---
{
  "id": "common-st-johnswort",
  "name": "Common St. Johnswort",
  "scientific": "Hypericum perforatum",
  "category": "invasive",
  "status": "Colorado noxious weed · List C — Manage widespread weeds",
  "status_sources": [
    "CO-rules-2025"
  ],
  "warning": "Photosensitivity and animal exposure concerns",
  "warning_tone": "danger",
  "aliases": [
    "St John’s wort",
    "Klamath weed"
  ],
  "taxonomy_note": "",
  "source_ids": [
    "CO-rules-2025",
    "N-hypericum-perforatum",
    "S27",
    "Fremont-list-definitions"
  ],
  "description": "Common St. Johnswort: Photosensitivity and animal exposure concerns. Identification and evidence for Colorado readers.",
  "claim_evidence": [
    {
      "label": "Recognize",
      "sources": [
        "N-hypericum-perforatum"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "common-st-johnswort-1"
    },
    {
      "label": "Plants & habitat",
      "sources": [
        "N-hypericum-perforatum"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "common-st-johnswort-2"
    },
    {
      "label": "People",
      "sources": [
        "N-hypericum-perforatum"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "common-st-johnswort-3"
    },
    {
      "label": "Dogs & cats",
      "sources": [
        "N-hypericum-perforatum"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "common-st-johnswort-4"
    },
    {
      "label": "Other animals",
      "sources": [
        "N-hypericum-perforatum"
      ],
      "scope": "Species-specific",
      "note": "",
      "id": "common-st-johnswort-5"
    },
    {
      "id": "common-st-johnswort-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": "Dogs & cats",
        "question": "Find species-specific dog and cat exposure evidence; do not infer safety from missing reports.",
        "related_claim_ids": [
          "common-st-johnswort-4"
        ]
      },
      {
        "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/common-st-johnswort.md"
}
---

# Common St. Johnswort

*Hypericum perforatum*

**Colorado noxious weed · List C — Manage widespread weeds.** [CO-rules-2025](https://www.sos.state.co.us/CCR/GenerateRulePdf.do?ruleVersionId=11948&fileName=8+CCR+1206-2)

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).

> **Photosensitivity and animal exposure concerns.** [Exposure help](https://denson.github.io/colorado-weed-field-guide/safety/).

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

This is a sourced working profile. Evidence is incomplete for **Dogs & cats**. 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.

![Hypericum perforatum: Five-petaled yellow flowers with numerous projecting stamens.](https://denson.github.io/colorado-weed-field-guide/assets/photos/common-priority-photos-hypericum-perforatum-05-01-flower.jpg)

**flower** — Five-petaled yellow flowers with numerous projecting stamens.

Photo: W. Bulach; [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0); [source record](https://commons.wikimedia.org/wiki/File:.00_3738_St._John%27s_wort_(Hypericum_perforatum).jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/6/68/.00_3738_St._John%27s_wort_%28Hypericum_perforatum%29.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.

![Hypericum perforatum: Erect branching stems with clusters of yellow flowers amid green foliage.](https://denson.github.io/colorado-weed-field-guide/assets/photos/common-priority-photos-hypericum-perforatum-05-07-habit.jpg)

**habit** — Erect branching stems with clusters of yellow flowers amid green foliage.

Photo: Sten Porse; [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0/); [source record](https://commons.wikimedia.org/wiki/File:Hypericon-perforatum.JPG); [original image](https://upload.wikimedia.org/wikipedia/commons/2/2f/Hypericon-perforatum.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.

![Hypericum perforatum: Backlit oval leaf showing numerous translucent pale gland dots and darker marginal dots.](https://denson.github.io/colorado-weed-field-guide/assets/photos/common-priority-photos-hypericum-perforatum-05-13-leaf.jpg)

**leaf** — Backlit oval leaf showing numerous translucent pale gland dots and darker marginal dots.

Photo: Michael Gasperl (Migas); [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0/); [source record](https://commons.wikimedia.org/wiki/File:Hypericum-perforatum(Blatt).jpg); [original image](https://upload.wikimedia.org/wikipedia/commons/c/cf/Hypericum-perforatum%28Blatt%29.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

Yellow five-petaled flowers with many stamens; small opposite leaves show translucent dots when backlit. [N-hypericum-perforatum](https://plants.ces.ncsu.edu/plants/hypericum-perforatum/)

## Plants & habitat

Seeds and spreading underground stems create persistent patches that can displace native plants. [N-hypericum-perforatum](https://plants.ces.ncsu.edu/plants/hypericum-perforatum/)

## People

Hypericin-containing plant material can cause photosensitivity and dermatitis. The guide does not assess herbal use or medication interactions. [N-hypericum-perforatum](https://plants.ces.ncsu.edu/plants/hypericum-perforatum/)

## Dogs & cats

NC State flags dogs and cats. This source does not establish a pet-specific safe amount. [N-hypericum-perforatum](https://plants.ces.ncsu.edu/plants/hypericum-perforatum/)

## Other animals

Horses are also flagged by the source. Seek veterinary advice before allowing access to contaminated forage. [N-hypericum-perforatum](https://plants.ces.ncsu.edu/plants/hypericum-perforatum/)

## 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-hypericum-perforatum** — [hypericum perforatum — Plant Toolbox](https://plants.ces.ncsu.edu/plants/hypericum-perforatum/), NC State Extension. Accessed 2026-09-07T01:04:55+00:00. Locator: Description; Attributes; Problems; Poisonous to Humans.
- **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=common-st-johnswort) — choose this profile as a possible match and prepare your observations for Colorado Plants & Practical Care.

# Appendix for agents

## Open research questions

- **Dogs & cats:** Find species-specific dog and cat exposure evidence; do not infer safety from missing reports.
- **Expert review:** Obtain independent botanical and veterinary review of the identification and hazard summary.


## Identity and evidence boundaries

Match the scientific name and synonyms, not the common name alone.

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 |
|---|---|---|---|---|
| common-st-johnswort-1 | Recognize | Species-specific | N-hypericum-perforatum | No quantitative safe exposure established here. |
| common-st-johnswort-2 | Plants & habitat | Species-specific | N-hypericum-perforatum | No quantitative safe exposure established here. |
| common-st-johnswort-3 | People | Species-specific | N-hypericum-perforatum | No quantitative safe exposure established here. |
| common-st-johnswort-4 | Dogs & cats | Species-specific | N-hypericum-perforatum | No quantitative safe exposure established here. |
| common-st-johnswort-5 | Other animals | Species-specific | N-hypericum-perforatum | 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).

## 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/common-st-johnswort/

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

Markdown mirror: https://denson.github.io/colorado-weed-field-guide/plants/common-st-johnswort/index.md

Text fallback: https://denson.github.io/colorado-weed-field-guide/plants/common-st-johnswort/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