Weed of the Month

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

Field Bindweed

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December 2022

Field bindweed

Convolvulus arvensis

Field bindweed is a List C vining perennial in the morning glory family (Convolvulaceae).

Known to be in North America since the 1700s and in Colorado since 1872, it was introduced as a contaminant of seed.  It can now be found in all 50 states.  It is a native to Eurasia and Asia and naturalized in many other areas.

It has a thick taproot that can grow to 20-30 feet deep and multiple horizontal rhizomes with buds that form new plants.  Plants can easily regrow from root fragments. The root mass can reach 2½ to 5 tons per acre.

The trumpet shaped flowers form in the leaf axis.  Flowers form from late spring until frost.  The 1-inch-wide flowers are white to pink and have two small bracts that form ½ to 2 inches below the flower.

Each flower produces a roundish fruit that contains 2 to 4 seeds.  The seeds can stay viable in the soil for 20+ years.

Field bindweed stems are 5+ feet long.  They are twisted and are either prostrate or can climb and cover other plants, fences, and structures.

The 2-inch long and 1-inch-wide leaves are alternate, simple and arrow shaped, smaller towards the ends of the stems.  

A serious pest in wheat and bean crops, it also invades vineyards, orchards, degraded rangelands, landscaped areas, and lawns.  Field bindweed can harbor plant diseases (potato X disease, tomato spotted wilt, and vaccinium false bottom).

Control using cultural techniques and/or systemic herbicides.  It requires persistent efforts over multiple years.  The bindweed gall mite, Aceria malherbae has shown some good success in areas that are grazed or mowed.

Resources

Pacific Northwest Extension – Field bindweed (PDF) 

National Park Service 

Common St. Johnswort

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November 2022

Common St. Johnswort

Hypericum perforatum 

A native of Europe, Common St. Johnswort was introduced to North America in the late 1600s as an ornamental and medicinal plant.  It is a List C member of the Hypericaceae family.   Also known as Klamathweed or goatweed.  Our first record in Colorado is from 1834.

Plants are 1-3 feet tall but can reach up to 5 feet tall.  They are a somewhat shrubby perennial herbaceous forb.

Multiple rust-colored stems grow from the woody root crown.  Stems are woody below and herbaceous and branched above.  Leaves are sessile (no petiole), entire, opposite, up to 1.2 inches long, and narrow. The leaves have distinct translucent pores and black dots along the leaf margins. 

Flowers are bright yellow, about ¾ inch wide with numerous stamens. Flowers grow in clusters of up to 100 and are found at the end of the stems. The petal edges also have dark dots.  

Reproduction is from seed and rhizomes. Each plant produces 15,000 to 34,000 seeds that last anywhere from 3 to 50 years.  The vertical roots grow to 5 feet deep and lateral roots (rhizomes) extend about 3 feet from the crown and sprout new plants.  

Seeds can be spread by wind, water, and wildlife.  They germinate in fall and summer.

Goats and deer will feed on Common St. Johnswort but most animals will avoid it.  It contains chemicals that causes blistering and edema and causes light skinned animals to be sensitive to sunlight.  

The chemicals found in Common St. Johnswort include hyperforin and hypericin.  They show anti-depressant activity but both have been shown to interfere with other prescription medicines.

Common St Johnswort is found in grasslands and meadows, often as a result of overgrazing.

There are 500 species of Hypericum worldwide.  In Colorado our native Hypericum include Scouler's St. Johnswort (Hypericum scouleri) and Large St. Johnswort (Hypericum majus).  

Common St. Johnswort has been kept in check in many places by the biological control agents Chrysolina quadrigemma and C. hyperici.  These foliage feeding beetles were first released over 30 years ago and are well established in Colorado.  


RESOURCES

National Invasive Species Information Center

Status of Biological Control Agents Used for Managing Invasive Plants in Colorado (PDF)

Ohio Perennial and Biennial Weed Guide 

PHOTO CREDITS

Jefferson County Invasive Species Management

Yellow Flag Iris

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October 2022

Yellow flag iris 

Iris pseudacorus

Yellow flag iris is a Watch List species being considered for addition to the state’s weed list by the Colorado Department of Agriculture. 

It was imported to North America as an ornamental plant in the late-1700s but has escaped and now infests ditches, streams, and ponds.  In some areas it has formed large monocultures and changed the adjacent ecosystems.  Thick growths of yellow flag can clog irrigation systems and streams and, by trapping sediment in the roots, can narrow waterways. All parts of the plant are toxic to livestock and other animals.

Plants grow to about 3 feet tall with long sword-like leaves that grow from thick rhizomes in a fan-like arrangement.  Leaves are about ½ to 1¼ inch wide, flat with a pronounced midrib.  Plants resemble cattails when not in bloom.

The rhizomes can live for over 10 years in the soil and can remain viable for 3 months or more when dry.

Plants generally form flowers after three years.  The 2-3-inch-wide flowers are yellow to whitish with three upward facing petals and three downward facing sepals.  The sepals usually have dark purplish-brown streaking.  Flowering is summer through fall.

Seeds are formed in three-sided pods.  Each plant can form several hundred seeds that can survive and float for more than a year, enabling new infestations to establish long distances from existing occurrences.

PHOTO CREDITS

Jefferson County Invasive Species Management

Perennial pepperweed

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September 2022

Perennial pepperweed 

Lepidium latifolium

Perennial pepperweed is a member of the mustard family (Brassicaceae).  Populations form dense monocultures that are easily spread by root fragments and seed.  This plant can establish in a wide range of environments to including flood plains, pastures, roadsides, and residential sites.

Plants grow up to 6 feet tall.  Leaves are bright to gray-green with toothed to entire edges and have a prominent white mid-vein.  Basal leaves are larger than upper leaves.  

Multiple stems grow from the crown. Older stems are woody and can persist, forming a thicket that inhibits other plants from growing.

It has small white flowers that form in dense clusters near the end of the branches.  Two seeds form in rounded, flattened and slightly hairy reddish-brown fruits.

Perennial pepperweed probably entered the US prior to 1940 in a shipment of beet seed (Beta vulgaris) from Europe.

It prefers to grow in moist areas but can adapt where conditions are drier.  Perennial pepperweed is saline tolerant. It is also a salt pump, that is the rhizomes absorb salt from the soil, accumulate it into its leaves, and upon senescence its salty leaves fall to the soil. Salinity does not appear to affect seed germination.  This plant is known to thrive where Salt cedar (Tamarisk spp) populations exist.  

Herbicide treatment is best at the early bud stage or in the fall. Removal or cultivation is not very effective in large established sites because the root system extends deeply in the soil and fragments will give rise to new plants. 

Spotted knapweed

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August 2022

Spotted knapweed 

Centaurea stoebe

Spotted knapweed is a List B noxious weed thought to have been introduced to the US in 1893 as a contaminant in alfalfa seed and through soil in ship ballast.  Originally from Eurasia, this species has been found in most US states and 47 counties in Colorado.

It usually grows as a biennial but sometimes as a short-lived perennial.  It is found in rangeland, rights-of-way, and disturbed sites. 

Rosettes germinate in fall and spring and can easily be controlled by removal or herbicide treatment.

Plants are light green, 1 to 3 feet tall with 1 to 10 stems growing from the crown.

The compound light purple flowers, somewhat thistle-shaped, form at the ends of terminal and auxiliary stems.  The flower bracts are usually somewhat oval, with slightly spined edges and distinctive dark areas usually near the tip.

Each plant produces 450-4500+ seeds.  The majority of the seeds fall within a few feet of the mother plant.  Seeds can stay viable in the soil for about 8 years.  Spotted knapweed reproduces mainly by seed but occasionally plants will also grow from short lateral roots.  

Blair and Hufbauer’s 2009 study answered a lot of questions about knapweed hybridization.  They showed that spotted knapweed found in the US is not a hybrid but that most diffuse knapweed is. 

Identifying hybrids based on physical characteristics can be tricky in the field and a definitive ID really cannot be done without genetic assessment.

King County, WA 

University of Nebraska 

Blair, A., & Hufbauer, R. (2009). Geographic Patterns of Interspecific Hybridization between Spotted Knapweed (Centaurea stoebe) and Diffuse Knapweed (C. diffusa). Invasive Plant Science and Management, 2(1), 55-69. doi:10.1614/IPSM-08-105.1

Oxeye Daisy

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July 2022

Oxeye daisy

Leucanthemum vulgare

Oxeye daisy is a perennial member of the Sunflower family.  

It has 1 to 2-inch-wide compound flowers with 15-30 sterile white ray flowers and a central button made up of many yellow disk flowers.  Flowerheads are solitary at the ends of thin stems.  The flowerheads have narrow bracts with brown margins.

Leaves are lance-shaped with toothed edges and mostly hairless.  Stem leaves attach directly to the stem and are alternate.

Plants are from1 to 3 feet tall.  They are found along rights-of-way, in meadows, and in rangeland.  Sheep and goats will graze but most livestock and wildlife will not feed on it.

Oxeye daisy reproduces by spreading rhizomes and seed.  An escaped ornamental, it can often be found as an contaminate in wildflower seed mixes.

It is an escaped ornamental that was first recorded in the US in 1838.  Since then it has become widely spread and can be found in all 50 states.

Oxeye daisy can be easily controlled by pulling young plants or by treating with herbicide.

Musk Thistle

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June 2022

Musk thistle

Carduus nutans 

Musk thistle, aka Nodding thistle, is a biennial.  First year plants are a rosette and during the second year the plant bolts. Musk thistle reproduces solely by seed.  After seed set the plant dies.  Successful control of musk thistle is to prevent seed production.

This species can be a serious pasture weed.  Cut and bag seedheads if possible or sever root below soil in the rosette stage.  Seeds detach quickly and do not fall far from the parent plant.  Therefore, this plant can colonize rapidly in disturbed areas. It is unpalatable to wildlife and livestock.

Flower heads are large, purple, powder-puff shape, only with disk flowers.  Bracts are purple tinged, flower heads are on stalks that bend over, as if nodding.  A single flower head from musk thistle can produce up to 1200 seeds and a single plant up to 120,000 seeds. Seeds can be viable for over 10 years.

Musk thistle can grow up to 6 feet tall and has a taproot. The stem is covered with spiny wings.   

The earliest records of musk thistle in North America are from central Pennsylvania in 1852. Over 87,000 acres in Colorado are infested with musk thistle. 

Houndstongue

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May 2022

Houndstongue

Cynoglossum officinale

Houndstongue is a member of the Boraginaceae family.  It is usually a biennial but occasionally acts as a short-lived perennial. It has hairy oblong leaves that form a rosette the first year.  In early-spring to mid-summer in its second year, it starts to bolt.  Between 1-8 stems grow from the crown.  They can reach a height of 40+ inches. The stems have reduced leaves.  Purple to magenta, five-lobed flowers are arranged loosely along the flower stem.  Each flower is less than 4/10 inch wide and when mature, forms a four-parted seed head called a nutlet.  Each nutlet contains one seed.  The nutlets are covered with barbs that can latch onto fur, gear, and equipment.  

The plants have a deep (to 40 inches), thick taproot but are easy to pull when very young.  If the plants have started to bud, they should be securely bagged and thrown in the trash. Houndstongue is also easily controlled with herbicide if treated pre-flower.

Most (~75%) seeds fall within a few inches of the mother plant.  Longer dispersal distances are via animals.

Known since 1830 in the US and 1897 in Colorado, Houndstongue is originally from Eurasia.  It is thought to have arrived in North America in contaminated cereal seed.  It is not strongly competitive but takes advantage of disturbance.

Houndstongue contains pyrrolizidine alkaloids and is poisonous to livestock.  These alkaloids may be found in higher levels at the rosette stage.  Animals tend to avoid live plants but when it is found in dried hay livestock will consume enough to be affected.

Images

Root - John M. Randall, The Nature Conservancy, Bugwood.org

Shoe - K. George Beck and James Sebastian, Colorado State University, Bugwood.org

Flower - Joseph M. DiTomaso, University of California - Davis, Bugwood.org

Chinese clematis

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April 2022

Chinese clematis

Clematis orientalis

Chinese clematis, native to Asia, is a Colorado Noxious Weed List B species. This species is a perennial, herbaceous to woody vine with solitary flowers that have four yellow sepals.  It flowers late in the season on new wood. The seedheads that it produces are long tailed and feathery.  Seedheads are conspicuous all winter.  

Chinese clematis prefers a variety of habitats including riverbanks, riparian forests, scrub gullies, and slopes in hot dry valleys, especially on rocks by rivers and in scrub to 8500 feet in elevation.  

Leaves are opposite, pinnately compound and have 5 to 7 leaflets.   The vine climbs vigorously by petioles and rapidly overgrows trees, shrubs and other native species, ultimately killing them. Growth rate is 3 feet plus per year. 

Some, if not all members of this genus are mildly poisonous to humans and livestock.  All plant parts of Chinese clematis are toxic to humans, dogs, cats and horses. This species causes severe burning sensation and ulcers of the mouth.  

The listing of this species as a noxious weed includes all of the Clematis orientalis subspecies as well as any named cultivars.  Chinese clematis has been reported for sale as Clematis orientalis ‘Bill MacKenzie’.         

Flower color, and bloom dates are distinguishing features of Chinese clematis helping to separate it from the species native to Colorado.  Native Clematis species bear many more flowers on their petioles than Chinese clematis’ solitary flowers.  Western Virgins Bower (Clematis ligusticifolia) has white sepals while Blue clematis (Clematis occidentalis) has light blue-lavender sepals.  Rocky Mountain clematis (Clematis columbiana) also bears light blue-lavender sepals, yet Sugarbowls (Clematis hirsutissima), a bushy plant, has brownish-purple sepals.  Bloom dates help with the separation of species since Chinese clematis flowers can last well into October while the native species bloom from late April through August.

Good plant identification skills are required to discern the native Clematis from Chinese clematis especially when the native species are at the end of their floral phase for the season.

Invasive Plant Atlas

Washington State Noxious Weed Board

Colorado Department of Agriculture

Black henbane

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March 2022

Black henbane

Hyoscyamus niger 

Black henbane is a member of the Solanaceae family, the Nightshade or potato family.  This non-native invasive shrubby forb is originally from the Mediterranean region.  It contains tropane alkaloids (hyoscyamine and scopolamine).  It is poisonous to humans and animals and can be fatal if eaten.  Like many other members of this family, all parts are poisonous.

Known in the US since the 1600s, it was first introduced as an ornamental and for its herbal properties.  It had been used in Europe for thousands of years as an ingredient in herbal remedies, potions, and as a poison.

Black henbane grows as an annual and in some places as a biennial.  It can form dense 6-foot-tall patches.  Leaves are about 8 inches long and 6 inches wide.  When the leaves fall they form a litter layer that impedes desirable vegetation.  Both the leaves and stems have sticky hairs.  The plants smell especially bad.

Black henbane flowers during June to September.  The flowers form in two rows along racemes growing from the leaf axils.  The flowers are ¾ to 1 ¾ inches across, tubular-shaped, cream to light yellow colored with a deep purple throat.  The base of the flowers are surrounded by fused sepals forming an urn-shaped calyx which is retained on the stem and surrounds the seed pod.  Each plant can form 10,000-500,000 seeds that last in the soil for 5+ years.

Known in Colorado in limited areas, it can be found in hay fields, pastures, rights-of-way, and disturbed sites.


Photo Credits

Seedpods - Robert Vidéki, Doronicum Kft., Bugwood.org

Flower and Patch of Plants - Jan Samanek, Phytosanitary Administration, Bugwood.org

Others - Jeffco ISM

Rush skeletonweed

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February 2022

Rush skeletonweed

Chondrilla juncea

Rush skeletonweed was first reported in the US near Spokane, Washington in 1938.  This species is able to rapidly spread and establish.   Once established, it is difficult to eradicate.

Immature rosettes are difficult to distinguish from a dandelion.  And like a dandelion; leaves, stems, and roots all contain a milky sap.  Mature plants are multi-branched with wiry stems.  The stems are almost all leafless.  There are distinctive, stiff hairs on the lower portion of the stem that face downward.  The upper stems are nearly hairless.  

Rush skeletonweed blooms continuously throughout the season until first frost.  The flat flower heads are yellow and grow in the leaf axils and stem tips.  They can be single or in clusters.  A mature plant can produce up to 20,000 seeds in a growing season.  Seeds have a pappus, which allows seeds to be dispersed over long distances via the wind. Seeds are also ribbed with small teeth that allow them to attach to animals and other vectors.  

A perennial that mimics a biennial, Rush skeletonweed overwinters as a rosette.  The long thin taproot is distinctive and distinguishes it from similar plants that have short, stout taproots.  The deep taproot (7ft plus) allows access to soil moisture in semi-arid locations or during drought.  In addition to the deep taproot, the lateral roots produce daughter rosettes.  Rosettes can occur from root sprouts, seeds, or tiny fragments of the plant.  

This is a very difficult plant to control.  Roots severed up to five feet deep and still sprout a new plant.  Root segments as small as one inch can produce new plants.  Therefore, roots will continue to produce new plants after hand-pulling.

This plant has already infested many states in the southwest portion of the US.  While only found in a few small locations in Colorado, it is important to keep a watch out for this List A species.  

Resources

CABI

USDA Field Guide for Managing Rush Skeletonweed in the Southwest

Knotweed

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January 2022

Knotweed

Within North America the invasive knotweeds can be found throughout the northern Midwest and coastal regions.  Originally from Asia, knotweed has been planted as ornamentals since the 1800s.  These large plants have since moved into riparian areas and are forming huge thickets.  

The roots are rhizomatous and change the physical structure of creeks and rivers, which can result in severe erosion and collapse.  In areas like the states of Oregon and Washington, this has severely impacted some native fish spawning regions.  The plants can also grow through asphalt and building foundations.   Knotweeds are also known to be invasive and control efforts are undertaken in Europe, England, New Zealand and Australia.

In Colorado, Japanese and Bohemian knotweeds are A-List noxious weeds.  Giant knotweed is not well known in Colorado and it is no longer on the state’s weed list.  Previous reports of Giant knotweed may have been misidentified and are probably the hybrid.

Some features you can use to distinguish the different types include plant height, leaf size, leaf hairs, and size of flower clusters. (Knotweed Comparison Chart)

Bohemian knotweed is a hybrid between Japanese and Giant knotweed. The features are in between both parents and can sometimes be confusing.  

Reproduction includes vegetative through stem and rhizome fragments. There is some seed production but it is complicated.  Plants may have perfect or single sex flowers that may or may not be fertile.

Control requires repeated treatments because the extensive roots and rhizomes will resprout.  The roots can be 6 feet deep and the rhizomes 65 feet long.  First reported in Colorado in 1939, the knotweeds are now found in about 10 of our 63 counties.

Poison Hemlock

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December 2021

Poison hemlock

Conium maculatum

This species is in the carrot family (Apiaceae), the foliage and flowers resemble wild carrot and parsley.    

 All parts of poison hemlock are poisonous.  Many deaths have occurred because of people misidentifying it as one of the edible species of the Apiaceae family.  Even touching this plant can cause a serious reaction.  Poison hemlock ingestion by livestock is often fatal.  There is no antidote for hemlock poisoning.  

Poison hemlock is a biennial.  A rosette of fernlike leaves is produced the first year.  Year two, the plant can shoot up to 9 feet tall and then flowers.  It has smooth, hollow stems covered with purple splotches.  The leaves are fern-like, flowers are in tiny white clusters.

Poison hemlock was introduced in the 1800s.  It is usually found in moist soils but has adapted to dry.

As this plant reproduces solely by seed, the best control is to keep this plant from establishing, do not let it flower.  Do not burn, as the smoke also will contain the deadly toxins.   

Poison hemlock is a List C noxious weed in the State of Colorado.  Poison hemlock is often confused with water hemlock (Cicuta douglasii).  Both poison hemlock and water hemlock have a parsnip-like odor when the leaves are crushed..  Water hemlock is a perennial.  Reproduction can occur by seeds or roots.  Water hemlock is the most poisonous native species in Colorado.   According to the Center for Aquatic and Invasive Plants it is “One of the most toxic plants known to man.  While very similar in appearance to poison hemlock, the leaves of this plant are double to triple compound, giving them a hemp-like appearance.  

Always use caution when touching any plant that has not been properly identified! 

Diffuse knapweed

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November 2021

Diffuse Knapweed

Centaurea diffusa

Centaurea x psammogena

Diffuse knapweed is one of the most prevalent noxious weeds in western North America.  In some places it can be a short-lived perennial but acts as a biennial throughout most of Colorado.

Diffuse knapweed has white to lavender flowers with tan bracts below.  The bracts are fringed on the sides and have a terminal spine.  They will sometimes have darker spots at the base of the spine but not always.

Diffuse knapweed vegetation contains alkaloids that may act allelopathically to prevent other plants from germinating.

The cells at the base of the plant undergo abscission in the fall which allows the plants to break off and tumble across the landscape, dropping its many seeds as it goes.  

It is difficult to distinguish diffuse from the hybrid in the field.  In fact, the majority is thought to be the hybrid.  The only way to definitively tell whether what you have is diffuse or the hybrid is through DNA analysis, which is expensive and not practical for most weed management programs. 

Control is easy if you can access the site at the best growth stage.  The optimum stage for removal or herbicide treatment is at the rosette to pre-bud stage.  There are also several biological control agents.  Check with the Insectary to see if they will work on your site.

Resources

Blair, A.C. and Hufbauer, R.A. (2010), Hybridization and invasion: one of North America’s most devastating invasive plants shows evidence for a history of interspecific hybridization. Evolutionary Applications, 3: 40-51

Know Your Knapweeds - NDSU


Chamomile

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October 2021

Chamomile

Colorado has three species of invasive chamomile; Corn, Mayweed, and Scentless. Mayweed and Scentless are List B noxious weeds.

Generally, the invasive chamomiles have white daisy-like flowers that grow at the end of branched stems.   The leaves are alternate, 1-3 times pinnately divided.  All are annual but Scentless may sometimes also be a short-lived perennial. All are natives of Europe/Eurasia.  They can be found along roadsides, in fields, in disturbed areas, and cropland.

The leaves of Mayweed chamomile leaves are similar to corn chamomile but are more oblong and not as hairy.   Mayweed chamomile’s stems are ridged and sometimes reddish. Mayweed chamomile, also known as dog fennel, has a very unpleasant smell.  Corn chamomile is very limited in Colorado.  

Scentless chamomile is the more common of the three.  It has thread-like leaves.  The single stems branch about halfway up and the flowers tend to be all at the same height, forming a flattish inflorescence.

For comparison of the floristic features for the three species, please refer to this chart

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.

RESOURCES

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 solstitialis

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.


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 [email protected] 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.

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.

Saltcedar

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Saltcedar

Tamarix spp.

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.

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.

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.

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.

 Prevention:

  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:

  • 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

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

Prevention

  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

  • 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

Teasel

September 2020

Teasel

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.

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 esula 

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

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.

Prevention

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

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. 

Cheatgrass

January 2020

Cheatgrass

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.