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

The Boulder County Community Planning & Permitting office at 2045 13th St., Boulder is open to the public on Monday, Wednesday, and Thursday from 8 a.m. to 3 p.m., and on Tuesday from 10 a.m. to 3 p.m. Appointments are available on Tuesday but are not required. Schedule an appointment.

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.

Identification Guides

List A Species

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
Euphorbia cyparissias

Dyer’s woad
Isatis tinctoria

Giant reed
Arundo donax

Rush skeletonweed
Chondrilla juncea

More Than 10 Known Populations in Boulder County

Hairy willow-herb
Epilobium hirsutum

Knotweeds
Japanese, Giant, and Bohemian

Myrtle spurge
Euphorbia myrsinites

Mediterranean sage
Salvia aethiopis

Orange hawkweed
Hieracium aurantiacum

Purple loosestrife
Lythrum salicaria

Not Known in Boulder County

African rue
Peganum harmala

Camelthorn
Alhagi pseudalhagi

Hydrilla
Hydrilla verticillata

Common crupina
Crupina vulgaris

Giant salvinia
Elongated mustard
Brassica elongata

Flowering rush
Butomus umbellatus

Giant salvinia
Salvinia molesta

Meadow knapweed
Centaurea pratensis

Medusahead
Taeniatherum caput-medusae

Parrotfeather
Myriophyllum aquaticum

Squarrose knapweed
Centaurea virgata

Tansy ragwort
Senecio jacobaea

Yellow starthistle
Centaurea solstitialis

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

List B Species

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

Bull thistle
Cirsium vulgare

Canada thistle
Cirsium arvense

Common teasel
Dipsacus fullonum

Dalmatian toadflax
Linaria dalmatica & genistifolia

Diffuse knapweed
Centaurea diffusa

Houndstongue
Cynoglossum officinale

Leafy spurge
Euphorbia esula

Musk thistle
Carduus nutans

Russian knapweed
Acroptilon repens

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

Scotch thistle
Onopordum acanthium

Spotted knapweed
Centaurea stoebe

Treated as a List A Species in Boulder County

Yellow toadflax
Linaria vulgaris

Other List B Species

Absinth wormwood
Artemisia absinthium

Black henbane
Hyoscyamus niger

Bouncingbet
Saponaria officinalis

Chinese clematis
Clematis orientalis

Common tansy
Tanacetum vulgare

Cutleaf teasel
Dipsacus laciniatus

Dame's rocket
Dame’s rocket
Hesperis matronalis

Eurasian watermilfoil
Myriophyllum spicatum

Hoary cress
Cardaria draba

Hybrid knapweed
Centaurea x psammogena = C. stoebe x C. diffusa

Hybrid toadflax
Linaria vulgaris x L. dalmatica

Jointed goatgrass
Aegilops cylindrica

Mayweed chamomile
Anthemis cotula

Moth mullein
Verbascum blattaria

Oxeye daisy
Chrysanthemum leucanthemum

Perennial pepperweed
Lepidium latifolium

Plumeless thistle
Carduus acanthoides

Russian-olive
Elaeagnus angustifolia

Scentless chamomile
Matricaria perforata

Sulfur cinquefoil
Potentilla recta

Wild caraway
Carum carvi

Yellow nutsedge
Cyperus esculentus

List C Species

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
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

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