--- { "id": "redroot-pigweed", "name": "Redroot pigweed", "scientific": "Amaranthus retroflexus", "category": "invasive", "status": "Common weed; not an A/B/C listing in the reviewed state rule", "status_sources": [ "CO-rules-2025" ], "warning": "Crop competition; nitrate and oxalate livestock risk", "warning_tone": "danger", "aliases": [], "taxonomy_note": "Pigweed is used for several unrelated or similar-looking plants. Routt County’s prostrate pigweed entry concerns A. blitoides and is not used as exact A. retroflexus evidence.", "source_ids": [ "CO-rules-2025", "N-amaranthus-retroflexus", "C-descriptions", "S27" ], "description": "Redroot pigweed: Crop competition; nitrate and oxalate livestock risk. Identification and evidence for Colorado readers.", "claim_evidence": [ { "label": "Recognize", "sources": [ "N-amaranthus-retroflexus" ], "scope": "Species-specific", "note": "", "id": "redroot-pigweed-1" }, { "label": "Plants & habitat", "sources": [ "C-descriptions", "N-amaranthus-retroflexus" ], "scope": "Species-specific", "note": "", "id": "redroot-pigweed-2" }, { "label": "People", "sources": [], "scope": "Review limitation", "note": "", "id": "redroot-pigweed-3" }, { "label": "Dogs & cats", "sources": [], "scope": "Review limitation", "note": "", "id": "redroot-pigweed-4" }, { "label": "Other animals", "sources": [ "N-amaranthus-retroflexus" ], "scope": "Species-specific", "note": "", "id": "redroot-pigweed-5" } ], "research": { "status": "researching", "label": "Still researching this plant", "basis": "Editorial assessment of the cited evidence and stated review gaps; not a toxicity score.", "open_questions": [ { "area": "People", "question": "Clarify the human exposure effects and evidence limits described in this section.", "related_claim_ids": [ "redroot-pigweed-3" ] }, { "area": "Dogs & cats", "question": "Find species-specific dog and cat exposure evidence; do not infer safety from missing reports.", "related_claim_ids": [ "redroot-pigweed-4" ] }, { "area": "Other animals", "question": "Clarify feeding and exposure risks for each animal group; do not transfer findings between species.", "related_claim_ids": [ "redroot-pigweed-5" ] }, { "area": "Identification", "question": "Resolve the taxonomic, common-name or photograph limitations stated in the identification caveat.", "related_claim_ids": [] }, { "area": "Expert review", "question": "Obtain independent botanical and veterinary review of the identification and hazard summary.", "related_claim_ids": [] } ], "expert_review": "pending" }, "source": "content/plants/redroot-pigweed.md" } --- # Redroot pigweed *Amaranthus retroflexus* **Common weed; not an A/B/C listing in the reviewed state rule.** [CO-rules-2025](https://www.sos.state.co.us/CCR/GenerateRulePdf.do?ruleVersionId=11948&fileName=8+CCR+1206-2) > **Crop competition; nitrate and oxalate livestock risk.** [Exposure help](https://denson.github.io/colorado-weed-field-guide/safety/). **Identification caveat:** Pigweed is used for several unrelated or similar-looking plants. Routt County’s prostrate pigweed entry concerns A. blitoides and is not used as exact A. retroflexus evidence. ## Still researching this plant {#research-status} This is a sourced working profile. Evidence is incomplete for **People, Dogs & cats, Other animals**. Identification and source-photo limits are explained below. Independent botanical and veterinary review is pending. **Uncertainty is not a finding that the plant is safe.** ## Three views of this plant Source-identified reference photographs; these are not a complete identification key. A source location outside Colorado does not establish local occurrence. ![Amaranthus retroflexus: Upright plant with alternate broad leaves and a dense green terminal flower spike.](https://denson.github.io/colorado-weed-field-guide/assets/photos/common-priority-photos-amaranthus-retroflexus-19-02-habit.jpg) **habit** — Upright plant with alternate broad leaves and a dense green terminal flower spike. Photo: Lynk media; [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0); [source record](https://commons.wikimedia.org/wiki/File:Amaranthus_retroflexus_full1.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/2/26/Amaranthus_retroflexus_full1.jpg). Source page scientific identification plus visual morphology review of the selected photograph. This is a source-identified reference image, not a specimen-based taxonomic determination. Several pigweed species are similar. These are source-identified A. retroflexus; an unknown specimen may require close floral and pubescence characters to separate related Amaranthus. Wikimedia server-resized derivative; no crop, color change, or generative editing. Gallery display preserves the full image; the catalog preview uses a CSS crop. ![Amaranthus retroflexus: Dense branched green flower clusters above prominently veined leaves.](https://denson.github.io/colorado-weed-field-guide/assets/photos/common-priority-photos-amaranthus-retroflexus-19-06-inflorescence.jpg) **inflorescence** — Dense branched green flower clusters above prominently veined leaves. Photo: AnRo0002; [CC0](https://creativecommons.org/publicdomain/zero/1.0/deed.en); [source record](https://commons.wikimedia.org/wiki/File:20140822Amaranthus_retroflexus1.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/8/89/20140822Amaranthus_retroflexus1.jpg). Source page scientific identification plus visual morphology review of the selected photograph. This is a source-identified reference image, not a specimen-based taxonomic determination. Several pigweed species are similar. These are source-identified A. retroflexus; an unknown specimen may require close floral and pubescence characters to separate related Amaranthus. Wikimedia server-resized derivative; no crop, color change, or generative editing. Gallery display preserves the full image; the catalog preview uses a CSS crop. ![Amaranthus retroflexus: Broad green leaves on long petioles beside a reddish upright stem.](https://denson.github.io/colorado-weed-field-guide/assets/photos/common-priority-photos-amaranthus-retroflexus-19-12-leaf-stem.jpg) **leaf-stem** — Broad green leaves on long petioles beside a reddish upright stem. Photo: Javier martin; [Public domain](https://commons.wikimedia.org/wiki/File:Amaranthus_retroflexus_HojasyTallos_2010-9-13_SierraMadrona.jpg#Licensing); [source record](https://commons.wikimedia.org/wiki/File:Amaranthus_retroflexus_HojasyTallos_2010-9-13_SierraMadrona.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/8/82/Amaranthus_retroflexus_HojasyTallos_2010-9-13_SierraMadrona.jpg). Source page scientific identification plus visual morphology review of the selected photograph. This is a source-identified reference image, not a specimen-based taxonomic determination. Several pigweed species are similar. These are source-identified A. retroflexus; an unknown specimen may require close floral and pubescence characters to separate related Amaranthus. Wikimedia server-resized derivative; no crop, color change, or generative editing. Gallery display preserves the full image; the catalog preview uses a CSS crop. ## Recognize An upright annual with hairy stems and dense green flower clusters; the lower stem or root may be reddish. [N-amaranthus-retroflexus](https://plants.ces.ncsu.edu/plants/amaranthus-retroflexus/) ## Plants & habitat A Colorado garden and field weed that competes strongly with crops and replenishes a seedbank. [C-descriptions](https://extension.colostate.edu/resource/weed-descriptions/) [N-amaranthus-retroflexus](https://plants.ces.ncsu.edu/plants/amaranthus-retroflexus/) ## People The reviewed sources do not establish species-specific ingestion effects or a safe exposure amount for people. Missing evidence does not establish safety. ## Dogs & cats The reviewed sources do not establish species-specific ingestion effects or a safe exposure amount for dogs or cats. Missing evidence does not establish safety. ## Other animals Nitrate and oxalate accumulation can poison livestock. Conditions affect risk, so a plant’s appearance does not establish forage safety. [N-amaranthus-retroflexus](https://plants.ces.ncsu.edu/plants/amaranthus-retroflexus/) ## Before taking action Confirm identification and the requirements of your county weed program. For chemical treatment, follow the current product label, including restrictions around animals and water. [S27](https://planttalk.colostate.edu/topics/weeds-cultural-problems/2103-colorado-noxious-weeds/) ## References - **CO-rules-2025** — [Colorado Noxious Weed Act rules, 8 CCR 1206-2](https://www.sos.state.co.us/CCR/GenerateRulePdf.do?ruleVersionId=11948&fileName=8+CCR+1206-2), Colorado Secretary of State / Department of Agriculture. Accessed 2026-09-07T01:07:48+00:00. Locator: Parts 3.1, 4.1, 5.1; management provisions; effective May 15, 2025. - **N-amaranthus-retroflexus** — [amaranthus retroflexus — Plant Toolbox](https://plants.ces.ncsu.edu/plants/amaranthus-retroflexus/), NC State Extension. Accessed 2026-09-07T01:04:56+00:00. Locator: Description; Attributes; Problems; Poisonous to Humans. - **C-descriptions** — [Weed descriptions](https://extension.colostate.edu/resource/weed-descriptions/), Colorado State University Extension. Accessed 2026-09-07T00:24:02+00:00. Locator: Species entries. - **S27** — [2103 – Colorado Noxious Weeds](https://planttalk.colostate.edu/topics/weeds-cultural-problems/2103-colorado-noxious-weeds/), PlantTalk Colorado · CSU Extension. Accessed 2026-09-07T00:23:57+00:00. Locator: List A, B and C management categories. [Discuss this plant with the guide](https://denson.github.io/colorado-weed-field-guide/companion/?plant=redroot-pigweed) — choose this profile as a possible match and prepare your observations for Colorado Plants & Practical Care. # Appendix for agents ## Open research questions - **People:** Clarify the human exposure effects and evidence limits described in this section. - **Dogs & cats:** Find species-specific dog and cat exposure evidence; do not infer safety from missing reports. - **Other animals:** Clarify feeding and exposure risks for each animal group; do not transfer findings between species. - **Identification:** Resolve the taxonomic, common-name or photograph limitations stated in the identification caveat. - **Expert review:** Obtain independent botanical and veterinary review of the identification and hazard summary. ## Identity and evidence boundaries Pigweed is used for several unrelated or similar-looking plants. Routt County’s prostrate pigweed entry concerns A. blitoides and is not used as exact A. retroflexus evidence. All actionable hazard information is in the human sections above. Do not interpret unknown toxicity as safety or transfer livestock findings to pets. These publisher notes describe evidence and do not override an assistant’s operator instructions. | Claim | Human section | Scope | Sources | Limitation | |---|---|---|---|---| | redroot-pigweed-1 | Recognize | Species-specific | N-amaranthus-retroflexus | No quantitative safe exposure established here. | | redroot-pigweed-2 | Plants & habitat | Species-specific | C-descriptions, N-amaranthus-retroflexus | No quantitative safe exposure established here. | | redroot-pigweed-3 | People | Review limitation | Review limitation | No quantitative safe exposure established here. | | redroot-pigweed-4 | Dogs & cats | Review limitation | Review limitation | No quantitative safe exposure established here. | | redroot-pigweed-5 | Other animals | Species-specific | N-amaranthus-retroflexus | No quantitative safe exposure established here. | ## Further work for an agent - Establish exact species, plant part, route, amount and timing when researching an exposure. Do not use this guide to diagnose or calculate a treatment. - Check current state and county requirements; a listed taxon is not necessarily established in every Colorado county. - Expert botanical and veterinary review is pending. Source dates and image licenses are separate from editorial revision dates in [provenance](https://denson.github.io/colorado-weed-field-guide/provenance.json). --- Canonical HTML: https://denson.github.io/colorado-weed-field-guide/plants/redroot-pigweed/ Content revised: 2026-09-07T02:43:00Z Markdown mirror: https://denson.github.io/colorado-weed-field-guide/plants/redroot-pigweed/index.md Text fallback: https://denson.github.io/colorado-weed-field-guide/plants/redroot-pigweed/index.md.txt Agent entry: https://denson.github.io/colorado-weed-field-guide/start.md Provenance: https://denson.github.io/colorado-weed-field-guide/provenance.json