Annual sowthistle

Discuss this plant with the guide — choose this profile as a possible match and prepare your observations for Colorado Plants & Practical Care.

Sonchus oleraceus

Common introduced weed · not on state Lists A/B/C. C-floristic-2020 CO-rules-2025

Windborne seed and crop-pest hosting. Exposure help.

Identification caveat: Separate from prickly sowthistle (Sonchus asper) and the root-spreading perennial sowthistle (Sonchus arvensis). Seedlings can be difficult to separate.

Still researching this plant

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.

Sonchus oleraceus: Close view of a pale yellow head composed of many narrow strap-shaped florets. — **flower head** — Close view of a pale yellow head composed of many narrow strap-shaped florets.
Miwasatoshi · CC BY-SA 4.0
Photo source and identification
Original record · Original image
Source scientific identification plus visual morphology review of the selected photograph. This is a source-identified reference, not an independent specimen-based taxonomic determination. Sow-thistles can overlap in appearance. Source identification is retained; the leaf and fruit views complement the flower view rather than independently proving identity.
Wikimedia server-resized derivative; no local crop, colour change or generative editing. Gallery display preserves the full image; catalog preview may use a CSS crop.

Sonchus oleraceus: A deeply lobed middle leaf with a large pointed terminal lobe and toothed edges. — **leaf** — A deeply lobed middle leaf with a large pointed terminal lobe and toothed edges.
T.Voekler · CC BY-SA 3.0
Photo source and identification
Original record · Original image
Source scientific identification plus visual morphology review of the selected photograph. This is a source-identified reference, not an independent specimen-based taxonomic determination. Sow-thistles can overlap in appearance. Source identification is retained; the leaf and fruit views complement the flower view rather than independently proving identity.
Wikimedia server-resized derivative; no local crop, colour change or generative editing. Gallery display preserves the full image; catalog preview may use a CSS crop.

Sonchus oleraceus: Several brown fruits with fine white pappus hairs photographed against a dark background. — **fruits** — Several brown fruits with fine white pappus hairs photographed against a dark background.
Ayotte, Gilles, 1948- · CC BY-SA 4.0
Photo source and identification
Original record · Original image
Source scientific identification plus visual morphology review of the selected photograph. This is a source-identified reference, not an independent specimen-based taxonomic determination. Sow-thistles can overlap in appearance. Source identification is retained; the leaf and fruit views complement the flower view rather than independently proving identity.
Wikimedia server-resized derivative; no local crop, colour change or generative editing. Gallery display preserves the full image; catalog preview may use a CSS crop.

Identification

An upright annual with hollow stems, milky sap, yellow flower heads and fluffy seedheads. Upper leaves clasp the stem with pointed, clawlike basal lobes. U-sowthistle

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

The reviewed sources do not establish species-specific ingestion effects or a safe exposure amount for livestock. Missing evidence does not establish safety.

Plants & habitat

It occupies disturbed gardens and cropland and can host aphids and other crop pests. The cited examples are from California; they do not prove a particular Colorado pest outbreak. U-sowthistle

Evidence scope: Species ecology; location-limited pest examples.

Management & timing

Prevent plants from producing and dispersing seed. Confirm the species before treating a recurring colony as annual sowthistle. U-sowthistle

Evidence scope: Management inference from annual seed reproduction.

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

References

C-floristic-2020 — Colorado Floristic Quality Assessment: 2020 Update, 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, 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.
U-sowthistle — Annual sowthistle, University of California Statewide Integrated Pest Management. Accessed 2026-09-07T02:26:10+00:00. Locator: Named species or genus; identification, distribution and hazard sections where available.
S27 — 2103 – Colorado Noxious Weeds, PlantTalk Colorado · CSU Extension. Accessed 2026-09-07T00:23:57+00:00. Locator: List A, B and C management categories.

Content revised 2026-09-07 02:46:00Z · Source checks dated separately in the references.