Giant ragweed

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

Ambrosia trifida

Native Colorado plant · not on state Lists A/B/C. C-floristic-2020 CO-rules-2025

Allergenic pollen and strong competition in disturbed ground. Exposure help.

Identification caveat: Giant ragweed is Ambrosia trifida. A mismatched Latin name in the compared manufacturer guide is not adopted. Common ragweed and western ragweed have their own profiles.

Still researching this plant

This is a sourced working profile. Evidence is incomplete for 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.

Ambrosia trifida: An upright giant ragweed plant beside a fence, with large three- to five-lobed leaves. — **habit** — An upright giant ragweed plant beside a fence, with large three- to five-lobed leaves.
GTBacchus · 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. Giant ragweed leaves may be simple, three-lobed or five-lobed. The photographs retain the exact source species identification.
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.

Ambrosia trifida: Large lobed, toothed leaves arranged along a sturdy green stem. — **leaves and stem** — Large lobed, toothed leaves arranged along a sturdy green stem.
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. Giant ragweed leaves may be simple, three-lobed or five-lobed. The photographs retain the exact source species identification.
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.

Ambrosia trifida: Several tall leafy shoots carrying slender terminal spikes of green flower heads. — **flowering habit** — Several tall leafy shoots carrying slender terminal spikes of green flower heads.
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. Giant ragweed leaves may be simple, three-lobed or five-lobed. The photographs retain the exact source species identification.
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 annual with rough stems and opposite leaves, usually with three to five deep lobes. Early leaves may be unlobed. Male flower clusters stand at the stem tips. I-giant-ragweed

People

Windborne pollen is a significant hay-fever trigger for susceptible people. Colorado extension describes late-season ragweed exposure; local timing varies with weather. I-giant-ragweed C-ragweed-native

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

Some animals browse the foliage; that does not establish a safe feed amount or a pet-safety rating. I-giant-ragweed

Evidence scope: Forage observation; safety limitation.

Plants & habitat

Early emergence and tall growth can help it dominate disturbed sites. Resistance to some herbicide groups is documented in particular populations. I-giant-ragweed

Management & timing

Remove young plants before flowering and seed formation. For a large or persistent infestation, consult a local weed specialist about identification and resistance. C-ragweed-native

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.
I-giant-ragweed — Giant ragweed, Iowa State University Extension. Accessed 2026-09-07T02:26:09+00:00. Locator: Named species or genus; identification, distribution and hazard sections where available.
C-ragweed-native — Ragweed, 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, 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.