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 of Noxious Weeds
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
Not Known in Boulder County
African rue
Peganum harmala
Camelthorn
Alhagi pseudalhagi
Hydrilla
Hydrilla verticillata
Common crupina
Crupina vulgaris
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 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
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
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
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 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
Gypsophila paniculata
Caucasian bluestem
Bothriochloa bladhii
Common bugloss
Anchusa officinalis
Common reed
Phragmites australis
Garlic mustard
Alliaria petiolata
Garden loosestrife
Lysimachia vulgaris
Himalayan blackberry
Rubus armeniacus
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
Photos are provided courtesy of the Colorado Department of Agriculture.