---
{
  "id": "poison-hemlock",
  "name": "Poison hemlock",
  "scientific": "Conium maculatum",
  "category": "invasive",
  "status": "Colorado List C — Manage widespread weeds",
  "status_sources": [
    "S16"
  ],
  "warning": "Poisonous to people & animals",
  "warning_tone": "danger",
  "aliases": [],
  "taxonomy_note": "",
  "source_ids": [
    "S16",
    "S26",
    "S04",
    "Fremont-list-definitions"
  ],
  "description": "Poison hemlock: Poisonous to people & animals. Colorado identification, habitat and exposure evidence.",
  "claim_evidence": [
    {
      "label": "Recognize",
      "sources": [
        "S16"
      ],
      "scope": "Species-specific",
      "note": "",
      "reviewed": "2026-09-06",
      "id": "poison-hemlock-1"
    },
    {
      "label": "Plants & habitat",
      "sources": [
        "S16"
      ],
      "scope": "Species-specific + garden judgment",
      "note": "Access restriction and referral are cautious editorial advice based on documented poisoning.",
      "reviewed": "2026-09-06",
      "id": "poison-hemlock-2"
    },
    {
      "label": "People",
      "sources": [
        "S16",
        "S26"
      ],
      "scope": "Species-specific",
      "note": "",
      "reviewed": "2026-09-06",
      "id": "poison-hemlock-3"
    },
    {
      "label": "Dogs & cats",
      "sources": [
        "S16",
        "S04"
      ],
      "scope": "Broad animal warning; pet precaution",
      "note": "Pet recommendation applies general animal toxicity plus ASPCA's prompt-contact guidance; it is not a species-specific dose estimate.",
      "reviewed": "2026-09-06",
      "id": "poison-hemlock-4"
    },
    {
      "label": "Other animals",
      "sources": [
        "S16"
      ],
      "scope": "Species-specific + interpretation",
      "note": "Risk interpretation follows the county page listing both List C and toxicity.",
      "reviewed": "2026-09-06",
      "id": "poison-hemlock-5"
    },
    {
      "id": "poison-hemlock-list-meaning",
      "label": "State-list meaning",
      "sources": [
        "Fremont-list-definitions"
      ],
      "scope": "Colorado management-category definition; species classification retains its original status sources",
      "note": "Plain-language editorial label for a management category; not a toxicity or danger rating.",
      "reviewed": "2026-09-07"
    }
  ],
  "noxious_class": "C",
  "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"
  },
  "noxious_class_label": "C — Manage widespread weeds",
  "source": "content/plants/poison-hemlock.md"
}
---

# Poison hemlock

*Conium maculatum*

**Colorado List C — Manage widespread weeds.** [S16](https://www.jeffco.us/2230/Poison-Hemlock)

Widespread and well established. Control is recommended by the state; local governments may require management. This is a management category, separate from danger ratings. [Category source](https://fremontcountyco.gov/weed-control/weed-identification) · [What the lists mean](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists).

> **Poisonous to people & animals.** 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.

![Conium maculatum: Tall branching plant with many small white compound umbels above finely divided foliage.](https://denson.github.io/colorado-weed-field-guide/assets/photos/invasive-conium-maculatum-habit.jpg)

**habit** — Tall branching plant with many small white compound umbels above finely divided foliage.

Photo: Krzysztof Ziarnek, Kenraiz; [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0); [source record](https://commons.wikimedia.org/wiki/File:Conium_maculatum_kz01_2021-06-23_R%C3%B3wne.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/6/65/Conium_maculatum_kz01_2021-06-23_R%C3%B3wne.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.   Wikimedia server-resized derivative; no crop, color change, or generative editing. Gallery display preserves the full image; the catalog preview uses a CSS crop.

![Conium maculatum: Smooth green stem with conspicuous purple-red blotches beside finely divided leaves.](https://denson.github.io/colorado-weed-field-guide/assets/photos/invasive-conium-maculatum-stem.jpg)

**stem** — Smooth green stem with conspicuous purple-red blotches beside finely divided leaves.

Photo: MPF; [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0); [source record](https://commons.wikimedia.org/wiki/File:Conium_maculatum_Hauxley_2.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/f/f7/Conium_maculatum_Hauxley_2.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.   Wikimedia server-resized derivative; no crop, color change, or generative editing. Gallery display preserves the full image; the catalog preview uses a CSS crop.

![Conium maculatum: Small white flowers in compound umbels, with finely divided leaves visible beside them.](https://denson.github.io/colorado-weed-field-guide/assets/photos/invasive-conium-maculatum-flower.jpg)

**flower** — Small white flowers in compound umbels, with finely divided leaves visible beside them.

Photo: Nicolas Ramirez; [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0); [source record](https://commons.wikimedia.org/wiki/File:Conium_maculatum_007.jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/0/03/Conium_maculatum_007.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.   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

Finely divided leaves, stems with purple spots, and white umbrella-like flower clusters. Often near streams, ditches, and pasture edges. [S16](https://www.jeffco.us/2230/Poison-Hemlock)

## Plants & habitat

An introduced, colony-forming biennial. Garden judgment: exclude access and seek local management help once identification is confirmed. [S16](https://www.jeffco.us/2230/Poison-Hemlock)

## People

All parts are poisonous. Do not eat or taste it to identify it. Suspected ingestion warrants immediate Poison Control advice. [S16](https://www.jeffco.us/2230/Poison-Hemlock) [S26](https://www.poison.org/first-aid-for-poisonings)

## Dogs & cats

The county warning covers animals broadly. Treat suspected ingestion by a pet as urgent and contact a veterinarian. [S16](https://www.jeffco.us/2230/Poison-Hemlock) [S04](https://www.aspca.org/pet-care/aspca-poison-control/toxic-and-non-toxic-plants/milkweed)

## Other animals

Poisonous to livestock; do not use grazing as a control method. A List C designation does not indicate low toxicity. [S16](https://www.jeffco.us/2230/Poison-Hemlock)

## 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

- **S16** — [Poison Hemlock](https://www.jeffco.us/2230/Poison-Hemlock), Jefferson County, Colorado. Accessed 2026-09-07T00:23:54+00:00. Locator: List C designation; General; Habitat; Vegetation; Flower.
- **S26** — [First aid guidelines](https://www.poison.org/first-aid-for-poisonings), National Capital Poison Center. Accessed 2026-09-07T00:23:56+00:00. Locator: Poison center phone; emergency symptoms; do not induce vomiting.
- **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=poison-hemlock) — 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 |
|---|---|---|---|---|
| poison-hemlock-1 | Recognize | Species-specific | S16 | No quantitative exposure threshold established here. |
| poison-hemlock-2 | Plants & habitat | Species-specific + garden judgment | S16 | Access restriction and referral are cautious editorial advice based on documented poisoning. |
| poison-hemlock-3 | People | Species-specific | S16, S26 | No quantitative exposure threshold established here. |
| poison-hemlock-4 | Dogs & cats | Broad animal warning; pet precaution | S16, S04 | Pet recommendation applies general animal toxicity plus ASPCA's prompt-contact guidance; it is not a species-specific dose estimate. |
| poison-hemlock-5 | Other animals | Species-specific + interpretation | S16 | Risk interpretation follows the county page listing both List C and toxicity. |

## 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).

## State-list label provenance

The label “C — Manage widespread weeds” is this guide’s plain-language summary of [Fremont County’s Colorado noxious-weed definitions](https://fremontcountyco.gov/weed-control/weed-identification) [Fremont-list-definitions]. The letter and species assignment retain the status sources above. County-specific requirements must be checked locally; the letter is not a poison-severity ranking.


---
Canonical HTML: https://denson.github.io/colorado-weed-field-guide/plants/poison-hemlock/

Content revised: 2026-09-07T03:50:00Z

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

Text fallback: https://denson.github.io/colorado-weed-field-guide/plants/poison-hemlock/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