Weeds 101

According to the Colorado Noxious Weed Act, a noxious weed is an alien plant or parts of an alien plant that have been designated by rule as being noxious or has been declared a noxious weed by a local advisory board, and meets one or more of the following criteria:

• Aggressively invades or is detrimental to economic crops or native plant communities;
• Is poisonous to livestock;
• Is a carrier of detrimental insects, diseases, or parasites;
• The direct or indirect effect of the presence of this plant is detrimental to the environmentally sound management of natural or agricultural ecosystems.

Noxious weeds threaten valuable wildlife habitat and natural resources, cause economic hardships to agricultural producers, and are a nuisance for recreational activities. The Noxious Weed Act requires all Colorado residents to control noxious weeds using integrated methods to manage noxious weeds if the same is likely to be materially damaging to the land of neighboring landowners.

Want more information about noxious weeds and how they affect land use and recreation?

Visit our Education & Outreach Materials website to view the Colorado Noxious Weed Awareness Campaign materials.

The Colorado Department of Agriculture classifies noxious weeds into four distinct categories: List A, List B, List C, and the Watch List. This tiered system helps prioritize management efforts based on the weed's current distribution and the feasibility of control or eradication. Here’s a breakdown of the differences:

List A: If you find it, eradicate it.

• Goal: Complete eradication wherever found.
• Status: These are rare, non-native species with a limited presence or are not yet known to be in Colorado but are a problem in neighboring states.
• Legal Mandate: Management of these weeds is required by law for all landowners. The goal is to eliminate them before they can become established and widespread.
• Example: Myrtle spurge, giant salvinia, and dyer's woad are examples of plants that are or have been on this list.

List B: Help stop the Spread.

• Goal: To stop the continued spread of these species.
• Status: These weeds are established in certain areas of the state but are not yet so widespread that eradication is impossible.
• Legal Mandate: The Colorado Commissioner of Agriculture develops management plans for these species in consultation with local governments. In areas where the weed is uncommon, eradication is required. In more heavily infested areas, the goal is containment and suppression to prevent them from spreading further.
• Example: Canada thistle, leafy spurge, and spotted knapweed are common List B weeds in Colorado.

List C: Here to stay and supported through management plans.

• Goal: To provide educational, research, and biological control resources to local governments.
• Status: These are widespread and well-established weeds throughout the state.
• Legal Mandate: Control of these weeds is recommended but not required by state law. However, local governments (such as counties or municipalities) can choose to create their own ordinances to require control on public and private lands within their jurisdiction.
• Example: Field bindweed, bull thistle, and puncturevine are examples of weeds on this list.

Watch List: Report to EDDMapS! More information needed to assess the threat.

• Goal: To advise and educate the public and land managers about potential threats.
• Status: These species have been identified as potential threats to Colorado's environment and agriculture but their invasiveness in the state has not yet been fully determined.
• Legal Mandate: The Watch List is for advisory and educational purposes only. There is no legal requirement for control. The purpose of this list is to encourage people to identify and report new populations to help the state determine if they should be officially designated as noxious weeds and moved to List A, B, or C.
• Example: A plant like Tree-of-heaven is on the Watch List.

In summary, the key difference lies in the management objective and the legal requirement for control. List A is about eradication, List B is about containment and suppression, List C is about local control, and the Watch List is about early detection and education.

This is an excellent and common question in Colorado, especially in agricultural areas. While kochia and Russian thistle are widespread and troublesome weeds, they are not on the official Colorado Noxious Weed Act list for a few key reasons, which relate to how the state defines and classifies noxious weeds.

Why Kochia and Russian Thistle Don't Meet the Criteria

Even though they can be a major nuisance, kochia and Russian thistle don't meet the state's specific criteria to be legally designated as "noxious." Here's why:

• Ubiquity and Widespread Distribution: Both kochia and Russian thistle are already incredibly widespread and well-established throughout Colorado. In contrast, many plants on the state's noxious weed list (especially List A) are targeted because they are not yet widespread, and there is a realistic chance of eradication or containment. It would be an immense and likely impossible task to legally mandate the control of kochia and Russian thistle across the entire state.
• Management Practices Exist: While they are difficult to manage, a variety of control methods are known and used by agricultural producers, including tillage, hand-pulling, and a range of herbicides. This is in contrast to some noxious weeds that are resistant to common control methods or have no known effective management strategies.
• Economic Impact is Localized: While they can significantly impact crop yields and livestock forage in specific fields, the state's classification system focuses on plants that have a broader, statewide economic and environmental impact. For example, weeds like Canada thistle or Russian knapweed are known to dominate vast areas of rangeland and pastures, rendering them unusable.

While not on these lists, kochia and Russian thistle are still considered a major nuisance and are often included on "troublesome weed" lists by county extension offices and weed management districts. While they may not be legally "noxious," they are still a significant agricultural and land management concern in Colorado.

Contact your County Weed Manager, Colorado State University Extension office, or Conservation District. They will be able to confirm the species of the plant and give you an idea of how to treat it. If it's a List A or List B noxious weed, they will report it to the Noxious Weed Program; if it's a List C species, the county may have resources to help treat the more serious infestations.
 

Plant of Concern:

Have you found an unidentified plant that you believe is a concern to the environment or human health? Please fill out our Plant of Concern Form.

You can also find a direct link on the Early Detection Rapid Response Crew/Boots-on-the-Ground Assistance website.

Copies are sold through the Colorado Weed Management Association or your county weed manager may have a copy of this guide.

The Colorado Department of Agriculture (CDA) is a state government agency that serves as the primary regulatory authority for noxious weed management in Colorado, responsible for establishing official weed lists, developing statewide management plans, and enforcing the Colorado Noxious Weed Act. 

In contrast, the Colorado Weed Management Association (CWMA) is a non-profit membership organization that supports professionals and others involved in invasive species management through education, training, advocacy for supportive policies, and fostering professional networking, rather than direct regulation. While the CDA sets the foundational rules and oversight, the CWMA complements these efforts by empowering and connecting those working on the ground to implement effective weed control.
 

There is no one best way to rid the state of noxious weeds. Weeds, like all plants, vary in how they reproduce, have varying root structures (extensive root systems or a single taproot), and how they respond to herbicides. That's why an integrated weed management approach is necessary. This approach assesses the best techniques for a given species, from a choice of manual (mowing, pulling, digging up), cultural (land management practices such as irrigation, cultivation, types of cover or crops), biological (using plant pests that are native to the source of the weed); or chemical (herbicides). Other tools and techniques can be used on larger, woody weeds such as Russian olive and tamarisk.

Noxious weed management utilizes Integrated Weed Management (IWM), combining various strategies for effective and sustainable control. These methods include:

• Cultural Treatment: Manipulating the environment to favor desirable plants and suppress weeds, such as establishing competitive vegetation, proper grazing, mulching, prescribed burning, and optimizing soil health.
• Biological Treatment: Using living organisms, or biocontrol agents (e.g., specific insects or plant pathogens), to naturally suppress weed populations by reducing their vigor or reproduction.
• Mechanical Treatment: Physical removal or disruption of weeds, which includes hand-pulling, mowing, tilling, grubbing, or girdling.
• Chemical Treatment: Applying herbicides to kill or inhibit weed growth, typically through methods like foliar sprays, basal bark treatments, cut-stump applications, or soil treatments.

The most successful weed management programs strategically combine these approaches, considering the specific weed, infestation size, and environmental conditions.
 

When discussing noxious weed management, specific terms describe the goals and intensity of control efforts. Here's a breakdown of common definitions:

• Eradicate: This is the most ambitious goal, meaning the complete and permanent removal of every individual plant of a noxious weed species from a defined area, ensuring no viable seeds or plant parts remain. This is typically feasible only for new, small, or isolated infestations and aims to drive the population to zero.
• Eliminate: Often used interchangeably with "eradicate" in general conversation, in technical weed management, elimination can sometimes refer to reducing a population to a level where it no longer poses a significant threat, even if a few individuals might persist. However, in many policy contexts, "eradicate" and "eliminate" carry the same meaning: total removal.
• Control: This is the most common and often realistic goal for widespread noxious weed infestations. Control involves reducing the weed population to an acceptable or tolerable level, minimizing its negative impacts. This doesn't mean complete removal, but rather keeping the weed's spread and density below an economic or ecological threshold, requiring ongoing effort.
• Containment: This strategy aims to prevent the further spread of a noxious weed beyond its current known boundaries. It focuses on managing the edges of an infestation to stop it from expanding into uninfested areas and is often used when eradication isn't feasible but preventing expansion is critical.
• Suppression: Similar to control, suppression focuses on reducing the vigor, density, or reproductive capacity of a weed population to lessen its impact. It's about knocking back the weed's ability to thrive and dominate, often through repeated treatments.
• Prevention: This refers to proactive measures taken to stop a new noxious weed from establishing in an area where it doesn't currently exist. Examples include cleaning equipment, using weed-free forage, and educating the public. Prevention is considered the most cost-effective long-term strategy for weed management.

Understanding these terms helps in setting realistic goals, developing appropriate strategies, and effectively communicating about noxious weed management efforts.

The life cycle of a weed is one of the most important factors for effective management. For noxious weeds, knowing the difference between annual, biennial, and perennial plants helps you choose the right control method and timing. The core difference is how long the weed lives and reproduces.

Annual Weeds

• Life Cycle: One year. They germinate from a seed, grow, produce new seeds, and die all within a single growing season.
• Reproduction: Exclusively by seed. The plant's goal is to produce as many seeds as possible before it dies, ensuring its survival for the next year.
• Control: Control methods for annual weeds are most effective when they target the plant before it produces seed. Pulling, tilling, or applying pre-emergent herbicides can prevent the plant from completing its life cycle and leaving behind a new generation.

Biennial Weeds

• Life Cycle: Two years. They are "bi-annual" plants.
• Reproduction: Exclusively by seed, but they take two years to do so.
  First Year: The plant grows as a rosette, a cluster of leaves close to the ground, and stores energy in its root system. It does not produce a flowering stem.
• Second Year: The plant "bolts," meaning it sends up a tall flowering stalk, produces seeds, and then dies.
• Control: The best time to control biennial weeds is during their first year, when they are still in the rosette stage. At this point, they are small and have not yet produced a flowering stem. Once they are in their second year and have a flowering stalk, it is much harder to stop them from producing seeds. Common examples include musk thistle and bull thistle.

Perennial Weeds

• Life Cycle: More than two years. These weeds live for multiple growing seasons.
• Reproduction: Perennial weeds can reproduce in multiple ways, making them much more difficult to control. They can reproduce by:
  • Seed: Just like annuals and biennials, they produce seeds that can spread and create new plants.
  • Vegetative Reproduction: They can also spread through horizontal roots (rhizomes), horizontal stems (stolons), or root fragments. This means a single plant can form a large, interconnected colony.
• Control: Due to their hardy and extensive root systems, perennial weeds are the most challenging to eradicate. Simply pulling or mowing the top growth is often ineffective because the plant can regrow from its roots. Effective control requires a long-term strategy, often involving a combination of herbicide applications and mechanical methods to kill the entire root system. Canada thistle and field bindweed are classic examples of persistent perennial noxious weeds.

The Colorado Department of Agriculture's Noxious Weed Team has adopted a Plant Assessment Form, originally developed by the State of Wyoming, to guide its decision-making process for listing new noxious weeds. This form evaluates a plant through a series of questions that examine various factors, including its biogeography and distribution, reproductive biology and characteristics of spread, and its economic and ecological impacts. The information gathered helps the team determine whether a species poses a sufficient threat to be legally designated as a noxious weed in Colorado.

The Early Detection and Rapid Response (EDRR) concept is a proactive strategy for managing invasive species. It's based on the principle that the most effective and affordable way to deal with an invasive species is to find it early and act quickly, before it becomes widely established. The Invasion Curve visually demonstrates this, showing that management costs increase dramatically as an invasive species spreads, while the chances of successful eradication rapidly decrease.

Visit the NEW Certified Pesticide Applicator search tool!
The Colorado Department of Agriculture (CDA) has a new Certified Pesticide Applicator search tool to help you find qualified businesses in your area.
You can access the tool via the CDA's Plants, Pesticide Program website. To refine your search for noxious weed management, click on "Business" and apply filters.