---
{
  "id": "western-ragweed",
  "name": "Western ragweed",
  "scientific": "Ambrosia psilostachya",
  "category": "native",
  "status": "Native Colorado plant · not on state Lists A/B/C",
  "status_sources": [
    "C-floristic-2020",
    "CO-rules-2025"
  ],
  "warning": "A persistent perennial ragweed; allergy detail still under review",
  "warning_tone": "caution",
  "aliases": [
    "Cuman ragweed",
    "perennial ragweed"
  ],
  "taxonomy_note": "Unlike annual common and giant ragweeds, this species is perennial. Ambrosia psilostachya is not treated as a synonym of either.",
  "source_ids": [
    "C-floristic-2020",
    "CO-rules-2025",
    "F-ambrosia-psilostachya",
    "C-ragweed-native",
    "S27"
  ],
  "description": "Western ragweed: A persistent perennial ragweed; allergy detail still under review. Identification and evidence for Colorado readers.",
  "claim_evidence": [
    {
      "label": "Identification",
      "sources": [
        "F-ambrosia-psilostachya"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "western-ragweed-1"
    },
    {
      "label": "People",
      "sources": [
        "C-ragweed-native"
      ],
      "scope": "Related-species context; species-specific evidence unresolved",
      "note": "",
      "id": "western-ragweed-2"
    },
    {
      "label": "Dogs & cats",
      "sources": [],
      "scope": "Review limitation",
      "note": "",
      "id": "western-ragweed-3"
    },
    {
      "label": "Other animals",
      "sources": [
        "F-ambrosia-psilostachya"
      ],
      "scope": "Species forage discussion; broader treated-ragweed warning",
      "note": "",
      "id": "western-ragweed-4"
    },
    {
      "label": "Plants & habitat",
      "sources": [
        "F-ambrosia-psilostachya"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "western-ragweed-5"
    },
    {
      "label": "Management & timing",
      "sources": [
        "F-ambrosia-psilostachya"
      ],
      "scope": "Management inference from perennial growth",
      "note": "",
      "id": "western-ragweed-6"
    }
  ],
  "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": [
          "western-ragweed-2"
        ]
      },
      {
        "area": "Dogs & cats",
        "question": "Find species-specific dog and cat exposure evidence; do not infer safety from missing reports.",
        "related_claim_ids": [
          "western-ragweed-3"
        ]
      },
      {
        "area": "Other animals",
        "question": "Clarify feeding and exposure risks for each animal group; do not transfer findings between species.",
        "related_claim_ids": [
          "western-ragweed-4"
        ]
      },
      {
        "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"
  },
  "colorado_relevance": {
    "source_id": "C-floristic-2020",
    "locator": "Appendix B; PDF page 32; Ambrosia psilostachya",
    "basis": "Included in the 2020 statewide flora assessment. This is not a current county abundance survey.",
    "source_taxon": "Ambrosia psilostachya"
  },
  "source": "content/plants/western-ragweed.md"
}
---

# Western ragweed

*Ambrosia psilostachya*

**Native Colorado plant · not on state Lists A/B/C.** [C-floristic-2020](https://cnhp.colostate.edu/download/documents/2020/CO_FQA_2020_Final_Report.pdf) [CO-rules-2025](https://www.sos.state.co.us/CCR/GenerateRulePdf.do?ruleVersionId=11948&fileName=8+CCR+1206-2)

> **A persistent perennial ragweed; allergy detail still under review.** [Exposure help](https://denson.github.io/colorado-weed-field-guide/safety/).

**Identification caveat:** Unlike annual common and giant ragweeds, this species is perennial. Ambrosia psilostachya is not treated as a synonym of either.

## 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.

![Upright western ragweed stems with divided green leaves and elongated terminal flower spikes.](https://denson.github.io/colorado-weed-field-guide/assets/photos/coverage-range-photos-ambrosia-psilostachya-habit-311838775.jpeg)

**habit** — Upright western ragweed stems with divided green leaves and elongated terminal flower spikes.

Photo: Zihao Wang; [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/); [source record](https://www.inaturalist.org/observations/179141365); [original image](https://inaturalist-open-data.s3.amazonaws.com/photos/311838775/original.jpeg). Explicit iNaturalist observation taxon and Research Grade community identification; photograph visually reviewed for depicted features. This is source identification, not a new independent botanical determination. Source identifies this plant to species; no variety or subspecies determination is asserted. Downloaded iNaturalist large rendition; downloaded bytes retained unchanged. No local crop, retouching, or generative editing. Gallery display preserves the full image; catalog preview may use a CSS crop.

![Close view of small green, downward-facing male flower heads along a western ragweed spike.](https://denson.github.io/colorado-weed-field-guide/assets/photos/coverage-range-photos-ambrosia-psilostachya-male-flower-heads-312069077.jpeg)

**male-flower-heads** — Close view of small green, downward-facing male flower heads along a western ragweed spike.

Photo: Zihao Wang; [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/); [source record](https://www.inaturalist.org/observations/179141365); [original image](https://inaturalist-open-data.s3.amazonaws.com/photos/312069077/original.jpeg). Explicit iNaturalist observation taxon and Research Grade community identification; photograph visually reviewed for depicted features. This is source identification, not a new independent botanical determination. Source identifies this plant to species; no variety or subspecies determination is asserted. Downloaded iNaturalist large rendition; downloaded bytes retained unchanged. No local crop, retouching, or generative editing. Gallery display preserves the full image; catalog preview may use a CSS crop.

![Close view of the hairy surface and narrow lobes of a western ragweed leaf.](https://denson.github.io/colorado-weed-field-guide/assets/photos/coverage-range-photos-ambrosia-psilostachya-leaf-surface-312069114.jpeg)

**leaf-surface** — Close view of the hairy surface and narrow lobes of a western ragweed leaf.

Photo: Zihao Wang; [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/); [source record](https://www.inaturalist.org/observations/179141365); [original image](https://inaturalist-open-data.s3.amazonaws.com/photos/312069114/original.jpeg). Explicit iNaturalist observation taxon and Research Grade community identification; photograph visually reviewed for depicted features. This is source identification, not a new independent botanical determination. Source identifies this plant to species; no variety or subspecies determination is asserted. Downloaded iNaturalist large rendition; downloaded bytes retained unchanged. No local crop, retouching, or generative editing. Gallery display preserves the full image; catalog preview may use a CSS crop.


## Identification

A native perennial with relatively slender branching stems and an extensive underground system. Male flower heads occur at the tips; female flowers occur in leaf axils. [F-ambrosia-psilostachya](https://research.fs.usda.gov/feis/species-reviews/ambpsi)

## People

Ragweeds are associated with allergenic windborne pollen. The reviewed Colorado allergy discussion focuses on common and giant ragweed; western ragweed-specific allergy severity still needs review. [C-ragweed-native](https://sam.extension.colostate.edu/wp-content/uploads/sites/44/2019/08/August-31-2019-Ragweed.pdf)

*Evidence scope: Related-species context; species-specific evidence unresolved.*

## 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

The Forest Service describes forage and wildlife use and does not classify the untreated plant as poisonous in that discussion. It separately reports possible nitrate accumulation in herbicide-treated ragweeds; a safe feeding amount is not established. [F-ambrosia-psilostachya](https://research.fs.usda.gov/feis/species-reviews/ambpsi)

*Evidence scope: Species forage discussion; broader treated-ragweed warning.*

## Plants & habitat

It occupies native grasslands and disturbed sites and provides food or cover for birds and small mammals. Underground spread can make garden colonies persistent. [F-ambrosia-psilostachya](https://research.fs.usda.gov/feis/species-reviews/ambpsi)

## Management & timing

Plan follow-up for underground regrowth. Seed prevention alone does not address an established perennial colony. [F-ambrosia-psilostachya](https://research.fs.usda.gov/feis/species-reviews/ambpsi)

*Evidence scope: Management inference from perennial growth.*

## 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

- **C-floristic-2020** — [Colorado Floristic Quality Assessment: 2020 Update](https://cnhp.colostate.edu/download/documents/2020/CO_FQA_2020_Final_Report.pdf), Colorado Natural Heritage Program, Colorado State University. Accessed 2026-09-07T02:27:33+00:00. Locator: Appendix B, Colorado species list: scientific name and native/non-native fields.
- **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.
- **F-ambrosia-psilostachya** — [Western ragweed](https://research.fs.usda.gov/feis/species-reviews/ambpsi), USDA Forest Service, Fire Effects Information System. Accessed 2026-09-07T02:24:49+00:00. Locator: Taxonomy, distribution, botanical description, wildlife/livestock values and management.
- **C-ragweed-native** — [Ragweed](https://sam.extension.colostate.edu/wp-content/uploads/sites/44/2019/08/August-31-2019-Ragweed.pdf), Colorado State University Extension. Accessed 2026-09-07T02:26:09+00:00. Locator: Named species or genus; identification, distribution and hazard sections where available.
- **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=western-ragweed) — choose this profile as a possible match and prepare your observations for Colorado Plants & Practical Care.

# Appendix for agents

Colorado inclusion: [C-floristic-2020](https://cnhp.colostate.edu/download/documents/2020/CO_FQA_2020_Final_Report.pdf), Appendix B; PDF page 32; Ambrosia psilostachya. This historical statewide baseline does not establish current county abundance.

## 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

Unlike annual common and giant ragweeds, this species is perennial. Ambrosia psilostachya is not treated as a synonym of either.

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 |
|---|---|---|---|---|
| western-ragweed-1 | Identification | Species-specific | F-ambrosia-psilostachya | No quantitative safe exposure established here. |
| western-ragweed-2 | People | Related-species context; species-specific evidence unresolved | C-ragweed-native | No quantitative safe exposure established here. |
| western-ragweed-3 | Dogs & cats | Review limitation | Review limitation | No quantitative safe exposure established here. |
| western-ragweed-4 | Other animals | Species forage discussion; broader treated-ragweed warning | F-ambrosia-psilostachya | No quantitative safe exposure established here. |
| western-ragweed-5 | Plants & habitat | Species-specific | F-ambrosia-psilostachya | No quantitative safe exposure established here. |
| western-ragweed-6 | Management & timing | Management inference from perennial growth | F-ambrosia-psilostachya | 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/western-ragweed/

Content revised: 2026-09-07T02:46:00Z

Markdown mirror: https://denson.github.io/colorado-weed-field-guide/plants/western-ragweed/index.md

Text fallback: https://denson.github.io/colorado-weed-field-guide/plants/western-ragweed/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