---
{
  "id": "western-whorled-milkweed",
  "name": "Western whorled milkweed",
  "scientific": "Asclepias subverticillata",
  "category": "native",
  "status": "Colorado native",
  "status_sources": [
    "S06"
  ],
  "warning": "High livestock toxicity",
  "warning_tone": "danger",
  "aliases": [],
  "taxonomy_note": "",
  "source_ids": [
    "S06",
    "S05",
    "S04"
  ],
  "description": "Western whorled milkweed: High livestock toxicity. Colorado identification, habitat and exposure evidence.",
  "claim_evidence": [
    {
      "label": "Recognize",
      "sources": [
        "S06"
      ],
      "scope": "Species-specific",
      "note": "",
      "reviewed": "2026-09-06",
      "id": "western-whorled-milkweed-1"
    },
    {
      "label": "Plants & habitat",
      "sources": [
        "S06"
      ],
      "scope": "Species-specific",
      "note": "",
      "reviewed": "2026-09-06",
      "id": "western-whorled-milkweed-2"
    },
    {
      "label": "People",
      "sources": [
        "S05"
      ],
      "scope": "Genus-level",
      "note": "",
      "reviewed": "2026-09-06",
      "id": "western-whorled-milkweed-3"
    },
    {
      "label": "Dogs & cats",
      "sources": [
        "S04"
      ],
      "scope": "Genus-level",
      "note": "",
      "reviewed": "2026-09-06",
      "id": "western-whorled-milkweed-4"
    },
    {
      "label": "Other animals",
      "sources": [
        "S06"
      ],
      "scope": "Species-specific",
      "note": "",
      "reviewed": "2026-09-06",
      "id": "western-whorled-milkweed-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": "Expert review",
        "question": "Obtain independent botanical and veterinary review of the identification and hazard summary.",
        "related_claim_ids": []
      }
    ],
    "expert_review": "pending"
  },
  "source": "content/plants/western-whorled-milkweed.md"
}
---

# Western whorled milkweed

*Asclepias subverticillata*

**Colorado native.** [S06](https://extension.colostate.edu/resource/managing-western-whorled-milkweed/)

> **High livestock toxicity.** Native status, garden use and noxious-weed class are separate from toxicity. [Exposure help](https://denson.github.io/colorado-weed-field-guide/safety/).

## Still researching this plant {#research-status}

This is a sourced working profile. The cited findings are available below. Independent botanical and veterinary review is pending. **Uncertainty is not a finding that the plant is safe.**


## Three views of this plant

Photos are source-identified examples, not a complete identification key. Check leaves, stems, flowers and fruit together. Locations are not assumed to be in Colorado.

![Asclepias subverticillata: Several slender flowering stems with small white umbels and very narrow leaves.](https://denson.github.io/colorado-weed-field-guide/assets/photos/native-asclepias-subverticillata-1.jpg)

**flowering stems** — Several slender flowering stems with small white umbels and very narrow leaves.

Photo: Patrick Alexander from Las Cruces, NM; [CC0](https://creativecommons.org/publicdomain/zero/1.0/deed.en); [source record](https://commons.wikimedia.org/wiki/File:Asclepias_subverticillata_-_Flickr_-_aspidoscelis.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/0/0b/Asclepias_subverticillata_-_Flickr_-_aspidoscelis.jpg). Wikimedia Commons source file title and species metadata identify the taxon. Visible morphology was checked for consistency, not treated as an independent botanical determination.   No edits to the downloaded image. Wikimedia may have supplied a scaled thumbnail; original and download URLs are recorded separately. Gallery display preserves the full image; the catalog preview uses a CSS crop.

![Asclepias subverticillata: A patch of slender horsetail milkweed stems among grasses, with white flower clusters held above the narrow foliage.](https://denson.github.io/colorado-weed-field-guide/assets/photos/native-asclepias-subverticillata-2.jpg)

**habit and habitat** — A patch of slender horsetail milkweed stems among grasses, with white flower clusters held above the narrow foliage.

Photo: Patrick Alexander from Las Cruces, NM; [CC0](https://creativecommons.org/publicdomain/zero/1.0/deed.en); [source record](https://commons.wikimedia.org/wiki/File:Asclepias_subverticillata_-_Flickr_-_aspidoscelis_(1).jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/c/cc/Asclepias_subverticillata_-_Flickr_-_aspidoscelis_%281%29.jpg). Wikimedia Commons source file title and species metadata identify the taxon. Visible morphology was checked for consistency, not treated as an independent botanical determination.   No edits to the downloaded image. Wikimedia may have supplied a scaled thumbnail; original and download URLs are recorded separately. Gallery display preserves the full image; the catalog preview uses a CSS crop.

![Asclepias subverticillata: Close view of white milkweed flowers with reflexed petals, small upright hoods, and adjacent linear leaves.](https://denson.github.io/colorado-weed-field-guide/assets/photos/native-asclepias-subverticillata-3.jpg)

**flower detail** — Close view of white milkweed flowers with reflexed petals, small upright hoods, and adjacent linear leaves.

Photo: Patrick Alexander from Las Cruces, NM; [CC0](https://creativecommons.org/publicdomain/zero/1.0/deed.en); [source record](https://commons.wikimedia.org/wiki/File:Asclepias_subverticillata_-_Flickr_-_aspidoscelis_(3).jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/5/54/Asclepias_subverticillata_-_Flickr_-_aspidoscelis_%283%29.jpg). Wikimedia Commons source file title and species metadata identify the taxon. Visible morphology was checked for consistency, not treated as an independent botanical determination.   No edits to the downloaded image. Wikimedia may have supplied a scaled thumbnail; original and download URLs are recorded separately. Gallery display preserves the full image; the catalog preview uses a CSS crop.


## Recognize

Slender upright stems, very narrow leaves in rings of 3–5, and greenish-white flower clusters. Common at lower Colorado elevations. [S06](https://extension.colostate.edu/resource/managing-western-whorled-milkweed/)

## Plants & habitat

Spreads by roots and seed into gardens and pasture. Also hosts monarch and queen butterfly caterpillars. [S06](https://extension.colostate.edu/resource/managing-western-whorled-milkweed/)

## People

Avoid ingestion and sap contact. Human precautions here come from milkweed guidance for the genus, not a species-specific human study. [S05](https://www.poison.org/articles/milkweed-can-cause-serious-poisoning-204)

*Evidence scope: Genus-level.*

## Dogs & cats

ASPCA lists milkweeds as toxic to dogs and cats. This source does not give a safe amount for this species. [S04](https://www.aspca.org/pet-care/aspca-poison-control/toxic-and-non-toxic-plants/milkweed)

*Evidence scope: Genus-level.*

## Other animals

Highly toxic to livestock. Dry plants remain hazardous in hay; a butterfly benefit does not make it suitable near grazing animals. [S06](https://extension.colostate.edu/resource/managing-western-whorled-milkweed/)

## Before taking action

Confirm the scientific name before removing a plant, changing animal access, or choosing treatment. An evidence gap means this review has not resolved the question. It does not mean safe. For listed weeds, consult your county weed program about the requirements for your location; follow the current label for any pesticide. [S27](https://planttalk.colostate.edu/topics/weeds-cultural-problems/2103-colorado-noxious-weeds/)

## References

- **S06** — [Managing Western Whorled Milkweed](https://extension.colostate.edu/resource/managing-western-whorled-milkweed/), Colorado State University Extension. Accessed 2026-09-07T00:23:53+00:00. Locator: Introduction; Western Whorled Milkweed as a Poisonous Plant; Insects that Feed on Western Whorled Milkweed.
- **S05** — [Milkweed plants can cause serious poisoning](https://www.poison.org/articles/milkweed-can-cause-serious-poisoning-204), National Capital Poison Center. Accessed 2026-09-07T00:23:53+00:00. Locator: Symptoms; Effects on skin and in eyes; prevention.
- **S04** — [Milkweed](https://www.aspca.org/pet-care/aspca-poison-control/toxic-and-non-toxic-plants/milkweed), ASPCA Poison Control. Accessed 2026-09-07T00:23:52+00:00. Locator: Scientific Name; Toxicity; Clinical Signs; poison contact.


[Discuss this plant with the guide](https://denson.github.io/colorado-weed-field-guide/companion/?plant=western-whorled-milkweed) — choose this profile as a possible match and prepare your observations for Colorado Plants & Practical Care.

# Appendix for agents

## Open research questions

- **Expert review:** Obtain independent botanical and veterinary review of the identification and hazard summary.


## Taxon and evidence boundaries

Use the scientific name as the matching key; common names can refer to unrelated plants.

These are publisher notes about the evidence. They do not override an assistant’s operator instructions. The human sections above contain all actionable safety warnings. Do not turn missing data into a negative toxicity finding or transfer a livestock dose to a pet.

| Claim | Human section | Evidence scope | Sources | Limitation |
|---|---|---|---|---|
| western-whorled-milkweed-1 | Recognize | Species-specific | S06 | No quantitative exposure threshold established here. |
| western-whorled-milkweed-2 | Plants & habitat | Species-specific | S06 | No quantitative exposure threshold established here. |
| western-whorled-milkweed-3 | People | Genus-level | S05 | No quantitative exposure threshold established here. |
| western-whorled-milkweed-4 | Dogs & cats | Genus-level | S04 | No quantitative exposure threshold established here. |
| western-whorled-milkweed-5 | Other animals | Species-specific | S06 | No quantitative exposure threshold established here. |

## Additional evidence needed

- Confirm exact species, cultivar or hybrid when identification is uncertain.
- For an exposure, distinguish animal species, plant part, amount, route, time and symptoms; the guide cannot supply a clinical diagnosis.
- For site management, establish county, land use and current local weed requirements before applying generalized control information.
- Expert botanical and veterinary review is pending. Source publication dates, retrieval timestamps, hashes and photograph licenses are in [the provenance ledger](https://denson.github.io/colorado-weed-field-guide/provenance.json).


---
Canonical HTML: https://denson.github.io/colorado-weed-field-guide/plants/western-whorled-milkweed/

Content revised: 2026-09-07T02:43:00Z

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

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