A field observation worth sharing

Recently, while going about my day, I noticed a tall, pigweed-like plant in an agricultural field. It brought me back to a farm visit around the same time last year. I had gone there for another reason, but while walking through the field, I saw a similar weed and asked the producer, “Do you know what this could mean for your field?” She did not.

I explained that the concern was not simply the one plant standing in front of us. If a pigweed is allowed to flower and mature seed, a small patch can become a much larger problem in future crops. The producer wondered whether irrigation water could have deposited the seed on her property. That was a reasonable question. When I noticed a similar plant again this year, I felt the observation deserved wider attention.

Know the different types of pigweed

Pigweed is not one plant. It is a common name used for several warm-season annual weeds in the genus Amaranthus. Four important look-alikes in agricultural fields are redroot pigweed (Amaranthus retroflexus), smooth pigweed (Amaranthus hybridus), Palmer amaranth (Amaranthus palmeri), and waterhemp (Amaranthus tuberculatus). They can resemble one another, particularly before flowering, but several features help narrow the identification:

1. Redroot pigweed: fine hairs occur on the stems and often the leaves; leaves are usually oval to egg-shaped; and the dense flower or seedhead has relatively short, thick branches.
2. Smooth pigweed: stems are usually sparsely hairy; leaves are oval; and the flower cluster generally has longer, more slender spikes than redroot pigweed.
3. Palmer amaranth: stems and leaves generally lack hairs; leaves are oval to diamond-shaped; some mature leaves have a petiole as long as or longer than the leaf blade; and mature female flower heads have long, sharp bracts.
4. Waterhemp: stems and leaves are hairless; leaves tend to be longer, narrower, and glossier; and the flower or seedhead usually has slender branches. Like Palmer amaranth, waterhemp has separate male and female plants.

Begin with stem hairiness: a hairy stem points toward redroot or smooth pigweed, while a hairless stem points toward Palmer amaranth or waterhemp. Then compare leaf shape, petiole length, and flower or seedhead characteristics. Because these traits can vary and overlap, use several features together rather than relying on one. 

Why can one pigweed plant matter?

Let us take an example of redroot pigweed, a warm-season annual that reproduces by seed. It grows rapidly, competes for light, water, and nutrients, and can emerge over an extended part of the growing season. Published estimates of seed production vary widely from about 5,000 to 300,000 seeds per plant because plant size, emergence time, crop competition, moisture, temperature, and genetics all matter. The useful message is not one universal number; it is that a mature escape can add a consequential amount of seed. 

Do not assume that drought or late emergence makes a surviving plant harmless. In a controlled Australian study, redroot pigweed produced an average of 3,070 seeds per plant at full field capacity and 370 seeds per plant at 25% field capacity. A separate experiment found that later-emerging plants still produced seeds. Those exact values should not be transferred directly to every Nevada field, but they demonstrate an important principle: stress can reduce fecundity without eliminating it. 

Pigweed seeds can also persist in soil, so preventing new seeds is one of the most valuable long-term actions. Once a large seedbank develops, management becomes a multi-year commitment rather than a one-day job. 

Could irrigation water have brought it?

Yes, irrigation water is a scientifically plausible pathway, but the weed’s location alone cannot prove how it arrived. A western Nebraska study recovered viable seeds representing 77 weed species from surface irrigation water and estimated that tens of thousands of weed seeds per hectare entered sampled fields in one season. Weed seed loads increased as water moved through some canals, consistent with inputs from fields and ditch banks. 

There is also an important limitation. In a recent irrigation-pond experiment, redroot pigweed was among the least buoyant species tested; only 1% to 16% of its seeds remained afloat after 24 hours. That does not rule out short-distance movement in flowing or turbulent water, or movement with sediment and plant debris. It does mean that “it came through the water” should be treated as a hypothesis to investigate, not a fact to assume. 

Other possible pathways include soil carried on tillage, mowing or harvest equipment; seed in manure, feed or contaminated planting material; and seed already present along fence lines, roads, ditch banks or other disturbed ground. Often, more than one pathway is operating.

What should you do when you find it?

1. Confirm and map it. Mark the location, estimate the patch size, and record the crop or site, plant stage, and nearby water or equipment pathways. Record the identifying features. Then scout beyond the obvious plant; the visible escape may not be the only one.
2. Remove isolated plants before seed matures. Pull or hoe small plants when soil conditions allow, removing the root crown so the plant cannot recover. If flowers or seedheads are present, handle the whole plant carefully, bag it, and remove it from the production area. Do not shake mature heads, mow through them, or leave them on a ditch bank. Use only a disposal or composting method that reliably prevents viable seed from returning to the field.
3. Scout the irrigation pathway. Walk headgates, inlets, ditch banks, low spots, and the ends of irrigation runs. Recheck shortly after water moves and again later in the season. If seed appears to be entering from an upstream source, coordinate with the ditch operator or neighboring land manager.
4. Keep seed from hitchhiking. Clean soil and plant material from tillage, mowing, and harvest equipment before moving from an infested area to a clean field. Work clean areas first when practical. Pay special attention to combines and other machines that contact mature seedheads.
5. Make the crop or desirable vegetation competitive. Reduce bare, disturbed ground and use an appropriate crop stand, canopy, irrigation schedule, and nutrient plan for the site. In gardens and noncrop planting beds, a sufficiently deep, weed-free mulch can suppress new seedlings. Competition is a prevention tool, not a substitute for removing seed-producing escapes.
6. Choose control for the crop, site, and growth stage. Young pigweeds are generally easier to control than large or flowering plants. There is no responsible one-herbicide recommendation for every crop, pasture, ditch, and noncrop site. Product choice depends on the confirmed species, crop, growth stage, resistance history, irrigation setting, and current label. Combine cultural, mechanical, and, when appropriate, chemical tactics. Always follow the pesticide label, including site restrictions and grazing, haying, and preharvest intervals. Never apply a product to an irrigation ditch or water body unless the label explicitly permits that use.
7. Return and monitor. Redroot pigweed can emerge in multiple flushes, and soil seed can persist. Revisit mapped sites after each management action, after irrigation, and in following seasons. Record what worked and remove new escapes before they replenish the seedbank.

Do not overlook livestock and hay risks

Pigweeds can accumulate nitrate under some conditions, particularly with drought stress and high nitrogen fertility, and redroot pigweed can also contain oxalates. Finding one plant does not automatically make an entire pasture or hay lot unsafe; risk depends on concentration, plant material consumed, and animal factors. However, if pigweed makes up a meaningful portion of forage, especially after drought, frost, or heavy nitrogen fertilization, do not guess. Keep hungry animals from suddenly entering weed-heavy forage, obtain a representative forage test, and consult a veterinarian, livestock nutritionist, or the nearest Extension office. 

References

1. Behnken, L. M., & Durgan, B. R. (2024). Plants poisonous to livestock. University of Minnesota Extension. https://extension.umn.edu/agriculture/crop-production/forages/plants-poisonous-livestock 
2. Costea, M., Weaver, S. E., & Tardif, F. J. (2004). The biology of Canadian weeds. 130. Amaranthus retroflexus L., A. powellii S. Watson and A. hybridus L. Canadian Journal of Plant Science, 84(2), 631–668. https://doi.org/10.4141/P02-183 
3. Hartzler, B., & Anderson, M. (2026, June 8). Smooth pigweed. Iowa State University Extension and Outreach. https://crops.extension.iastate.edu/encyclopedia/smooth-pigweed 
4. Khan, A. M., Mobli, A., Werth, J. A., & Chauhan, B. S. (2021a). Effect of emergence time on growth and fecundity of redroot pigweed (Amaranthus retroflexus) and slender amaranth (Amaranthus viridis): Emerging problem weeds in Australian summer crops. Weed Science, 69(3), 333–340. https://doi.org/10.1017/wsc.2021.9 
5. Khan, A. M., Mobli, A., Werth, J. A., & Chauhan, B. S. (2021b). Effect of soil moisture regimes on the growth and fecundity of slender amaranth (Amaranthus viridis) and redroot pigweed (Amaranthus retroflexus). Weed Science, 69(1), 82–87. https://doi.org/10.1017/wsc.2020.89 
6. Qi, Y., Yan, B., Fu, G., Guan, X., Du, L., & Li, J. (2017). Germination of seeds and seedling growth of Amaranthus retroflexus L. following sublethal exposure of parent plants to herbicides. Scientific Reports, 7, Article 157. https://doi.org/10.1038/s41598-017-00153-4 
7. Ray, A. O., LeBude, A., Altland, J., Harlow, C. D., & Neal, J. C. (2026). Buoyancy and survival of weed seeds in container nursery irrigation ponds. Weed Technology, 40, e27. https://doi.org/10.1017/wet.2026.10101 
8. University of California Statewide Integrated Pest Management Program. (2026, January). Redroot pigweed. University of California Agriculture and Natural Resources. https://ipm.ucanr.edu/weeds-identification-gallery/redroot-pigweed/ 
9. University of Minnesota Extension. (n.d.). Annual broadleaf weeds. Retrieved August 19, 2026, from https://extension.umn.edu/agriculture/crop-production/weed-management/annual-broadleaf-weeds 
10. Utah State University Extension. (n.d.). Redroot pigweed. Retrieved August 19, 2026, from https://extension.usu.edu/planthealth/ipm/ornamental-pest-guide/weeds/w_redroot-pigweed 
11. Wilson, R. G., Jr. (1980). Dissemination of weed seeds by surface irrigation water in western Nebraska. Weed Science, 28(1), 87–92. https://doi.org/10.1017/S0043174500027831 

A field observation worth sharing

Recently, while going about my day, I noticed a tall, pigweed-like plant in an agricultural field. It brought me back to a farm visit around the same time last year. I had gone there for another reason, but while walking through the field, I saw a similar weed and asked the producer, “Do you know what this could mean for your field?” She did not.

I explained that the concern was not simply the one plant standing in front of us. If a pigweed is allowed to flower and mature seed, a small patch can become a much larger problem in future crops. The producer wondered whether irrigation water could have deposited the seed on her property. That was a reasonable question. When I noticed a similar plant again this year, I felt the observation deserved wider attention.

Know the different types of pigweed

Pigweed is not one plant. It is a common name used for several warm-season annual weeds in the genus Amaranthus. Four important look-alikes in agricultural fields are redroot pigweed (Amaranthus retroflexus), smooth pigweed (Amaranthus hybridus), Palmer amaranth (Amaranthus palmeri), and waterhemp (Amaranthus tuberculatus). They can resemble one another, particularly before flowering, but several features help narrow the identification:

1. Redroot pigweed: fine hairs occur on the stems and often the leaves; leaves are usually oval to egg-shaped; and the dense flower or seedhead has relatively short, thick branches.
2. Smooth pigweed: stems are usually sparsely hairy; leaves are oval; and the flower cluster generally has longer, more slender spikes than redroot pigweed.
3. Palmer amaranth: stems and leaves generally lack hairs; leaves are oval to diamond-shaped; some mature leaves have a petiole as long as or longer than the leaf blade; and mature female flower heads have long, sharp bracts.
4. Waterhemp: stems and leaves are hairless; leaves tend to be longer, narrower, and glossier; and the flower or seedhead usually has slender branches. Like Palmer amaranth, waterhemp has separate male and female plants.

Begin with stem hairiness: a hairy stem points toward redroot or smooth pigweed, while a hairless stem points toward Palmer amaranth or waterhemp. Then compare leaf shape, petiole length, and flower or seedhead characteristics. Because these traits can vary and overlap, use several features together rather than relying on one. 

Why can one pigweed plant matter?

Let us take an example of redroot pigweed, a warm-season annual that reproduces by seed. It grows rapidly, competes for light, water, and nutrients, and can emerge over an extended part of the growing season. Published estimates of seed production vary widely from about 5,000 to 300,000 seeds per plant because plant size, emergence time, crop competition, moisture, temperature, and genetics all matter. The useful message is not one universal number; it is that a mature escape can add a consequential amount of seed. 

Do not assume that drought or late emergence makes a surviving plant harmless. In a controlled Australian study, redroot pigweed produced an average of 3,070 seeds per plant at full field capacity and 370 seeds per plant at 25% field capacity. A separate experiment found that later-emerging plants still produced seeds. Those exact values should not be transferred directly to every Nevada field, but they demonstrate an important principle: stress can reduce fecundity without eliminating it. 

Pigweed seeds can also persist in soil, so preventing new seeds is one of the most valuable long-term actions. Once a large seedbank develops, management becomes a multi-year commitment rather than a one-day job. 

Could irrigation water have brought it?

Yes, irrigation water is a scientifically plausible pathway, but the weed’s location alone cannot prove how it arrived. A western Nebraska study recovered viable seeds representing 77 weed species from surface irrigation water and estimated that tens of thousands of weed seeds per hectare entered sampled fields in one season. Weed seed loads increased as water moved through some canals, consistent with inputs from fields and ditch banks. 

There is also an important limitation. In a recent irrigation-pond experiment, redroot pigweed was among the least buoyant species tested; only 1% to 16% of its seeds remained afloat after 24 hours. That does not rule out short-distance movement in flowing or turbulent water, or movement with sediment and plant debris. It does mean that “it came through the water” should be treated as a hypothesis to investigate, not a fact to assume. 

Other possible pathways include soil carried on tillage, mowing or harvest equipment; seed in manure, feed or contaminated planting material; and seed already present along fence lines, roads, ditch banks or other disturbed ground. Often, more than one pathway is operating.

What should you do when you find it?

1. Confirm and map it. Mark the location, estimate the patch size, and record the crop or site, plant stage, and nearby water or equipment pathways. Record the identifying features. Then scout beyond the obvious plant; the visible escape may not be the only one.
2. Remove isolated plants before seed matures. Pull or hoe small plants when soil conditions allow, removing the root crown so the plant cannot recover. If flowers or seedheads are present, handle the whole plant carefully, bag it, and remove it from the production area. Do not shake mature heads, mow through them, or leave them on a ditch bank. Use only a disposal or composting method that reliably prevents viable seed from returning to the field.
3. Scout the irrigation pathway. Walk headgates, inlets, ditch banks, low spots, and the ends of irrigation runs. Recheck shortly after water moves and again later in the season. If seed appears to be entering from an upstream source, coordinate with the ditch operator or neighboring land manager.
4. Keep seed from hitchhiking. Clean soil and plant material from tillage, mowing, and harvest equipment before moving from an infested area to a clean field. Work clean areas first when practical. Pay special attention to combines and other machines that contact mature seedheads.
5. Make the crop or desirable vegetation competitive. Reduce bare, disturbed ground and use an appropriate crop stand, canopy, irrigation schedule, and nutrient plan for the site. In gardens and noncrop planting beds, a sufficiently deep, weed-free mulch can suppress new seedlings. Competition is a prevention tool, not a substitute for removing seed-producing escapes.
6. Choose control for the crop, site, and growth stage. Young pigweeds are generally easier to control than large or flowering plants. There is no responsible one-herbicide recommendation for every crop, pasture, ditch, and noncrop site. Product choice depends on the confirmed species, crop, growth stage, resistance history, irrigation setting, and current label. Combine cultural, mechanical, and, when appropriate, chemical tactics. Always follow the pesticide label, including site restrictions and grazing, haying, and preharvest intervals. Never apply a product to an irrigation ditch or water body unless the label explicitly permits that use.
7. Return and monitor. Redroot pigweed can emerge in multiple flushes, and soil seed can persist. Revisit mapped sites after each management action, after irrigation, and in following seasons. Record what worked and remove new escapes before they replenish the seedbank.

Do not overlook livestock and hay risks

Pigweeds can accumulate nitrate under some conditions, particularly with drought stress and high nitrogen fertility, and redroot pigweed can also contain oxalates. Finding one plant does not automatically make an entire pasture or hay lot unsafe; risk depends on concentration, plant material consumed, and animal factors. However, if pigweed makes up a meaningful portion of forage, especially after drought, frost, or heavy nitrogen fertilization, do not guess. Keep hungry animals from suddenly entering weed-heavy forage, obtain a representative forage test, and consult a veterinarian, livestock nutritionist, or the nearest Extension office. 

References

1. Behnken, L. M., & Durgan, B. R. (2024). Plants poisonous to livestock. University of Minnesota Extension. https://extension.umn.edu/agriculture/crop-production/forages/plants-poisonous-livestock 
2. Costea, M., Weaver, S. E., & Tardif, F. J. (2004). The biology of Canadian weeds. 130. Amaranthus retroflexus L., A. powellii S. Watson and A. hybridus L. Canadian Journal of Plant Science, 84(2), 631–668. https://doi.org/10.4141/P02-183 
3. Hartzler, B., & Anderson, M. (2026, June 8). Smooth pigweed. Iowa State University Extension and Outreach. https://crops.extension.iastate.edu/encyclopedia/smooth-pigweed 
4. Khan, A. M., Mobli, A., Werth, J. A., & Chauhan, B. S. (2021a). Effect of emergence time on growth and fecundity of redroot pigweed (Amaranthus retroflexus) and slender amaranth (Amaranthus viridis): Emerging problem weeds in Australian summer crops. Weed Science, 69(3), 333–340. https://doi.org/10.1017/wsc.2021.9 
5. Khan, A. M., Mobli, A., Werth, J. A., & Chauhan, B. S. (2021b). Effect of soil moisture regimes on the growth and fecundity of slender amaranth (Amaranthus viridis) and redroot pigweed (Amaranthus retroflexus). Weed Science, 69(1), 82–87. https://doi.org/10.1017/wsc.2020.89 
6. Qi, Y., Yan, B., Fu, G., Guan, X., Du, L., & Li, J. (2017). Germination of seeds and seedling growth of Amaranthus retroflexus L. following sublethal exposure of parent plants to herbicides. Scientific Reports, 7, Article 157. https://doi.org/10.1038/s41598-017-00153-4 
7. Ray, A. O., LeBude, A., Altland, J., Harlow, C. D., & Neal, J. C. (2026). Buoyancy and survival of weed seeds in container nursery irrigation ponds. Weed Technology, 40, e27. https://doi.org/10.1017/wet.2026.10101 
8. University of California Statewide Integrated Pest Management Program. (2026, January). Redroot pigweed. University of California Agriculture and Natural Resources. https://ipm.ucanr.edu/weeds-identification-gallery/redroot-pigweed/ 
9. University of Minnesota Extension. (n.d.). Annual broadleaf weeds. Retrieved August 19, 2026, from https://extension.umn.edu/agriculture/crop-production/weed-management/annual-broadleaf-weeds 
10. Utah State University Extension. (n.d.). Redroot pigweed. Retrieved August 19, 2026, from https://extension.usu.edu/planthealth/ipm/ornamental-pest-guide/weeds/w_redroot-pigweed 
11. Wilson, R. G., Jr. (1980). Dissemination of weed seeds by surface irrigation water in western Nebraska. Weed Science, 28(1), 87–92. https://doi.org/10.1017/S0043174500027831