Noxious Weeds Identification
Cypress spurge (Euphorbia cyparissias), a noxious weed in Colorado.

Identification of Noxious Weeds

Noxious weeds are plants that are not native to the county. Most have come from Europe or Asia either accidentally or as ornamentals that have escaped. These plants have an advantage because the insects, diseases, and animals that would normally control them are not found here. Because these plants have developed specialized mechanisms to survive, they are able to spread at an alarming rate.

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List A weeds are rare weed species that are subject to eradication wherever detected in order to protect neighboring lands and the state as a whole.

Less Than 10 Known Species in Boulder County

Cypress spurge
Cypress spurge

Euphorbia cyparissias

Dyer's woad
Dyer’s woad

Isatis tinctoria

Giant reed
Giant reed

Arundo donax

Rush skeletonweed
Rush skeletonweed
Chondrilla juncea

More Than 10 Known Populations in Boulder County

Hairy willow-herb
Hairy willow-herb

Epilobium hirsutum

Knotweeds
Knotweeds

Japanese, Giant, and Bohemian

Myrtle spurge
Myrtle spurge
Euphorbia myrsinites

Mediterranean sage
Mediterranean sage
Salvia aethiopis

Orange hawkweed
Orange hawkweed
Hieracium aurantiacum

Purple loosestrife
Purple loosestrife
Lythrum salicaria

Not Known in Boulder County

African rue
African rue

Peganum harmala

Camelthorn
Camelthorn

Alhagi pseudalhagi

Hydrilla
Hydrilla
Hydrilla verticillata

Common crupina
Common crupina

Crupina vulgaris

Giant salvinia
Elongated mustard

Brassica elongata

Flowering rush
Flowering rush
Butomus umbellatus

Giant salvinia
Giant salvinia

Salvinia molesta

Meadow knapweed
Meadow knapweed
Centaurea pratensis

Medusahead
Medusahead
Taeniatherum caput-medusae

Parrotfeather
Parrotfeather

Myriophyllum aquaticum

Squarrose knapweed
Squarrose knapweed

Centaurea virgata

Tansy ragwort
Tansy ragwort

Senecio jacobaea

Yellow starthistle
Yellow starthistle

Centaurea solstitialis

Eradicated from Boulder County 2012. Watch for re-emergence.

List B Species Designated by the Board of County Commissioners for Required Management & Control

Bull thistle
Bull thistle

Cirsium vulgare

Canada thistle
Canada thistle

Cirsium arvense

Common teasel
Common teasel

Dipsacus fullonum

Dalmatian toadflax
Dalmatian toadflax

Linaria dalmatica & genistifolia

Diffuse knapweed
Diffuse knapweed

Centaurea diffusa

Houndstongue
Houndstongue

Cynoglossum officinale

Leafy spurge
Leafy spurge
Euphorbia esula

Musk thistle
Musk thistle
Carduus nutans

Russian knapweed
Russian knapweed
Acroptilon repens​

Salt cedar
Salt cedar
Tamarix chinensis, T. parviflora, and
T. ramosissima

Scotch thistle
Scotch thistle
Onopordum acanthium

Spotted knapweed
Spotted knapweed
Centaurea stoebe

Treated as a List A Species in Boulder County

Yellow toadflax
Yellow toadflax
Linaria vulgaris

Other List B Species

Absinth wormwood
Absinth wormwood

Artemisia absinthium

Black henbane
Black henbane

Hyoscyamus niger

Bouncingbet
Bouncingbet

Saponaria officinalis

Chinese clematis
Chinese clematis

Clematis orientalis

Common tansy
Common tansy

Tanacetum vulgare

Cutleaf teasel
Cutleaf teasel

Dipsacus laciniatus

Dame's rocket
Dame’s rocket

Hesperis matronalis

Eurasian watermilfoil
Eurasian watermilfoil

Myriophyllum spicatum

Hoary cress
Hoary cress

Cardaria draba

Hybrid knapweed
Hybrid knapweed

Centaurea x psammogena = C. stoebe x C. diffusa

Hybrid toadflax
Hybrid toadflax

Linaria vulgaris x L. dalmatica

Jointed goatgrass
Jointed goatgrass

Aegilops cylindrica

Mayweed chamomile
Mayweed chamomile
Anthemis cotula

Moth mullein
Moth mullein
Verbascum blattaria

Oxeye daisy
Oxeye daisy
Chrysanthemum leucanthemum

Perennial pepperweed
Perennial pepperweed
Lepidium latifolium

Plumeless thistle
Plumeless thistle
Carduus acanthoides

Russian-olive
Russian-olive
Elaeagnus angustifolia

Scentless chamomile
Scentless chamomile
Matricaria perforata

Sulfur cinquefoil
Sulfur cinquefoil
Potentilla recta

Wild caraway
Wild caraway
Carum carvi

Yellow nutsedge
Yellow nutsedge
Cyperus esculentus

Management is recommended, but not required, for List C Species.


Bulbous bluegrass

Poa bulbosa

Chicory

Cichorium intybus

Common burdock

Arctium minus

Common mullein

Verbascum thapsus
Common St. Johnswort
Common St. Johnswort

Hypericum perforatum

Downy brome

Bromus tectorum

Field bindweed

Convolvulus arvensis

Halogeton

Halogeton glomeratus

Johnsongrass

Sorghum halepense

Perennial sowthistle

Sonchus arvensis

Poison hemlock

Conium maculatum

Puncturevine

Tribulus terrestris

Quackgrass

Elymus repens

Redstem filaree

Erodium cicutarium

Velvetleaf

Abutilon theophrasti

Wild-proso millet

Panicum miliaceum

Watch List Species that have been determined to pose a potential threat to the agricultural productivity and environmental values of the lands of the state. The Watch List is intended to serve advisory and educational purposes only. Its purpose is to encourage the identification and reporting of these species in order to facilitate the collection of information to assist in determining which species should be designated as noxious weeds.

Baby's breath
Baby’s breath

Gypsophila paniculata
Baby's breath
Caucasian bluestem

Bothriochloa bladhii

Common bugloss

Anchusa officinalis

Common reed

Phragmites australis

Garlic mustard

Alliaria petiolata
Garden loosestrife
Garden loosestrife

Lysimachia vulgaris
Himalayan blackberry
Himalayan blackberry

Rubus armeniacus
Himalayan blackberry
Hoary alyssum

Berteroa incana

Meadow hawkweed

Hieracium caespitosum

Onionweed

Asphodelus fistulosus

Siberian elm

Ulmus pumila

Scotch broom

Cytisus scoparius

Swainsonpea

Sphaerophysa salsula

Syrian beancaper

Zygophyllum fabago

Tree of Heaven

Ailanthus altissima

Ventenata grass

Ventenata dubia

White bryony

Bryonia alba

Yellow bluestem

Bothriochloa ischaemum

Yellow flag iris

Iris pseudacorus

Photos are provided courtesy of the Colorado Department of Agriculture.

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