Weed of the Month

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