigma
Glomerules
Sosnoskie, 2018.Female Waterhemp Flowering Progression 7 Days Before Pollination (DBP) 6 DBP 5 DBP 4 DBP
Female Waterhemp Flowering Progression 3 DBP 2 DBP 1 DBP Start of Pollination
Start of Pollination
Stigma are the upper part of the female
flowers that receives pollen.
Stigma appearFemale Waterhemp Flowering Progression 1 Day After 2 DAP Pollination (DAP) 3 DAP 4 DAP
Female Waterhemp Flowering Progression 5 DAP 6 DAP 7 DAP 8 DAP
Female Waterhemp Flowering Progression 9 DAP 10 DAP Start of Seed Viability; Plants Need to Be Removed 11 DAP 12 DAP
Female Waterhemp Flowering Progression 13 DAP 14 DAP 15 DAP 16 DAP
Male Waterhemp Flowering Description
1. Flowering begins in terminal (top of plant) bud. A flower- 2. Flowering spikes elongate and new ones continue to grow
ing spike is developed. Then flowering continues in the leaf out of leaf axils.
axils as new flowering spikes are developed there.
New flowering spikes orig- New flowering spikes
Terminal Bud
inating in the leaf axils
4. Flowering spikes elongate throughout the flowering process.
3. As the spike grows, flowers group together in clusters
Generally, glomerules are further apart on male flower spikes
called (glomerules). Anthers with pollen will appear when
than on female plants.
the flower opens up. This indicates the start of pollination.
Anthers
Glomerule
Hartzler, 2020.Male Waterhemp Flowering Progression 7 Days Before Pollination (DBP) 6 DBP 5 DBP 4 DBP
Male Waterhemp Flowering Progression 3 DBP 2 DBP 1 DBP Star of Pollination
Male Waterhemp Flowering Progression 1 Day After Pollination 2 DAP 3 DAP 4 DAP
Male Waterhemp Flowering Progression 5 DAP 6 DAP 7 DAP 8 DAP
9 DAP 10 DAP 11 DAP 12 DAP
Female and Male Waterhemp Comparison—Start of Pollination Female Example 1 Male Example 1 Female Example 2 Male Example 2
Female and Male Waterhemp Comparison—5 days after pollination Female Example 1 Male Example 1 Female Example 2 Male Example 2
Female and Male Waterhemp Comparison—9 days after pollination Female Example 1 Male Example 1 Female Example 2 Male Example 2
References AgWeb. 2015. “The Worst Weed, According to Farmers.” August 20. Accessed November 6, 2019. https://www.agweb.com/article/the-worst- weed-according-to-farmers-NAA-ben-potter. Bell, Michael S., and Patrick J. Tranel. "Time Requirement from Pollination to Seed Maturity in Waterhemp ( Amaranthus Tuberculatus )." Weed Science 58, no. 2 (2010): 167-73. Hager, Aaron, And Sprague, Christy. 2001. “Waterhemp--Biology, Identification, and Management Considerations.” April 27. Accesed December 29, 2020. http://bulletin.ipm.illinois.edu/pastpest/articles/200105l.html#:~:text=Specifically%2C%20the%20only%20way%20to,(utricle)% 20fractures%20when%20separated. Hartzler, Bob. “Waterhemp: A 'Friendly' Native Evolves into the Cornbelt’s Worst Weed Problem.” Integrated Crop Managment. Iowa State Uni- versity, November 11, 2019. https://crops.extension.iastate.edu/blog/bob-hartzler/waterhemp-friendly-native-evolves-cornbelts-worst-weed- problem#:~:text=Waterhemp%20(Amaranthus%20tuberculatus)%20is%20native,commonly%20found%20in%20crop%20fields. Heap, Ian. “Herbicide Resistant Tall Waterhemp Globally (Amaranthus Tuberculatus (=A. Rudis)).” International Herbicide-Resistant Weed Data- base. WeedScience.org, January 1, 2021. http://weedscience.org/Pages/Species.aspx. Hilty, John. Water Hemp (Amaranthus rudis). Illinois Wildflowers. Accessed February 1, 2021. http://www.illinoiswildflowers.info/weeds/plants/ water_hemp.htm. Liu, Jianyang, Adam S. Davis, and Patrick J. Tranel. "Pollen Biology and Dispersal Dynamics in Waterhemp (Amaranthus Tuberculatus)." Weed Sci- ence 60, no. 3 (2012): 416-22. Norsthworthy et al. "Tall Waterhemp (Amaranthus Tuberculatus) and Palmer Amaranth (Amaranthus Palmeri) Seed Production and Retention at Soybean Maturity." Weed Technology 30, no. 1 (2016): 284-90. Pratt, Donald B., and Lynn G. Clark. “Amaranthus Rudis and A. Tuberculatus, One Species or Two?” Journal of the Torrey Botanical Society 128, no. 3 (2001): 282–96. https://doi.org/10.2307/3088718. Sosnoskie, Lynn. “Pigweed Identification (a Quick Guide).” ANR Blogs. Univiersity of California Ag and Natural Resources, June 13, 2018. https:// ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=27466. Waselkov, Katherine E, and Kenneth M Olsen. "Population Genetics and Origin of the Native North American Agricultural Weed Waterhemp (Amaranthus Tuberculatus; Amaranthaceae)." American Journal of Botany 101, no. 10 (2014): 1726-1736. Wu, Chenxi, and Owen, Micheal D. K. "When Is the Best Time to Emerge: Reproductive Phenology and Success of Natural Common Waterhemp (Amaranthus Rudis) Cohorts in the Midwest United States?" Weed Science 62, no. 1 (2014): 107-17.
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