Weed of the Month

Each month we will feature a noxious weed to help landowners identify weeds they may encounter on their property.

Yellow toadflax

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September 2021

Yellow toadflax

Linaria vulgaris

Yellow toadflax, sometimes called butter-and-eggs, is a member of the Plantaginaceae family (formerly Scrophulariaceae).  This perennial spreads by seed and extensive spreading underground roots.  Large colonies can be found in disturbed areas, meadows, pastures, and rights-of-way.  It is often one of the first plants to return after a forest fire.

The yellow flowers are snapdragon-shaped with a distinctive long spur and often have an orange throat.   The flowers grow in groups at the end of the stems and can be found from June through September.  

Multiple stems grow from the crown.  Plants can grow to two feet tall.  Leaves are narrow, waxy and attach directly to the stems.  Each plant can produce 15,000 to 30,000 seeds that will last for about 10 years in the soil.

The taxonomy of Linaria species can be challenging because there is a lot of hybridization in the field, especially Yellow toadflax and Dalmatian toadflax here in Colorado.  Yellow toadflax is native to Britain, Europe, and Asia.  In the US, it is an escaped ornamental and has been grown here since the late 1600s.  It has also been used for its herbal properties and as a source for dye.


Flora of North America

Invasive Plant Atlas

National Invasive Species Information Center  

Weed Control in Natural Areas in the Western United States

Wisconsin Horticulture Division of Extension

Yellow starthistle

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Photo credit: Fred Doran

July 2021

Yellow starthistle

Centaurea solstitalis

Yellow starthistle is a member of the sunflower family (Asteraceae). In most locations it is a winter annual. It has yellow tufted flowers that grow on the end of branched stems. The flowers have bracts below with a central sharp stiff spine that drops off after the seedhead matures. Older flowers drop their petals and only cottony chaff remains, known as the Q-tip stage. Flowering occurs early summer through fall.

Each plant can produce up to 100,000 seeds that can last 2-10 years. Most seeds are dropped within a few feet of the plant. There have been reports that infestations can produce up to 100 million seeds per acre. There are both plumed and plumeless seeds.

Rosettes leaves are pea-green with toothed edges. Once the plant begins to bolt, the rosette leaves dry up. The mature stems are winged and covered with fine hairs, giving the plant a grayish-green appearance.

Plants grow to 3.3 feet tall and have a taproot that can be over three feet long.

Plants can be controlled by removal, but in areas that get multiple germination flushes, you must return frequently to ensure no plants mature. In most areas, using a selective herbicide is the best choice.

Yellow starthistle is known to utilize a large portion of available soil moisture which gives it an advantage over other plants. It prefers dry, open, sunny locations. It invades rangeland, disturbed sites, rights-of-way, and occasionally cropland. Large stands prevent wildlife movement and reduce biodiversity.

Yellow starthistle can cause chewing disease in horses and if they ingest enough, it can be fatal. Other livestock do not appear to be affected.

Although native to Eurasia, it first showed up in California in the early to mid-1800s and is thought to have arrived as a contaminate in alfalfa seed from Chile. At the time, only alfalfa seed from Chile was planted in California. The yellow starthistle in Chile is thought to have been introduced from Spain. Herbarium records for Colorado show that it was first reported in Boulder County in 1949. Since 1999, has been found in isolated spots in other Front Range and West Slope locations.


Stewardship Abstract (PDF)

Fire Effects Information System (FEIS)

Eurasian Watermilfoil

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Photo credit: Elizabeth Brown

June 2021

Eurasian watermilfoil

Myriophyllum spicatum

This exotic species is an aquatic submerged plant that is one of the most destructive aquatic weeds known. Eurasian watermilfoil (EWM) has leaves whorled around the stem with 14 or more leaflets on each side. Fruiting stems with small yellow flowers form spikes above the water. EWM is often confused with native milfoils. Native species typically have fewer than 14 (5 to 10) leaflet pairs per leaf. The exotic species can hybridize with the native milfoils. Hybrids may require DNA testing to distinguish from non-hybrids.

EWM aggressively forms dense mats that block the ability of sunlight to enter the water and stop the growth of native species, displaces wildlife habitat, and impede all forms of water-based recreation. After fruiting, EWM makes fragments of the plant, these fragments are responsible for new colonies as they float to other areas, sink, and start new plants. Transport on boating equipment is a significant means of plant establishment in new bodies of water. Any equipment used on or in the water can collect and spread this aquatic species.

This aggressive invader establishes in moving and standing waters and grows at a rate of approximately one foot per week. It can infest an entire lake within a couple of years after introduction, EWM has less value as a food source for waterfowl than the native plants that it replaces.

Remember to always clean, drain, and dry your boats, trailer, gear, and any equipment when playing or working in aquatic systems. It is not legal to transport watercraft over land with water drain plugs or aquatic vegetation on board per Colorado Parks and Wildlife (CPW) Aquatic Nuisance Species (ANS) regulations.

According to CPW, this is the highest priority aquatic noxious weed in Colorado. Millions of dollars are spent nationwide for control efforts. If you detect EWM or any other ANS, please report it to Invasive.Species@state.co.us. The CPW ANS Program will verify the detection per regulation and will collaborate with CDA, the County, and other pertinent landowners or stakeholders to determine and implement the appropriate management response.


U.S. Geological Survey, Nonindigenous Aquatic Species Database

Minnesota DNR EWM

Absinth wormwood

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May 2021

Absinth wormwood

Artemisia absinthium

Absinth wormwood (Artemisia absinthium), is a Colorado List B member of the Sunflower family (Asteraceae). This aromatic perennial grows to 3-5 feet tall. It has drooping drab-yellow disk (rayless) flowerheads that grow in the axis of leaves at the ends of its many branches. It flowers mid to late summer.

Ridged stems grow from the crown of its taproot. The stems dry out in the fall and persist through the winter. The dried stems help to more easily spot infestations in the spring as new growth starts at the base.

Both the leaves and stems are covered with fine hairs giving the plant a grayish appearance. The leaves are up to 5 inches long, deeply lobed, olive green above and white below. Leaves attach directly to the stem and are arranged alternately. The leaves towards the ends of the stems are smaller and linear.

It reproduces by seed and each plant can produce up to 50,000 seeds a year. The seed last in the soil for 3-4 years, or longer.

Absinth wormwood invades range, pasture, and disturbed areas. It is toxic to wildlife and livestock. Maintaining a healthy site can be effective in preventing its establishment.

Originally from Europe, Absinth wormwood has been used to make Absinthe, a spirit popular in the 19th and early 20th century. It had been thought to cause hallucinations and madness so was banned in many areas of the world. More recent studies have shown that the level of thujone, the absinth wormwood compound found in Absinthe, was probably not high enough to cause the effects which were due instead, to effects of alcohol over-indulgence.


High Plains Integrated Pest Management

Fire Effects Information System (FEIS)



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Tamarix chinensis and T. ramosissima

Saltcedar, also known as Tamarisk, was introduced into the United States in the 1800’s.  Saltcedar was planted as an ornamental, to stabilize riverbanks and also as a windbreak.  Saltcedar has displaced native vegetation on approximately 1.6 million acres of land.  Seedlings can survive flooding and drought.  This species secretes salt at a high rate on the ground surface and into the soil.  This inhibits native plants from growing in its vicinity.  

This plant is a deciduous shrub or small tree.  Petals and sepals are arranged in groups of five, they are mostly pink, sometimes white.  The plants flower anytime between April and August. The leaves are small, scale-like and bluish-green in color.   

Salt cedar reproduces by seed (up to 600,000 a year) and vegetatively 

Integrated management techniques include chemical, mechanical and biocontrol.   


RiversEdge West

USDA Saltcedar Field Guide

Colorado Department of Agriculture

Common mullein

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Common mullein

Verbascum thapsus

Common mullein, also known as woolly mullein, was introduced into the United States in the mid 1700’s. It was used as a fish poison for easy collection of fish. Introduction into Colorado is not known but was noted as a common weed in Boulder in 1905. This species occurs in all states to include Alaska and Hawaii and is often mistaken as a native plant.

First year plants have soft, fuzzy leaves that are gray-green. Second year plants can grow to more than 7 feet tall, leaves and stem are covered with woolly hairs.

Plants flower from June to August. Flowers are yellow, saucer-shaped and attached to an erect stem.

Common mullein is a biennial plant that reproduces by seed. It can produce 250,000+ seeds per plant. Seeds have been known to germinate after more than 100 years. Common mullein seed was found in archaeological soil in Denmark, this seed was dated from 1300 AD and germinated.

Integrated management techniques include chemical, and mechanical. If flowering, bag and dispose of plants to prevent the spread of seeds.

Common mullein is a Colorado List C noxious weed and control is recommended.

Dalmatian toadflax flower

January 2021

Broad-leaved Dalmatian Toadflax Linaria dalmatica

Narrow-leaved Dalmatian Toadflax Linaria genistifolia

Dalmatian toadflax is an herbaceous perennial found in rangeland, pastures, rights-of-way, and disturbed areas.  An escaped ornamental, it was first brought to North America in the 18th century.  Records show that it has been in Colorado since 1905

Plants grow to about 3 feet tall.  Leaves grasp the stem and can be heart-shaped (Broad-leaved) or more narrow (narrow-leaved).  Leaves reduce in size as they near the tip of the stems.  Foliage and stems are grayish-green and have a waxy surface.

Plants flower between May-October.  Flowers are snapdragon-shaped, pale to bright yellow with a spur.  

Broad-leaved Dalmatian toadflax can produce up to 500,000 seeds per plant.  Seeds may remain dormant for up to 10 years in the soil. 

Broad-leaved Dalmatian toadflax has a deep taproot and lateral secondary roots that produce new plants from buds.  Narrow-leaved toadflax has rhizomes.  Both reproduce by seed and vegetatively.

Integrated management techniques include biological, chemical, and mechanical.

Dalmatian toadflax is known to hybridize with Yellow toadflax (Linaria vulgaris). The hybrid is fertile and shows varying characteristics of the parents.


Montana’s Noxious Weed Field Guide

CABI - Invasive Species Compendium

Russian olive

December 2020

Russian olive

Elaeagnus angustifolia

Russian olive is a perennial, deciduous, multi-stemmed shrub or tree that is 16-40 foot tall and can live 30+ years.  Plants can grow as much as 6 feet a year.  

Young plants have reddish bark that turns silvery as they mature.  Older plants have fissured bark that may slough off as they age.  Branches are flexible and may have stiff 1-2 inch long thorns.

Leaves are light green with star-shaped hairs on the upper surface and scales on the lower surface.  The leaves are alternate and lance-shaped, 4 in long.  

Russian olive matures at about 3 years old and begins to bloom during May-June.  Flowers are light-yellow, four-lobed, and insect pollinated.  Fruit is a light-colored drupe and about ¾ inch long.  Fruit matures August-October.  Seed is moved by birds and animals and may be moved by water.

The roots are deep and extensive.  They may fix nitrogen if associated with certain bacteria.

It is native to eastern Europe and western and central Asia but has naturalized throughout North America including areas throughout most of Colorado.  First introduced in the 1900’s, it originally was planted for wind breaks, soil stabilization, and as an ornamental.  It is now found in riparian areas, shelter belts, ornamental plantings, and rights-of-way.

Russian olive forms dense thickets in some areas.  Outcompetes native vegetation, changes the structure of an area, displaces wildlife.  Alters nutrient cycling and hydrology.  Withstands disturbance, can change the structure of waterways which in turn changes how an area regenerates after disturbance. 

To understand the control requirements in your area, refer to the Weed Rule.


Element Abstract – Russian olive

Weed Control Methods Handbook: Tools and Techniques for Use in Natural Areas

National Invasive Species Information Center

Fire Effects Information System – Russian olive

Russian knapweed

November 2020

Russian knapweed

Rhaponticum repens

Russian knapweed is a herbaceous perennial in the sunflower family (Asteraceae).  Originally from Eurasia, it was introduced to North America in the late 1800’s as a seed contaminant in alfalfa. This List B noxious weed is found in many disturbed areas such as pastures, grasslands, and roadsides. Russian knapweed has been found in most counties in Colorado.

The leaves are generally 1½ - 4 inches long, attached to erect and branched stems. It has pinkish to lavender colored flowers and blooms from June to August. The flower heads have tan, rounded bracts with papery tips. Russian knapweed reproduces by seed but more often by its creeping roots.  The roots can grow horizontally to about 15 feet and vertically to about 22 feet.   New plants can easily form from root fragments as small as an inch.

Currently, the most effective control method is herbicide, which can be applied during the bud to flower stage (Spring to Summer) or when plants are in the rosette stage. Hand pulling is not recommended because plants regrow from the small root fragments left behind. Grazing is not recommended.  Animals avoid Russian knapweed due to its bitter flavor.  The plant contains neurotoxins that causes chewing disease in horses which can be fatal.

Recently biological control has been used in areas where Russian knapweed is very prevalent.   Biocontrol for Russian knapweed includes the gall midge, Jaapiella ivannikovi, and a gall wasp, Aulacidea acroptilonica.


  1. Buy high quality seed
  2. Do not walk pets in areas infested with Russian knapweed
  3. Learn to identify plants in their early stages

 What You Can Do:

  1. Inform your neighbors and community if you spot this weed
  2. Survey your property in the early spring to identify rosettes and treat accordingly
  3. Revisit known sites each year to continue control methods


Russian Knapweed and Yellow Star-Thistle Poisoning of Horses - New Mexico State University

Guide to Poisonous Plants - CSU

UC IPM – Russian knapweed

Canada thistle

October 2020

Canada thistle

Cirsium arvense

Canada thistle is a perennial plant that is part of the Asteraceae family. These plants can grow anywhere from 2 to 5 feet tall. Canada thistle reproduces by seed and root propagation which results in the colony formation of this plant. Roots can grow 15 feet or more horizontally and 6 to15 feet vertically.

Leaves are 4 to 8 inches long and dark green with spiny, serrated edges. Canada thistle flowers from June to August with purple, pink or white flowers.

Canada thistle can be found in fields, riparian areas, pastures, roadsides, and lawns and gardens. Due to growing behaviors and root systems this noxious weed is one of the most widespread noxious weeds in Colorado.

Canada thistle is a List B noxious weed and in Jeffco it is slated for suppression.  Canada thistle is best controlled with mowing monthly June through September and then treating with herbicide in the fall. Canada thistle rust is a new biological control method that is showing good success in parts of Colorado.  It is available from CDA's Palisade Insectary


  1. Identify nearby populations that may spread to your property
  2. Limit disturbances on your property that can allow Canada thistle to encroach
  3. Educate your neighbors on Canada thistle so they also understand why it is important to control this noxious weed

What You Can Do

  1. Treat your Canada thistle populations and offer to help your neighbors treat theirs
  2. Report any populations you locate to your county weed coordinator
  3. Clean your gear before and after hikes to limit spread


September 2020


Dipsacus spp.

Colorado has two species of Teasel.  Both are Colorado List B noxious weeds.  These biennial members of the Teasel family (Dipsacaceae) are originally from Europe and Asia.  They have been known in the US since the 1700’s and in Colorado since 1896.

Colorado’s populations of Teasel have exploded in the last 20 years.  Found in rights-of-way, empty lots, prairies, savannas, and riparian areas; Teasel displaces native plants that wildlife need to survive.  It also changes ecosystem services and modifies water movement.

Both species have bright green leaves that are opposite and may form a cup surrounding the prickly stem.  Plants are 6+ feet tall and have taproots.

Cutleaf teasel (Dipsacus sativus) rosette leaves are distinctly lobed.  Common teasel (Dipsacus fullonum, syn. Dipsacus sylvestris) rosette leaves are often crenulated with a pronounced midrib.  Common teasel leaf surfaces are prickly and edges more entire and smooth.

Both species flower from June through September and each have small flowers arranged in flowerheads at the end of stems.  Cutleaf teasel has predominantly white flowers arranged with bracts that are shorter than the flowerhead.  Common teasel has small pink to purple flowers with bracts that are normally longer than the flowerhead.

Both species reproduce by seed with 2000 – 3000 seeds per plant.  Most seed falls within 5 feet of the plant and some are spread by birds or animals.  Another way Teasel is spread is through ornamental enthusiasts who use the seedheads for decorations or in crafts.

Control options include herbicide, removal, and prevention.

Purple loosestrife

August 2020

Purple loosestrife

Lythrum salicaria

Purple loosestrife is a List A noxious weed.  It grows as a semi-aquatic perennial forb.  Originally from Eurasia, it was brought to North America as an ornamental but has escaped.   It grows to 6-10 feet tall and can be found along the edges of water bodies, creeks, and rivers.  It is often found growing within cattails and can be hard to spot when new to an area.

The flowers are purple, ¼-½ inch wide, with 5-7 petals in long racemes (flower-heads).  Plants begin to flower in June.  Each plant can produce 2.7 million seeds that may stay viable for 2-3 years.  

Plants have lance-shaped leaves that are about ½ inch wide and 1-3 inches long.  Leaves have distinctive veins parallel to the leaf edge. Stems are 4 to 6-sided, erect, and branch at the ends. Plants may have as many as 30 stems.  Roots are fibrous and extensive.

Purple loosestrife reproduces by seed and plant fragments.  The small seeds can easily be carried by water.  Once established, Purple loosestrife will displace native wetland species and diminish wildlife habitat.


What You Can Do

Garden responsibly. Don’t plant a pest.  

Keep ornamentals contained.

Report sightings of Purple loosestrife to your local weed manager.


The Northwest Invasive Plant Council (NWIPC)

CDA Purple Loosestrife Information

The Nature Conservancy – Element Stewardship Abstract

Invasive Species Compendium

Orange hawkweed

July 2020

Orange hawkweed

Hieracium aurantiacum

Orange hawkweed is a perennial member of the Sunflower family (Asteraceae).  This List A noxious weed can grow to about 24 inches tall.  Groups of ½ inch wide orange dandelion shaped flowers form at the end of slender, hairy, leafless stems.  The dark green, hairy leaves are lance-shaped and form a basal rosette.  It reproduces by seed, runners (stolons) and rhizomes.  It forms dense mats.  It can be found in forest meadows, along rights-of-way, waterways, lakes, and ponds. Originally from Europe, it is found in many areas of North America.  In Colorado, it is found in large areas of the Front Range and in Grand County.  And is found in lower numbers in a number of other counties.

Control is herbicide at the early to pre-bud stage.  Pulling is not recommended because the roots, stolons, and rhizomes fracture easily and can regrow.

CDA Fact Sheet

CO Distribution

Leafy spurge

April 2020

Leafy Spurge 

Euphorbia escula

Leafy spurge is a perennial member of the spurge family.  This List B noxious weed reproduces by seed and spreading roots.  Seeds are expelled up to 15 feet from ripened seed pods and have a sticky gel-like substance on their surface that allows seeds to stick onto wildlife, pets, humans, and equipment.  Roots grow horizontally to 15 feet and vertically to 30 feet.  Plants can grow from root fragments as small as 1/8-1/4 inch long.  Roots have growth buds along their length that give rise to new plants.

Leafy spurge flowers from May through July.  The flower parts are inconspicuous and are found in the middle of modified bracts that are bright yellowish green.  These form clusters at the ends of stems.

Leaves are narrow, about 1-4 inches long, with smooth surfaces and edges.

Plants grow to about 3 feet tall.  Stems are thin.  Plants have a milky latex sap that can be toxic.  It can cause blistering on the lips of livestock.  Plants may appear reddish in the fall.

In some areas of Colorado Leafy spurge populations have declined significantly, in part due to biological control beetles. Visit CDA’s Insectary website for more info.

Leafy spurge is a problem in rangeland, pastures, parklands and riparian corridors.

Control includes good land management, biological control, and chemical.  Mowing or grazing with goats combined with other methods has shown results in some areas.

What You Can Do

  • Choose weed-free hay when feeding hay to livestock
  • Survey your property in early spring to identify patches of leafy spurge so you can control it before it seeds
  • Do not walk or drive through patches of leafy spurge


Team Leafy Spurge


CSU Extension Fact Sheet

CDA Factsheet  

Hoary cress

March 2020

Hoary Cress

Cardaria draba

Other Names: Whitetop, Heart-podded whitetop

Hoary cress is a List B perennial. This member of the mustard family reproduces by seed and rhizomes and can quickly form a monoculture.

Plants can be up to about 18 to 24 inches tall but are generally smaller. The leaves are dark green. Lower leaves have a petiole but leaves higher on the plant clasp the stem. Dense clusters of four-petaled white flowers grow at the ends of upright branches. Seedpods are slightly inflated and heart-shaped. Each plant produces up to 4800 seeds per year.

Large patches can be seen along roadsides and in parks in early spring. Hoary cress is also an agricultural pest in rangeland, pastures and in crops such as wheat.

Hoary cress has been known in Colorado since 1898. There are also two other weedy whitetops known in the state. Lens-podded whitetop (Cardaria chalapensis) and Hairy whitetop (Cardaria pubescens).

What You Can Do

Talk to your local park manager and see if they would like help locating patches of noxious weeds.

If you have Hoary cress on your property, treat it before it blooms.

Tell your neighbors. We will all benefit when more people become aware of the problem and take action.


Clean your gear and equipment when moving between different sites. Whether you are a hiker, biker or work outdoors, weeds can easily be moved unintentionally by clinging to your clothing and equipment. Learn more at PlayCleanGo


Field Guide for Managing Whitetop in the Southwest

Weed Control in Natural Areas in the Western United States

CWMA Factsheet

CDA Factsheet

Myrtle spurge

February 2020

Myrtle Spurge

Euphorbia myrsinites

Myrtle spurge is a List A noxious weed commonly found in the urban and foothill areas of Colorado.  Initially sold as a xeriscape plant, this perennial of the spurge family, has escaped and naturalized in many areas including some remote and rugged areas.

Myrtle spurge shows up in early spring at lower elevations and begins to flower in March-April.

Grey-green leaves are fleshy, egg-shaped, and attach directly to the stem at their base.  Stems are stout and trail along the ground.  Leaves and stems contain a sticky, caustic sap that can cause severe rashes and blistering.

The inconspicuous flowers are surrounded by chartreuse bracts and develop at the ends of the stems.  When the seeds ripen, they are expelled from their capsule and are covered in a sticky gel that adheres the seeds to surfaces and passing animals.

Landowners are encouraged to start looking for this plant in mid-February.  It can be pulled as long as you remove the top few inches of the taproot.  Herbicides also work well.  The Colorado Department of Agriculture’s website has factsheets detailing control methods. 

More info:



January 2020


Bromus tectorum

Cheatgrass is a winter annual grass that emerges in the fall and/or early spring. It has shallow fibrous roots and reproduces solely by seed. Seedlings are dark green and may be seen growing through previous years’ thatch. Mature plants can reach 20+ inches high but most plants are about 12 inches tall. Older plants are straw to red colored.

It can be found on all continents except Antarctica. It was introduced to North America in the mid-1800’s from Eurasia and was first noted in Colorado in 1892.

Cheatgrass matures earlier than native perennial grasses and its shallow roots utilize soil moisture and nutrients before deeper rooted plants can access them.

Cheatgrass burns hotter, quicker and more frequently than our native grasslands. The change in regime makes it harder for native grasslands to recover, making them even more susceptible to cheatgrass invasion.

Removal, grazing, or chemical treatment prior to any seed development are recommended controls. See the Cheatgrass Management Handbook: Managing an invasive annual grass in the Rocky Mountain Region for more information.

Fact Sheet