The term “noxious weed” is not merely a descriptive term, but a legal term that mandates control of certain weeds at a Federal, State, or local level. In Colorado, noxious weed control is required by state law. The Colorado Noxious Weed Management Act was passed into law in July, 1990, (CRS 35.5.5-101 et. Seq). The law states that certain noxious weeds pose a threat to the continued economic and environmental value of the land in Colorado and they must be managed by all landowners in the state.

Biological wildfire, an explosion in slow motion, a vegetative cancer. These phrases refer to the ecological threat, noxious weeds. Noxious weeds are like biological litter, but imagine trash that grows and expands then it’s thrown along a trail or roadside. Noxious weeds share three general characteristics: nonnative, often poisonous or unpalatable to wildlife and livestock, and are superior competitors that have tremendous ability to displace native vegetation.

Noxious weeds establish themselves in soil disturbed by construction of highways, buildings, and trails. They tend to produce an abundant amount of seeds. The seeds of the original population are spread by livestock, machinery, vehicles, wind, water, people and wildlife.

Most of the noxious weeds of this area came from Eurasia, from habitats similar to those of the drier sites in the county. They are well suited to this climate. With no natural enemies in this country, noxious weeds quickly establish, and are difficult to eradicate. They severely impact ranching and agriculture, recreational values of natural areas, and most of all, they severely compromise the biointegrity and biodiversity of ecosystems in the state.

Identifying Noxious Weeds

Field bindweed (Convolvulus arvensis), is a perennial broadleaf plant that reproduces by seeds and horizontal roots. Leaves are 1 to 2 inches long and are shaped like arrowheads. Flowers are bell or trumpet-shaped, white or pink and about ¾ to 1 inch broad. Seeds remain viable for up to 40 years.

Musk thistle (Carduus nutans), is a biennial broadleaf plant that reproduces solely by seed. Leaves are 6 to 14 inches in length, dark green, and spiny with white margins. Flowering heads are terminal, solitary, usually nodding, and deep rose in color. Broad spine-tipped bracts are located underneath flowering heads. Mature plants can grow as tall as 6 feet and appear solitarily or with several stems from one base.

Scotch thistle (Onopordum acanthium), is another biennial broadleaf plant similar to musk thistle. Mature plants can grow up to 12 feet tall and have a large, fleshy taproot. Stems are numerous, branched, and have broad, spiny wings. Flower heads are numerous, violet to reddish in color, with spine-tipped bracts beneath. Leaves may be up to 2 feet long, and 1 foot wide, and covered with dense hair giving the leaves a gray-green appearance.

Purple loosestrife (Lythrum salicaria), is a perennial plant that reproduces by seeds and rhizomes. Leaves are lance-shaped, entire, and whorled. Showy flowers are rose-purple in color with 5 to 7 petals arranged in long racemes. Plants can grow 6 to 8 feet tall and become taller and bushier as the rootstock matures. A single flowering stalk can produce 300,000 seeds that remain viable for up to 20 years.

Leafy spurge (Euphorbia esula), is a deep-rooted perennial plant that reproduces by seeds and by roots (reaching about 30 foot depths in the soil). Capsules can send seeds up to 15 feet from the parent plant, while roots can produce as many as 300 vegetative buds. Leaves are alternate, linear, and 1 to 4 inches long. Flowers are yellowish-green, small, numerous clusters, enclosed by heart-shaped bracts. The entire plant contains a white, milky latex.

Canada thistle (Cirsium arvense), is a perennial broadleaf plant that reproduces primarily by roots but also by seed. Leaves are set close on the stem, dark green, and contain numerous, sharp spines. Flowers are small, about 1 centimeter in diameter, and white to purple in color. Mature plants grow 2 to 4 feet tall.

Management of Noxious Weeds

The most effective plan for managing noxious weeds combines several control methods in a consistent, integrated management program. The plan must take into account the needs of the desirable plants, the nature of the plant pests, and the needs of the property owner and land users. An Integrated Weed Management (IWM) program consists of a variety of combinations of the following methods:

• Prevention – Good management will help keep desirable vegetation healthy and weeds under control. Buy only weed-seed-free hay, plant only certified seed, wash your vehicle and equipment after being in a weed-infested area, monitor your property and respond quickly to new weed infestations.
• Cultural – Cultural controls seek to control weed problems by establishing desired plant species. Cultural techniques manipulate the plant community through cultivating (cutting through and turning over the soil), re-seeding, fertilizing and irrigating.
• Biological – Biological control agents are organisms (usually insects)that are deliberately introduced to an area to control noxious weeds. The aim of biological control is not eradication, but rather to exert enough pressure on a weed to reduce its abundance to acceptable levels. Biological control agents are most useful for reducing seed production or weakening plants in large, dense noxious weed infestations where other control methods are not cost-effective.

• Livestock Grazing – Landmanagers can use cattle, sheep and goats to selectively overgraze certain weed species, thereby weakening them. In cases where desirable native species are not attractive to livestock, grazing may favor growth of native species over weeds. Livestock and wildlife can carry and spread weed seed on their coats or in their feces; avoid moving livestock from weedy areas to weed-free areas when weeds are producing viable seed.
• Mechanical – Techniques like mowing, tilling, hand-pulling, or burning can physically disrupt plant growth.
• Herbicides – Herbicides are chemicals that kill or control targeted plants. They can be safe and effective when applied properly. Herbicides decrease growth, seed production, and competitiveness of susceptible weeds.

Noxious Weed Regulations

As mentioned in the beginning of this section, noxious weed control is required by state law – the Colorado Noxious Weed Management Act. All landowners in the state must manage noxious weeds. The law further directs local government (cities and counties) to develop integrated weed management plans for their jurisdiction. By complying with this law, you are not only retaining or possibly increasing your property value, you are being a good neighbor and following the Code of the West as well. Team up with neighbors in the fight against noxious weeds.

Adams County-Adams County established a Noxious Weed Management Plan and a Noxious Weed Enforcement Policy in March 1997. The Policy mandates that all landowners in unincorporated Adams County control the designated noxious weeds on their property. The weed species designated as noxious in Adams County are; leafy spurge (Euphorbia esula), purple loosestrife (Lythrum salicaria), field bindweed (Convolvulus arvensis), Canada thistle (Cirsium arvense), musk thistle (Carduus nutans), Scotch thistle (Onopordum acanthium), diffuse knapweed (Centaurea diffusa), spotted knapweed (Centaurea maculosa), and Russian knapweed (Acroptilon repens).

Arapahoe County– Since January 1, 1998, Arapahoe County landowners are required to control any plants on the County’s noxious weed list. This list includes the same species as the Adams County’s noxious weed list but with the addition of whitetop (Cardaria draba), dalmatian toadflax (Linaria genistifolia), and yellow toadflax (Linaria vulgaris).

Weld County-Weld County established an Undesirable Plant Management Plan which was adopted as County Ordinance #169 in May, 1992. The ordinance was revised in April, 1996, as County Ordinance #169A. The County Ordinance mandates that all landowners in unincorporated Weld County control the designated noxious weeds on their property. The Weld County Noxious Weed List is similar to that of Adams Counties with one exception, the addition of dalmatian toadflax (Linaria dalmatica).

Additional Resources

Adams County Extension – Weed & Pest Information

County Weed Departments:

Adams County Weed Department: 303-637-8115

Arapahoe County Weed Department: 303-738-7861

Weld County Weed Department: 970-356-4000 ext.3770