Weed of the Month
Each month we will feature a noxious weed to help landowners identify weeds they may encounter on their property.
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.
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.
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.
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.
- Buy high quality seed
- Do not walk pets in areas infested with Russian knapweed
- Learn to identify plants in their early stages
What You Can Do:
- Inform your neighbors and community if you spot this weed
- Survey your property in the early spring to identify rosettes and treat accordingly
- Revisit known sites each year to continue control methods
UC IPM – Russian knapweed
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
- Identify nearby populations that may spread to your property
- Limit disturbances on your property that can allow Canada thistle to encroach
- Educate your neighbors on Canada thistle so they also understand why it is important to control this noxious weed
What You Can Do
- Treat your Canada thistle populations and offer to help your neighbors treat theirs
- Report any populations you locate to your county weed coordinator
- Clean your gear before and after hikes to limit spread
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 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.
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.
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
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
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.
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.