In Colorado, certain plants are illegal to grow due to their invasive nature and harmful environmental impact. These species, listed under the state’s “List A Noxious Weeds,” threaten native ecosystems, agriculture, and wildlife. Their rapid spread crowds out native plants, increasing risks like soil erosion and wildfires. To protect Colorado’s landscapes, the state has marked them as banned.
Squarrose Knapweed (Centaurea virgata)
Squarrose knapweed is a perennial with a taproot that grows 1 to 3 feet tall. Its rose-to-pink flowers appear singly or in pairs at the branch tips, surrounded by distinctive recurved bract tips with longer terminal spines. This invasive species spreads quickly, raising production costs for ranchers, harming wildlife habitats, reducing plant diversity, increasing soil erosion, and heightening wildfire risks.
Camelthorn (Alhagi pseudalhagi)
Camelthorn, a perennial shrub that grows between 1½ to 4 feet tall, originates from regions ranging from Cyprus and Egypt to Mongolia, India, and Saudi Arabia. Introduced to the U.S. before 1915 in packing material, it spreads through seeds in water, hay, and cattle. This aggressive species colonizes quickly via creeping roots, making it difficult to manage and eradicate due to its extensive root system.
African Rue (Peganum harmala)
African Rue is a drought-tolerant perennial herb that reach heights ranging from 1 to 2 feet tall, with deep taproots extending over 20 feet. Its multi-branched, erect stems support deeply dissected, fleshy leaves. Blooming from April to September, it produces tiny seeds in a three-chambered capsule. The plant secretes allelopathic chemicals that inhibit native plant growth and is highly toxic to livestock and humans. Its seeds and fruit are poisonous, and fires can exacerbate its harmful effects.
Dyer’s Woad (Isatis tinctoria)
Dyer’s woad is a plant that grows from 1 to 4 feet tall with a deep taproot, acting as a winter annual, biennial, or short-lived perennial. Its bluish-green leaves form a basal rosette, and yellow flowers bloom from April to June. The weed crowds out native plants, including grasses vital for wildlife, and invades agricultural land, reducing crop yields. Efforts to curb its spread have been ongoing for over a decade.
Yellow Flag Iris (Iris pseudacorus)
An invasive plant that forms dense clusters in wetlands, Yellow Flag Iris often overpowers the native vegetation. It disrupts ecosystems by degrading habitats for fish, birds, and other wildlife. This species quickly invades shallow ponds and stream edges, often clogging ditches and irrigation canals. In addition to being difficult to control, its sap potentially irritates the skin and is toxic to livestock. If you spot this plant, cut flower heads to reduce seed spread and dig up rhizomes entirely. For larger infestations, herbicides labeled for aquatic use are recommended.
Myrtle Spurge (Euphorbia myrsinites)
Myrtle Spurge is toxic to the skin, causing redness, swelling, and blisters due to its sap. When ingested, it may irritate the mouth, lead to excessive drooling, and cause gastrointestinal distress. This plant’s harmful effects make it a danger to both humans and animals, requiring careful handling and management to prevent exposure and ensure safety in outdoor spaces.
Hairy Willowherb (Epilobium hirsutum)
Hairy Willowherb is a toxic plant that poses a serious risk to both humans and animals. Its leaves are particularly harmful when ingested. They can lead to severe health issues such as convulsions in humans. Pets, including cats and dogs, are also at risk if they consume any part of the plant. Hairy Willowherb can also accumulate toxins and heavy metals, increasing its toxicity. Proper management is crucial to prevent exposure.
Orange Hawkweed (Hieracium aurantiacum)
Orange Hawkweed is a perennial weed native to Europe. It was initially introduced to the U.S. for ornamental purposes due to its striking orange-red flowers. However, its invasive nature has become a problem. The plant spreads rapidly through seeds, stolons, and rhizomes, threatening meadows, pastures, and roadsides. Its ability to outcompete native vegetation makes it a persistent issue across various landscapes in the U.S.
Tansy Ragwort (Senecio jacobaea)
Tansy Ragwort, a toxic weed native to Europe and Asia Minor, grows 1 to 6 feet tall. Its yellow flower clusters can produce up to 150,000 seeds. If uncontrolled, it forms dense stands that crowd out native plants, reducing food sources for wildlife. The plant is highly toxic to livestock, particularly cattle and horses, and significantly diminishes the habitat value by excluding beneficial plants.
Yellow Starthistle (Centaurea solstitialis)
Yellow Starthistle, a winter annual native to Northern Spain, grows up to 4 feet tall and features blue-green foliage and sharp spines around its vibrant yellow flowers. It spreads rapidly, displacing native plants, degrading wildlife habitats, and reducing livestock forage. Toxic to horses, it’s challenging to control and can dominate areas quickly. Its spiny flowers also limit human activity, making management crucial.
Cypress Spurge (Euphorbia cyparissias)
Cypress Spurge is a low-growing perennial that survives winter as root and crown tissue. It reproduces through seeds and lateral root buds. The leaves are narrow, ½ to 1¼ inches long, and yellow or yellowish-green near the flowers. The stems, 4 to 32 inches tall, are green to yellowish-green, producing a toxic, milky sap when broken. The flowers are clustered and turn reddish-green as they mature.