--- { "source_ids": [ "A-greasewood", "A-halogeton", "A-larkspur", "A-loco", "A-ragwort", "ARS-deathcamas", "ARS-larkspur", "B-deathcamas", "C-descriptions", "C-floristic-2020", "C-foxtail", "C-low-water-cactus", "C-ragweed-native", "C-routt-guide", "C-tumbleweeds", "C-wetland-guide", "CO-rules-2025", "F-ambrosia-psilostachya", "F-apocynum", "F-gutierrezia", "F-locoweed", "F-melilotus", "F-opuntia", "F-sarcobatus", "F-securigera", "F-sisymbrium", "F-urtica", "FDA-poison-ivy", "Fremont-list-definitions", "I-buffalobur", "I-giant-ragweed", "J-canada-thistle", "J-houndstongue", "J-leafy-spurge", "J-perennial-pepperweed", "J-russian-knapweed", "J-russian-olive", "J-saltcedar---tamarisk", "K-bindweed", "K-kochia", "M-quackgrass", "N-ailanthus-altissima", "N-amaranthus-retroflexus", "N-ambrosia-artemisiifolia", "N-apocynum-cannabinum", "N-arctium-minus", "N-arundo-donax", "N-centaurea-solstitialis", "N-convolvulus-arvensis", "N-cyperus-esculentus", "N-datura-stramonium", "N-erigeron-canadensis", "N-euphorbia-cyparissias", "N-euphorbia-esula", "N-euphorbia-maculata", "N-euphorbia-marginata", "N-galium-aparine", "N-hydrilla-verticillata", "N-hyoscyamus-niger", "N-hypericum-perforatum", "N-iris-pseudacorus", "N-lythrum-salicaria", "N-melilotus-officinalis", "N-myriophyllum-aquaticum", "N-myriophyllum-spicatum", "N-pastinaca-sativa", "N-ranunculus-acris", "N-reynoutria-japonica", "N-rhaponticum-repens", "N-salvinia-molesta", "N-securigera-varia", "N-solanum-dulcamara", "N-sorghum-halepense", "N-urtica-dioica", "N-xanthium-strumarium", "NPS-african", "P-buttercup", "P-dogbane", "S04", "S05", "S06", "S13", "S14", "S15", "S16", "S17", "S18", "S19", "S20", "S21", "S22", "S26", "S27", "S29", "U-buffalobur", "U-silverleaf", "U-water-hemlock", "UC-russian-thistle", "W-buffalobur", "WA-african-rue", "WA-bohemian-knotweed", "WA-flowering-rush", "WA-medusahead", "WA-tansy-ragwort", "WA-ventenata" ] } --- # Biggest concerns Start with the kind of problem you need to prevent. These are selected routes into the guide, not a universal severity ranking. Read each profile for the affected animal group and evidence limits. For suspected exposure, use the [safety guide](https://denson.github.io/colorado-weed-field-guide/safety/). State-list labels describe management requirements, separately from these hazards. [What Lists A, B and C mean](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). ## Serious ingestion hazards Plants with serious poisoning warnings. Affected species and evidence strength differ; use the individual profile before drawing conclusions about a person or animal. - [Poison hemlock](https://denson.github.io/colorado-weed-field-guide/plants/poison-hemlock/) — **Poisonous to people & animals**. State management: [List C — Manage widespread weeds](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/poison-hemlock/index.md). ![Conium maculatum: Tall branching plant with many small white compound umbels above finely divided foliage.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/poison-hemlock.webp) Photo: Krzysztof Ziarnek, Kenraiz; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/poison-hemlock/). - [Western water hemlock](https://denson.github.io/colorado-weed-field-guide/plants/western-water-hemlock/) — **Potentially fatal poisoning; wetland look-alike danger**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/western-water-hemlock/index.md). ![Cicuta douglasii: Upright leafy stems with pinnately divided, sharply toothed leaflets.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/western-water-hemlock.webp) Photo: Lee Cain; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/western-water-hemlock/). - [Jimsonweed](https://denson.github.io/colorado-weed-field-guide/plants/jimsonweed/) — **Serious poisoning risk to people and animals**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/jimsonweed/index.md). ![Datura stramonium: Bushy plant with coarsely toothed broad leaves and upright white flowers.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/jimsonweed.webp) Photo: H. Zell; CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/jimsonweed/). - [Black henbane](https://denson.github.io/colorado-weed-field-guide/plants/black-henbane/) — **Highly poisonous; keep animals away**. State management: [List B — Stop the spread](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/black-henbane/index.md). ![Pale yellow henbane flowers with purple veins and dark purple throats above lobed leaves.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/black-henbane.webp) Photo: H. Zell; CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/black-henbane/). - [Meadow death camas](https://denson.github.io/colorado-weed-field-guide/plants/meadow-death-camas/) — **Potentially fatal ingestion; onion-like bulbs**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/meadow-death-camas/index.md). ![Toxicoscordion venenosum: An upright cluster of small pale flowers above a basal group of narrow, strap-shaped leaves.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/meadow-death-camas.webp) Photo: NPS Photo; Public domain. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/meadow-death-camas/). - [African rue](https://denson.github.io/colorado-weed-field-guide/plants/african-rue/) — **Highly poisonous if eaten; deep-rooted invader**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/african-rue/index.md). ![Peganum harmala: A partly open white flower above narrow, deeply divided leaves.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/african-rue.webp) Photo: Alex Abair; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/african-rue/). - [Bittersweet nightshade](https://denson.github.io/colorado-weed-field-guide/plants/bittersweet-nightshade/) — **Poisonous berries and foliage**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/bittersweet-nightshade/index.md). ![Solanum dulcamara: Dense tangle of leafy scrambling stems with pointed green leaves.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/bittersweet-nightshade.webp) Photo: H. Zell; CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/bittersweet-nightshade/). - [Western whorled milkweed](https://denson.github.io/colorado-weed-field-guide/plants/western-whorled-milkweed/) — **High livestock toxicity**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/western-whorled-milkweed/index.md). ![Asclepias subverticillata: Several slender flowering stems with small white umbels and very narrow leaves.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/western-whorled-milkweed.webp) Photo: Patrick Alexander from Las Cruces, NM; CC0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/western-whorled-milkweed/). - [Hemp dogbane](https://denson.github.io/colorado-weed-field-guide/plants/hemp-dogbane/) — **Cardiac-glycoside poisoning hazard for people and animals**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/hemp-dogbane/index.md). ![Apocynum cannabinum: A dense leafy flowering stand beside a cultivated field.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/hemp-dogbane.webp) Photo: Ayotte, Gilles, 1948-; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/hemp-dogbane/). - [Silverleaf nightshade](https://denson.github.io/colorado-weed-field-guide/plants/silverleaf-nightshade/) — **Toxic leaves and berries; persistent spreading roots**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/silverleaf-nightshade/index.md). ![Upright silverleaf nightshade stem with gray-green leaves and purple flowers in a grassy field.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/silverleaf-nightshade.webp) Photo: Sam Kieschnick; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/silverleaf-nightshade/). - [Tall buttercup](https://denson.github.io/colorado-weed-field-guide/plants/tall-buttercup/) — **Fresh plant irritation and animal poisoning hazard**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/tall-buttercup/index.md). ![Meadow buttercup growing among grass, with yellow flowers held above deeply divided basal leaves.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/tall-buttercup.webp) Photo: Daniel Cahen; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/tall-buttercup/). - [Crownvetch](https://denson.github.io/colorado-weed-field-guide/plants/crownvetch/) — **Toxic to horses; spreading roots can smother vegetation**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/crownvetch/index.md). ![Sprawling crownvetch stems with many pink-and-white flower clusters and pinnate leaves among other vegetation.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/crownvetch.webp) Photo: Dale Knox; CC0 1.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/crownvetch/). ## Pets, paws and skin contact A route to awns, burs, sharp points and irritating plant contact, including poison-ivy oil carried on fur. - [Foxtail barley](https://denson.github.io/colorado-weed-field-guide/plants/foxtail-barley/) — **Barbed seed awns injure animals**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/foxtail-barley/index.md). ![Hordeum jubatum: A tuft of foxtail barley with slender leaves and pale pink, long-awned heads.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/foxtail-barley.webp) Photo: Diego Delso; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/foxtail-barley/). - [Puncturevine / goathead](https://denson.github.io/colorado-weed-field-guide/plants/puncturevine/) — **Sharp burs · Animal injury**. State management: [List C — Manage widespread weeds](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/puncturevine/index.md). ![Tribulus terrestris: Five-petaled pale yellow flower beside hairy leaves with paired oval leaflets.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/puncturevine.webp) Photo: Forest & Kim Starr; CC BY 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/puncturevine/). - [Common burdock](https://denson.github.io/colorado-weed-field-guide/plants/common-burdock/) — **Burs and fine bristles injure animals**. State management: [List C — Manage widespread weeds](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/common-burdock/index.md). ![Arctium minus: Green spherical burs covered with long hooked bracts.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/common-burdock.webp) Photo: Isidre blanc; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/common-burdock/). - [Common cocklebur](https://denson.github.io/colorado-weed-field-guide/plants/common-cocklebur/) — **Burs catch in fur; seedlings and seeds are hazardous**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/common-cocklebur/index.md). ![Xanthium strumarium: Broad triangular to shallowly lobed leaf with coarse teeth and prominent veins.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/common-cocklebur.webp) Photo: Franco Folini; CC BY 2.5. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/common-cocklebur/). - [Myrtle spurge](https://denson.github.io/colorado-weed-field-guide/plants/myrtle-spurge/) — **Poisonous · Eye & skin injury**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/myrtle-spurge/index.md). ![Euphorbia myrsinites: Close view of yellow-green bracts and small cup-shaped cyathia.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/myrtle-spurge.webp) Photo: Flocci Nivis; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/myrtle-spurge/). - [Cypress spurge](https://denson.github.io/colorado-weed-field-guide/plants/cypress-spurge/) — **Sap can injure skin and eyes**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/cypress-spurge/index.md). ![Euphorbia cyparissias: Yellow-green floral bracts and small cyathia above many narrow leaves.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/cypress-spurge.webp) Photo: Tom Scavo; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/cypress-spurge/). - [Western poison ivy](https://denson.github.io/colorado-weed-field-guide/plants/western-poison-ivy/) — **Skin-rash oil can travel on pet fur**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/western-poison-ivy/index.md). ![Toxicodendron rydbergii: Dense cluster of pale whitish berries among dry leaves and twigs.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/western-poison-ivy.webp) Photo: Andrey Zharkikh from Salt Lake City, USA; CC BY 2.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/western-poison-ivy/). - [Buffalobur](https://denson.github.io/colorado-weed-field-guide/plants/buffalobur/) — **Sharp spines; toxic to livestock**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/buffalobur/index.md). ![Solanum rostratum: Deeply lobed leaves and prickly stems, with a yellow flower visible at the edge.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/buffalobur.webp) Photo: Juan Carlos Fonseca Mata; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/buffalobur/). - [Wild parsnip](https://denson.github.io/colorado-weed-field-guide/plants/wild-parsnip/) — **Sap plus sunlight can cause severe skin burns**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/wild-parsnip/index.md). ![Pastinaca sativa: A tall branched stem with compound leaves and small yellow umbels, shown against the sky.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/wild-parsnip.webp) Photo: Stefan.lefnaer; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/wild-parsnip/). - [Stinging nettle](https://denson.github.io/colorado-weed-field-guide/plants/stinging-nettle/) — **Stinging hairs; this profile covers a taxonomic complex**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/stinging-nettle/index.md). ![Urtica dioica: An upright nettle stem bearing toothed opposite leaves and slender branching flower clusters.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/stinging-nettle.webp) Photo: Przykuta; CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/stinging-nettle/). - [Spotted spurge](https://denson.github.io/colorado-weed-field-guide/plants/spotted-spurge/) — **Milky sap can injure skin and eyes; keep pets from chewing**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/spotted-spurge/index.md). ![Spotted spurge: low radiating reddish stems and small opposite leaves form a mat on stony ground.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/spotted-spurge.webp) Photo: Stefan.lefnaer; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/spotted-spurge/). - [Snow-on-the-mountain](https://denson.github.io/colorado-weed-field-guide/plants/snow-on-the-mountain/) — **Latex can damage skin and eyes; toxic if eaten**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/snow-on-the-mountain/index.md). ![Tall, branching snow-on-the-mountain plant with green lower leaves and white-edged upper foliage.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/snow-on-the-mountain.webp) Photo: Mark G; CC0 1.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/snow-on-the-mountain/). - [Plains pricklypear](https://denson.github.io/colorado-weed-field-guide/plants/plains-pricklypear/) — **Sharp spines around paws, mouths and eyes**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/plains-pricklypear/index.md). ![Low cluster of plains pricklypear pads covered with long pale spines on dry ground.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/plains-pricklypear.webp) Photo: eigenmotion; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/plains-pricklypear/). - [Catchweed bedstraw](https://denson.github.io/colorado-weed-field-guide/plants/catchweed-bedstraw/) — **Hooked hairs and burrs cling to fur and clothing**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/catchweed-bedstraw/index.md). ![Galium aparine: Small white flowers and rounded young fruits at the tip of a prickly leafy shoot.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/catchweed-bedstraw.webp) Photo: Krzysztof Ziarnek, Kenraiz; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/catchweed-bedstraw/). ## Pasture, hay and livestock losses For landowners and animal keepers: forage poisoning and injury deserve their own review. Livestock findings are not pet exposure limits. - [Tall larkspur](https://denson.github.io/colorado-weed-field-guide/plants/tall-larkspur/) — **Serious livestock and pet poisoning hazard**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/tall-larkspur/index.md). ![Delphinium barbeyi: A blue-flowered larkspur stem with several spurred flowers and divided stem leaves.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/tall-larkspur.webp) Photo: Ms. Sticky; CC BY 2.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/tall-larkspur/). - [Low larkspur](https://denson.github.io/colorado-weed-field-guide/plants/low-larkspur/) — **Serious livestock and pet poisoning hazard**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/low-larkspur/index.md). ![Delphinium nuttallianum: A patch of low blue-flowered larkspur growing among stones and dry leaf litter.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/low-larkspur.webp) Photo: NPS; Public domain. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/low-larkspur/). - [Halogeton](https://denson.github.io/colorado-weed-field-guide/plants/halogeton/) — **Severe oxalate poisoning in livestock**. State management: [List C — Manage widespread weeds](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/halogeton/index.md). ![Halogeton glomeratus: Small fleshy cylindrical blue-green leaves cluster along a reddish stem.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/halogeton.webp) Photo: Matt Lavin from Bozeman, Montana, USA; CC BY-SA 2.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/halogeton/). - [Johnsongrass](https://denson.github.io/colorado-weed-field-guide/plants/johnsongrass/) — **Livestock cyanide risk; added fire fuel**. State management: [List C — Manage widespread weeds](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/johnsongrass/index.md). ![Sorghum halepense: Close view of grass spikelets with dangling reddish anthers.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/johnsongrass.webp) Photo: Jim Conrad; Public domain. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/johnsongrass/). - [Tansy ragwort](https://denson.github.io/colorado-weed-field-guide/plants/tansy-ragwort/) — **Poisonous to livestock; harmful alkaloids**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/tansy-ragwort/index.md). ![Jacobaea vulgaris: An upright flowering plant with yellow heads and deeply divided leaves.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/tansy-ragwort.webp) Photo: jamie-aa; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/tansy-ragwort/). - [Russian knapweed](https://denson.github.io/colorado-weed-field-guide/plants/russian-knapweed/) — **Toxic to horses**. State management: [List B — Stop the spread](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/russian-knapweed/index.md). ![Rhaponticum repens: Dense patch of upright gray-green leafy plants beneath trees.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/russian-knapweed.webp) Photo: Amirh. absnd; CC0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/russian-knapweed/). - [Yellow starthistle](https://denson.github.io/colorado-weed-field-guide/plants/yellow-starthistle/) — **Severe horse poisoning; sharp spines**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/yellow-starthistle/index.md). ![Centaurea solstitialis: A bright yellow flower head beside a dry head, showing the long radiating spines.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/yellow-starthistle.webp) Photo: Юрий Данилевский (Yuriy Danilevsky); CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/yellow-starthistle/). - [Houndstongue](https://denson.github.io/colorado-weed-field-guide/plants/houndstongue/) — **Toxic to livestock**. State management: [List B — Stop the spread](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/houndstongue/index.md). ![Cynoglossum officinale: Upright leafy flowering plant with broad lance-shaped foliage and branched upper flower stems.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/houndstongue.webp) Photo: Bernd Haynold; CC BY 2.5. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/houndstongue/). - [White locoweed](https://denson.github.io/colorado-weed-field-guide/plants/white-locoweed/) — **Toxic to livestock**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/white-locoweed/index.md). ![Oxytropis sericea: Dry tan pods clustered along a stalk, some split open near their tips.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/white-locoweed.webp) Photo: Matt Lavin from Bozeman, Montana, USA; CC BY-SA 2.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/white-locoweed/). - [Kochia](https://denson.github.io/colorado-weed-field-guide/plants/kochia/) — **Tumbling seed spread; livestock poisoning risk**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/kochia/index.md). ![Bassia scoparia: Upright bushy green plant with many narrow leaves on densely branched stems.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/kochia.webp) Photo: WildBoar; Public domain. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/kochia/). - [Redroot pigweed](https://denson.github.io/colorado-weed-field-guide/plants/redroot-pigweed/) — **Crop competition; nitrate and oxalate livestock risk**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/redroot-pigweed/index.md). ![Amaranthus retroflexus: Upright plant with alternate broad leaves and a dense green terminal flower spike.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/redroot-pigweed.webp) Photo: Lynk media; CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/redroot-pigweed/). - [Common St. Johnswort](https://denson.github.io/colorado-weed-field-guide/plants/common-st-johnswort/) — **Photosensitivity and animal exposure concerns**. State management: [List C — Manage widespread weeds](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/common-st-johnswort/index.md). ![Hypericum perforatum: Five-petaled yellow flowers with numerous projecting stamens.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/common-st-johnswort.webp) Photo: W. Bulach; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/common-st-johnswort/). - [Broom snakeweed](https://denson.github.io/colorado-weed-field-guide/plants/broom-snakeweed/) — **Livestock poisoning and abortion risk when heavily eaten**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/broom-snakeweed/index.md). ![Small branching broom snakeweed plant with yellow flower clusters above a woody base on rocky ground.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/broom-snakeweed.webp) Photo: CK2AZ; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/broom-snakeweed/). - [Black greasewood](https://denson.github.io/colorado-weed-field-guide/plants/black-greasewood/) — **Oxalate poisoning can be fatal to livestock**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/black-greasewood/index.md). ![Low, spreading greasewood shrub with dense green foliage in open dry ground.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/black-greasewood.webp) Photo: John Gaskin; CC0 1.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/black-greasewood/). - [Yellow sweetclover](https://denson.github.io/colorado-weed-field-guide/plants/yellow-sweetclover/) — **Moldy sweetclover hay can cause dangerous bleeding**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/yellow-sweetclover/index.md). ![Yellow sweetclover: small yellow pea-like flowers hang along a narrow elongated raceme.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/yellow-sweetclover.webp) Photo: Isidre blanc; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/yellow-sweetclover/). - [White sweetclover](https://denson.github.io/colorado-weed-field-guide/plants/white-sweetclover/) — **Moldy sweetclover hay can cause dangerous bleeding**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/white-sweetclover/index.md). ![Melilotus albus: Small white flowers on slender racemes above three-part leaves.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/white-sweetclover.webp) Photo: zebedeugalinha; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/white-sweetclover/). - [Crownvetch](https://denson.github.io/colorado-weed-field-guide/plants/crownvetch/) — **Toxic to horses; spreading roots can smother vegetation**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/crownvetch/index.md). ![Sprawling crownvetch stems with many pink-and-white flower clusters and pinnate leaves among other vegetation.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/crownvetch.webp) Photo: Dale Knox; CC0 1.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/crownvetch/). ## Dry grass, tumbleweeds and fire concerns Read the profiles for the evidence about dry vegetation, tumbling plants and effects on land management. - [Cheatgrass / downy brome](https://denson.github.io/colorado-weed-field-guide/plants/cheatgrass/) — **Grass awns · Fire hazard**. State management: [List C — Manage widespread weeds](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/cheatgrass/index.md). ![Bromus tectorum: Green to purple awned spikelets hang from slender drooping panicle branches.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/cheatgrass.webp) Photo: AnRo0002; CC0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/cheatgrass/). - [Medusahead](https://denson.github.io/colorado-weed-field-guide/plants/medusahead/) — **Barbed awns; persistent fire-promoting thatch**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/medusahead/index.md). ![Taeniatherum caput-medusae: Dense grass growth with numerous pale, spreading, long-awned heads.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/medusahead.webp) Photo: Matt Lavin from Bozeman, Montana, USA; CC BY-SA 2.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/medusahead/). - [Ventenata](https://denson.github.io/colorado-weed-field-guide/plants/ventenata/) — **Displaces forage and native grassland**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/ventenata/index.md). ![Ventenata dubia: An open, delicate panicle of pale grass spikelets held against a mountain landscape.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/ventenata.webp) Photo: Matt Lavin; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/ventenata/). - [Russian thistle](https://denson.github.io/colorado-weed-field-guide/plants/russian-thistle/) — **Prickly tumbleweeds; livestock and fire hazards**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/russian-thistle/index.md). ![Salsola tragus: Rounded, densely branched green plant with slender leaves and spiny-looking branch tips.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/russian-thistle.webp) Photo: Stefan.lefnaer; CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/russian-thistle/). - [Tumble mustard](https://denson.github.io/colorado-weed-field-guide/plants/tumble-mustard/) — **Another tumbleweed: brittle plants scatter persistent seed**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/tumble-mustard/index.md). ![Tall, finely branched tumble mustard plant carrying many small yellow flowers.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/tumble-mustard.webp) Photo: Ben Keen; CC0 1.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/tumble-mustard/). ## Persistent roots and habitat displacement Plants whose spread or persistence can make restoration and routine garden management difficult. Methods depend on the site and confirmed identity. - [Field bindweed](https://denson.github.io/colorado-weed-field-guide/plants/field-bindweed/) — **Smothers plants; livestock concern**. State management: [List C — Manage widespread weeds](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/field-bindweed/index.md). ![Convolvulus arvensis: Pale pink funnel-shaped flowers with darker pink bands and nearby arrow-shaped leaves.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/field-bindweed.webp) Photo: Svklimkin; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/field-bindweed/). - [Canada thistle](https://denson.github.io/colorado-weed-field-guide/plants/canada-thistle/) — **Sharp spines; competes with vegetation**. State management: [List B — Stop the spread](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/canada-thistle/index.md). ![Cirsium arvense: Branching stem carrying numerous small pink-purple flower heads and buds.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/canada-thistle.webp) Photo: Ivar Leidus; CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/canada-thistle/). - [Leafy spurge](https://denson.github.io/colorado-weed-field-guide/plants/leafy-spurge/) — **Irritating sap; eye injury risk**. State management: [List B — Stop the spread](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/leafy-spurge/index.md). ![Euphorbia esula: Dense upright leafy stems topped with yellow-green flower clusters.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/leafy-spurge.webp) Photo: H. Zell; CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/leafy-spurge/). - [Japanese knotweed](https://denson.github.io/colorado-weed-field-guide/plants/japanese-knotweed/) — **Root and stem fragments regrow**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/japanese-knotweed/index.md). ![Reynoutria japonica: Broad pointed leaves and pale flower clusters viewed against the light.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/japanese-knotweed.webp) Photo: Michael Gasperl ( Migas ); CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/japanese-knotweed/). - [Bohemian knotweed](https://denson.github.io/colorado-weed-field-guide/plants/bohemian-knotweed/) — **Hybrid regrows from small fragments**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/bohemian-knotweed/index.md). ![Reynoutria × bohemica: Young rounded leaves with reddish-purple tones and a folded emerging leaf.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/bohemian-knotweed.webp) Photo: Patrick Hacker; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/bohemian-knotweed/). - [Perennial pepperweed](https://denson.github.io/colorado-weed-field-guide/plants/perennial-pepperweed/) — **Deep roots form persistent wetland colonies**. State management: [List B — Stop the spread](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/perennial-pepperweed/index.md). ![A fruiting perennial-pepperweed branch bearing many small green oval fruits.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/perennial-pepperweed.webp) Photo: Ayotte, Gilles, 1948-; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/perennial-pepperweed/). - [Tree of heaven](https://denson.github.io/colorado-weed-field-guide/plants/tree-of-heaven/) — **Root suckers form persistent thickets**. State management: [List C — Manage widespread weeds](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/tree-of-heaven/index.md). ![Ailanthus altissima: Mature tree with a broad rounded leafy crown and single trunk.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/tree-of-heaven.webp) Photo: Darkone; CC BY-SA 2.5. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/tree-of-heaven/). - [Russian olive](https://denson.github.io/colorado-weed-field-guide/plants/russian-olive/) — **Thorns; riparian habitat concern**. State management: [List B — Stop the spread](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/russian-olive/index.md). ![Elaeagnus angustifolia: Small yellow flowers and narrow silvery leaves on a twig.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/russian-olive.webp) Photo: Georg Slickers; CC BY-SA 2.5. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/russian-olive/). - [Tamarisk](https://denson.github.io/colorado-weed-field-guide/plants/tamarisk/) — **Alters riparian vegetation**. State management: [List B — Stop the spread](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/tamarisk/index.md). ![Tamarix ramosissima: Slender pink flower racemes on fine twiggy branches.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/tamarisk.webp) Photo: Jerzy Opioła; CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/tamarisk/). - [Yellow nutsedge](https://denson.github.io/colorado-weed-field-guide/plants/yellow-nutsedge/) — **Underground tubers sustain infestations**. State management: [List B — Stop the spread](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/yellow-nutsedge/index.md). ![Yellow nutsedge with upright leafy stems and radiating yellow-green spikelet clusters.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/yellow-nutsedge.webp) Photo: Krzysztof Ziarnek, Kenraiz; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/yellow-nutsedge/). - [Quackgrass](https://denson.github.io/colorado-weed-field-guide/plants/quackgrass/) — **Rhizome fragments regrow in lawns and gardens**. State management: [List C — Manage widespread weeds](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/quackgrass/index.md). ![Elymus repens: Dense upright grass clump with long green blades and slender flower spikes.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/quackgrass.webp) Photo: Rasbak; CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/quackgrass/). - [Giant reed](https://denson.github.io/colorado-weed-field-guide/plants/giant-reed/) — **Dense stands displace riparian plants**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/giant-reed/index.md). ![Arundo donax: A dense stand of tall, jointed canes and long leaves near water.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/giant-reed.webp) Photo: zebedeugalinha; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/giant-reed/). - [Horseweed](https://denson.github.io/colorado-weed-field-guide/plants/horseweed/) — **Fast windborne spread; some populations resist herbicides**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/horseweed/index.md). ![Erigeron canadensis: A tall upright stem topped by a broad, finely branched pale mature head cluster.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/horseweed.webp) Photo: Enrico Blasutto; CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/horseweed/). - [Silverleaf nightshade](https://denson.github.io/colorado-weed-field-guide/plants/silverleaf-nightshade/) — **Toxic leaves and berries; persistent spreading roots**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/silverleaf-nightshade/index.md). ![Upright silverleaf nightshade stem with gray-green leaves and purple flowers in a grassy field.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/silverleaf-nightshade.webp) Photo: Sam Kieschnick; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/silverleaf-nightshade/). - [Crownvetch](https://denson.github.io/colorado-weed-field-guide/plants/crownvetch/) — **Toxic to horses; spreading roots can smother vegetation**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/crownvetch/index.md). ![Sprawling crownvetch stems with many pink-and-white flower clusters and pinnate leaves among other vegetation.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/crownvetch.webp) Photo: Dale Knox; CC0 1.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/crownvetch/). ## Waterways and aquatic habitats Use the individual accounts to understand spread and identification before moving plants, water or equipment from an affected site. - [Hydrilla](https://denson.github.io/colorado-weed-field-guide/plants/hydrilla/) — **Dense aquatic growth; wildlife disease association**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/hydrilla/index.md). ![Hydrilla verticillata: A wet stem tip with short leaves in whorls and toothed leaf margins.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/hydrilla.webp) Photo: Stanislav Murashkin; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/hydrilla/). - [Giant salvinia](https://denson.github.io/colorado-weed-field-guide/plants/giant-salvinia/) — **Floating mats harm aquatic habitat**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/giant-salvinia/index.md). ![Salvinia molesta: Close view of floating, folded green leaves with textured upper surfaces.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/giant-salvinia.webp) Photo: Ixitixel; CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/giant-salvinia/). - [Eurasian watermilfoil](https://denson.github.io/colorado-weed-field-guide/plants/eurasian-watermilfoil/) — **Fragments spread dense aquatic growth**. State management: [List B — Stop the spread](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/eurasian-watermilfoil/index.md). ![A watermilfoil shoot at the water surface with a small flower spike rising above the water.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/eurasian-watermilfoil.webp) Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org; CC BY 3.0 us. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/eurasian-watermilfoil/). - [Parrotfeather](https://denson.github.io/colorado-weed-field-guide/plants/parrotfeather/) — **Aquatic mats obstruct waterways**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/parrotfeather/index.md). ![Myriophyllum aquaticum: Dense green, feather-leaved shoots emerging above the water surface.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/parrotfeather.webp) Photo: Quillipede; CC0 1.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/parrotfeather/). - [Flowering rush](https://denson.github.io/colorado-weed-field-guide/plants/flowering-rush/) — **Small fragments spread aquatic colonies**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/flowering-rush/index.md). ![Butomus umbellatus: A cluster of pale pink flowers and buds held on radiating stalks beside open water.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/flowering-rush.webp) Photo: Jacek Pietruszewski; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/flowering-rush/). - [Purple loosestrife](https://denson.github.io/colorado-weed-field-guide/plants/purple-loosestrife/) — **Crowds out wetland vegetation**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/purple-loosestrife/index.md). ![Lythrum salicaria: Close view of a purple flower with narrow petals and projecting stamens.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/purple-loosestrife.webp) Photo: Douglas Goldman; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/purple-loosestrife/). - [Yellow flag iris](https://denson.github.io/colorado-weed-field-guide/plants/yellow-flag-iris/) — **Ingestion concern; invasive wetland colonies**. State management: [List A — Eradicate](https://denson.github.io/colorado-weed-field-guide/safety/#weed-lists). [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/yellow-flag-iris/index.md). ![Iris pseudacorus: A yellow iris flower with dark markings on the large drooping segment.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/yellow-flag-iris.webp) Photo: Kailey Alderfer; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/yellow-flag-iris/). ## Pollen and seasonal allergies Ragweed profiles separate documented allergy concerns from species-specific questions still under research. This is not a local pollen forecast. - [Common ragweed](https://denson.github.io/colorado-weed-field-guide/plants/common-ragweed/) — **Pollen allergy; skin irritation**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/common-ragweed/index.md). ![Ambrosia artemisiifolia: A low stand of common ragweed flowering along the edge of a paved path, with other vegetation nearby.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/common-ragweed.webp) Photo: Ayotte, Gilles, 1948-; CC BY-SA 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/common-ragweed/). - [Giant ragweed](https://denson.github.io/colorado-weed-field-guide/plants/giant-ragweed/) — **Allergenic pollen and strong competition in disturbed ground**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/giant-ragweed/index.md). ![Ambrosia trifida: An upright giant ragweed plant beside a fence, with large three- to five-lobed leaves.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/giant-ragweed.webp) Photo: GTBacchus; CC BY-SA 3.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/giant-ragweed/). - [Western ragweed](https://denson.github.io/colorado-weed-field-guide/plants/western-ragweed/) — **A persistent perennial ragweed; allergy detail still under review**. [Evidence and agent notes](https://denson.github.io/colorado-weed-field-guide/plants/western-ragweed/index.md). ![Upright western ragweed stems with divided green leaves and elongated terminal flower spikes.](https://denson.github.io/colorado-weed-field-guide/assets/thumbs/western-ragweed.webp) Photo: Zihao Wang; CC BY 4.0. [Photo credits and sources](https://denson.github.io/colorado-weed-field-guide/plants/western-ragweed/). ## Early detection matters All **List A — Eradicate** entries are included in the [coverage checklist](https://denson.github.io/colorado-weed-field-guide/coverage/). Statewide eradication requirements make these reporting priorities even where a plant is not yet widespread. [CO-rules-2025](https://www.sos.state.co.us/CCR/GenerateRulePdf.do?ruleVersionId=11948&fileName=8+CCR+1206-2). # Appendix for agents Group membership is an editorial selection based on the linked profile warnings. It is not an incidence estimate, dose comparison or complete toxic-plant list. The [concerns.json](https://denson.github.io/colorado-weed-field-guide/concerns.json) mapping preserves these choices; claim-level references remain with each plant. --- Canonical HTML: https://denson.github.io/colorado-weed-field-guide/biggest-concerns/ Content revised: 2026-09-21T19:48:00Z Markdown mirror: https://denson.github.io/colorado-weed-field-guide/biggest-concerns/index.md Text fallback: https://denson.github.io/colorado-weed-field-guide/biggest-concerns/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