Identification

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.

 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.

African rue (Peganum harmala)

African rue (Peganum harmala)

A perennial plant that grows 1-2 feet tall. The roots of the plant can reach a depth of up to 20 feet, if the soil where it is growing is very dry. Flowers are white with five petals and are present April to September.

Camelthorn (Alhagi pseudalhagi)

Camelthorn (Alhagi pseudalhagi)

An erect perennial shrub that grows from 1 1/2 to 4 feet tall. The stems are hairy when the plant is young and as it matures they become smooth and branched with sharp yellow tipped spines up to 1 3/4 inches long. Leaves are blue-green in color with smooth edges and are lance-shaped.

Common crupina (Crupina vulgaris)

Common crupina (Crupina vulgaris)

An erect winter annual growing one to three feet in height. The leaves are alternate, finely dissected, and develop short, stiff spines along the edges. Flowers are pink to purple in color and grow one to two on a branch with each plant having several branches and up to 40 flowering heads per plant.

Common crupina is not known to occur in Colorado but is still considered a List A species.

Cypress spurge (Euphorbia cyparissias)

Cypress spurge (Euphorbia cyparissias)

A low growing biennial with narrow linear or lance-shaped leaves and yellowish-green flowers. Only known infestations in Boulder County are on private lands.

Dyer's woad (Isatis tinctoria)

Dyer's woad (Isatis tinctoria)

A deep tap rooted winter annual, biennial or short lived perennial that grows from 1-4 feet tall. Blooms from April to early June with tiny numerous yellow flowers clustered on upper multi-branched stems.

Giant salvinia (Salvinia molesta)

Giant salvinia (Salvinia molesta)

A submersed aquatic freshwater perennial species that is native to South America. It starts as free floating pieces with leaves. The plant consists of horizontal stems that float just below the water surface. At each node it produces a pair of floating or emergent leaves that are green in color with rows of white, bristly hairs.

Giant salvinia is not known to occur in Colorado but is still considered a List A species.

Hydrilla (Hydrilla verticillata)

Hydrilla (Hydrilla verticillata)

A submersed invasive aquatic plant. It can grow to the surface and form dense mats. It may be found in all types of water bodies. The stems are slender, branched, and up to 25 feet long. Hydrilla's small leaves are strap-like and pointed.

Hydrilla is not known to occur in Colorado but is still considered a List A species.

Meadow knapweed (Centaurea pratensis)

Meadow knapweed (Centaurea pratensis)

A perennial that grows from a woody crown. The upright stems, grow from 20 to 40 inches tall and branch near the middle. Flower heads are solitary at tips of the branches, pink to purple in color, and 3/4 of an inch in size.

Mediterranean sage (Salvia aethiopis)

Mediterranean sage (Salvia aethiopis)

A biennial in the mint family with hairy bluish-green lobed leaves and a pungent odor when crushed. It grows an erect branched flower stalk with small white blooms, and breaks off in the fall forming a tumbleweed. Common in Boulder County.

Medusahead (Taeniatherum caput-medusae)

Medusahead (Taeniatherum caput-medusae)

A winter annual grass that can grow to heights of 6 to 24 inches. Stems are wiry and slender containing a few short leaves. The leaf blades are narrow and rolled in the stalk. The yellowish-green sheen of dense stands are highly visible after other annual grasses turn brown.

Medusahead is not known to occur in Colorado but is still considered a List A species.

Myrtle spurge (Euphorbia myrsinites)

Myrtle spurge (Euphorbia myrsinites)

Has fleshy blue-green alternate leaves and inconspicuous yellow-green flowers. This plant has a toxic milky sap and can cause severe skin irritations, blistering and is poisonous if ingested. Quite common and a real threat in Boulder County found especially in the foothills.

Purple loosestrife (Lythrum salicaria)

Purple loosestrife (Lythrum salicaria)

An invasive tap-rooted perennial forb from Europe, and an escaped ornamental inhabiting natural areas wetland margins. It can grow from 2 - 8 feet tall on four-sided stems with 5 inch lance-shaped leaves and has tight clustered pinkish-purple flowers on long vertical heads. Common in Boulder County

Rush skeletonweed (Chondrilla juncea)

Rush skeletonweed (Chondrilla juncea)

A perennial from Asia and the Mediterranean reproduces by seed and by an extensive root system. Rosettes closely resemble common dandelion and mature plants grow from 1-4 feet on wiry stems with toothed leaves and blooms with small yellow flowers from mid-July into early fall.

Sericea lespedeza (Lespedeza cuneata)

Blooms summer to fall and can grow 3 to 6 feet tall. Leaves are thin, alternate, abundant and three-parted.

Squarrose knapweed

Squarrose knapweed (Centaurea virgata)

A taprooted long-lived perennial that reaches 1 to 3 feet in height. It has bract tips that are recurved or spreading with terminal spine longer than lateral spines. Flowers are borne in singles or pairs at the tip of each branch. The flowers are rose to pink colored and usually the plant only produces 3 to 4 seeds per head.

Tansy ragwort (Senecio jacobaea)

Tansy ragwort (Senecio jacobaea)

A poisonous weed native to Europe and Asia minor, that is responsible for the deaths of thousands of livestock. It is a taprooted biennial or short lived perennial reaching 1 to 6 feet in height. The stems are stout and erect, with slightly branching characteristics. The leaves are 2 to 8 inches long, alternate and equally distributed mostly pinnately lobed, with the terminal lobe generally larger than the lateral ones.

Yellow starthistle (Centaurea solstitialis)

Yellow starthistle (Centaurea solstitialis)

A winter annual from Spain with a vigorous taproot and bright yellow flowers with sharp spines around base of flower. A mature plant can grow up to 4 feet tall and can produce 100,000 seeds.


Some photos are provided courtesy of the Colorado Department of Agriculture.

 List B Species

List B weeds are species with discrete statewide distributions that are subject to eradication, containment, or suppression in portions of the state designated by the Commissioner in order to stop the spread of these species.

Absinth wormwood (Artemisia absinthium)

More information coming soon.

Black henbane (Hyoscyamus niger)

More information coming soon.

Bouncingbet (Saponaria officinalis)

Bouncingbet (Saponaria officinalis)

A perennial forb with flowers that are crowded at the ends of branches, and have five petals that are generally light pink to white and slightly notched at the apex. Flowering begins in July and continues until September.

Bull thistle (Cirsium vulgare)

Bull thistle (Cirsium vulgare)

A biennial forb with gumdrop-shaped flowers that are pinkish to dark purple in color and 1 ½ to 2 inches in diameter. The flower bracts are somewhat tapered and covered with spines.

Canada thistle (Cirsium arvense)

Canada thistle (Cirsium arvense)

A deep-rooted perennial that can grow 2 to 4 feet in height. The leaves are oblong, spiny, bright green in color, and are only slightly hairy on the under-surface.

Chinese clematis (Clematis orientalis)

More information coming soon.

Common tansy (Tanacetum vulgare)

More information coming soon.

Common teasel (Dipsacus fullonum)

Common teasel (Dipsacus fullonum)

A biennial or sometimes monocarpic perennial forb. The fruits are a four-angled achene, each containing a single seed. The flowers are purple or white with spiny, awned bracts at the base.

Corn chamomile (Anthemis arvensis)

More information coming soon.

Cutleaf teasel (Dipsacus laciniatus)

More information coming soon.

Dalmatian toadflax

Dalmatian toadflax, broad-leaved & narrow-leaved (Linaria dalmatica & Linaria genistifolia)

A perennial forb that grows to 3 feet, and has bright yellow snapdragon-like flowers with an orange throat on elongated racemes.

Dame's rocket

Dame's rocket (Hesperis matronalis)

A biennial or short lived perennial forb with flowers that are white to purple with four petals and are clustered in loose terminal stalks. Flowers appear from May to August and the plant can produce seeds and flowers on any flower cluster at the same time.

Diffuse knapweed

Diffuse knapweed (Centaurea diffusa)

A biennial forb. During the first year of growth appears as a rosette in spring or fall. During the second year the stem bolts, flowers, sets seed, and the plant dies. Once the plant dries up, it breaks off at ground level and becomes a tumbleweed which disperses the still viable seeds over long distances.

Eurasian watermilfoil (Myriophyllum spicatum)

More information coming soon.

Hoary cress (Cardaria draba)

More information coming soon.

Houndstongue (Cynoglossum officinale)

More information coming soon.

Leafy spurge (Euphorbia esula)

More information coming soon.

Mayweed chamomile (Anthemis cotula)

More information coming soon.

Moth mullein (Verbascum blattaria)

More information coming soon.

Musk thistle (Carduus nutans)

More information coming soon.

Orange hawkweed (Hieracium aurantiacum)

More information coming soon.

Oxeye daisy (Chrysanthemum leucanthemum)

More information coming soon.

Perennial pepperweed (Lepidium latifolium)

More information coming soon.

Plumeless thistle (Carduus acanthoides)

More information coming soon.

Quackgrass (Elytrigia repens)

More information coming soon.

Russian knapweed (Acroptilon repens)

More information coming soon.

Russian-olive (Elaeagnus angustifolia)

More information coming soon.

Salt cedar (Tamarix chinensis, parviflora, & ramosissima)

More information coming soon.

Scentless chamomile (Matricaria perforata)

More information coming soon.

Scotch thistle (Onopordum acanthium)

More information coming soon.

Scotch thistle (Onopordum tauricum)

More information coming soon.

Spotted knapweed (Centaurea maculosa)

More information coming soon.

Spurred anoda (Anoda cristata)

More information coming soon.

Sulfur cinquefoil (Potentilla recta)

More information coming soon.

Venice mallow (Hibiscus trionum)

More information coming soon.

Wild caraway (Carum carvi)

More information coming soon.

Yellow nutsedge (Cyperus esculentus)

More information coming soon.

Yellow toadflax (Linaria vulgaris)

More information coming soon.

 Watch List

Watch List weed species 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.


Contacts

Parks & Open Space
Steve Sauer
303-678-6110
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