2023 Weed of the Month

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

Arundo

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December 2023

Arundo (aka Giant Reed)

Arundo donax

Giant reed is a perennial grass that can grow to 25+ feet tall.  Originally imported to the US as an erosion control and horticultural plant in the 1800s, it has escaped and moved into many riparian areas in the west.  It is native to the Mediterranean region and eastern Asia.

It spreads by rhizomes and stem nodes and can form large clumps resembling bamboo.  It grows in full sun and in lightly shaded areas and is very drought tolerant.

The leaves can be 2 inches wide and up to 36 inches long.  Stems are ¼ -2 inches wide.

Its distinctive auricles clasp the stem.  The ligules are large and papery with small hairs along the margin. 

The flowers are arranged in a large plume 1-2 feet long.  Within North America, it is not believed that the seeds are viable. 

Giant reed forms huge colonies along waterways.  It disrupts the water flow, increases sedimentation, and changes the structure of water channels that can lead to erosion.  Dead and dry stands can pose a fire hazard. 

Colorado has less than 500 known locations and this List A noxious weed needs to be completely eliminated.  Because it easily spreads by plant fragments removal is not an option.  Use of a systemic herbicide will give the best control.

Control includes treating with herbicide .  Visit us at www.jeffco.us/jcism for more information.

RESOURCES

Colorado Weed Management Association  Noxious Weeds of Colorado 14th Edition

PHOTO CREDITS

Alicia Doran

JCISM

Weed of the Month is a joint effort between Jefferson County Invasive Species Management, CWMA, and Archuleta County.

Common burdock

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November 2023

Common Burdock (Arctium minus)

Common burdock is a List C biennial species and belongs to the sunflower family (Asteraceae).

A native to Europe, it is found throughout much of North America. Common burdock invades riparian corridors, irrigation ditches, pastures, and rangeland. 

Common burdock has seeds that are covered in small barbs that stick to clothing and fur, which aids in seed dispersal. Each plant can produce up to 15,000 seeds. Its basal leaves are large with undulating edges and it can be mistaken for rhubarb. The upper-side of the leaves can be covered in fine hairs, and the underside can be woolly. Leaves typically wither by the time of flowering. The stem is stout and branched and can grow to 6 feet long. Superficially, the blooms may resemble those of bull thistle. 

Common burdock is a secondary host plant for diseases of economically important crops.  It can reduce the value of sheep wool by getting entangled in the fibers, and can taint milk if it is heavily grazed.

Control includes removal of seedlings in the fall and spring or treating with herbicide in the spring.  Visit us at www.jeffco.us/jcism for more information.

RESOURCES

Invasive Plant Atlas

Colorado Weed Management Association  Noxious Weeds of Colorado 14th Edition

PHOTO CREDITS

Ethan Proud, Archuleta County

Weed of the Month is a joint effort between Jefferson County Invasive Species Management, CWMA, and Archuleta County.

Bouncingbet

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October 2023

Bouncingbet 

Saponaria officinalis

Bouncingbet has become naturalized in many parts of North America.  It can often be found near abandoned homesteads.  Originally from Europe and Asia, this member of the Pink Family (Caryophyllaceae) has been known in North America since at least 1802 and in Colorado since 1890.  

Its pink to white flowers are about an inch wide and are grouped at the end of branched stems.  Some horticultural varieties have double flowers but most have 4-5 petals.  Flowers have a five-pointed calyx that has reddish edges.

These List B perennial plants grow to about three feet tall with oval to lance shaped leaves that attach to the stem. The leaves are about two inches wide and four inches long with three distinct parallel veins.

The stems and rhizomes contain saponins that lather when water is added.  It has been used as a soap and as an additive to beer.

Control includes removal of seedlings or using a systemic herbicide.  The best time to treat Bouncingbet is at the bolting to early bud stage in late spring to mid-summer.

References

Jefferson County Invasive Species Management

CWMA Nox Weeds of Colorado

Photo Credits

All images by JCISM 


Weed of the Month is a joint effort between Jefferson County Invasive Species Management, CWMA, and Archuleta County.

Bull thistle

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September 2023

Bull thistle

Cirsium vulgare

Bull thistle is one of Colorado’s List B thistles.  Originally from Europe, Asia, and Northern Africa, it is now found in all 50 states.  It invades native grasslands and rangelands, reducing the forage that wildlife and livestock depend on.

Its dark green deeply lobbed leaves distinguish it from the other native and invasive thistles.  Its flowers are urn-shaped and light purple with spiny bracts and can be seen June-September.  The 1.5-inch-wide flowers grow at the end of branches, usually singly but sometimes in small groups.

Because they are biennials, the plants spend their first year as a rosette (seedling).  In the second year, they form a central stalk that branches as it matures.  Plants can grow to seven feet tall but are usually 3-4 feet tall.  Stems are winged and spiny.

Herbicide is most effective in the fall at the rosette stage or in the spring through the early bolt stage.  Treating older plants is not effective because once the flowers form, seeds will still be viable.  Control this time of year is limited to removal.  Flowers need to be cut, bagged, and put in the trash.



References

Jefferson County Invasive Species Management

CWMA Nox Weeds of Colorado


Photo Credits

All images by JCISM 


Weed of the Month is a joint effort between Jefferson County Invasive Species Management, CWMA, and Archuleta County.

Common tansy

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August 2023

Common tansy 

Tanacetum vulgare

Common tansy is a List B perennial member of the sunflower family (Asteraceae).  Originally from Europe and Asia, it is an escaped ornamental.  It has been known in Colorado since 1890 and the US since the 1600s.  Also known as bitter buttons, cow bitter, or golden buttons.

Common tansy forms thick patches.  It reproduces by seed and rhizomes and grows to 1.5-6 feet tall.  Stems grow from the root crown and are persistent.  The stems branch near top and are sometimes purplish.  

Leaves are alternate, pinnately compound, deeply divided, 1-6 in wide and 2-12 in long.  They have a strong smell when crushed.

Common tansy has rayless button shaped flowers.  Each flower is ¼ - ½ inch wide and are in groups at the end of the branches.  July-Sept.

Common tansy repels insects and is sometimes used as a companion plant.  It is poisonous to livestock.

It has been used as an herbal remedy but is toxic at high levels.  It contains thujone, a neurotoxin.  Do not confuse it with Blue Tansy (Tanacetum annuum).

Control includes removal of new populations.  For established populations, mow monthly and treat with herbicide in the fall.

Photo Credits

All images by JCISM 

Puncturevine

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July 2023

Puncturevine 

(Tribulus terrestris)

AKA caltrop, goathead, or devil’s thorn

Puncturevine is a List C species and is a part of the Caltrop family (Zygophyllaceae). 

Flowers are small and yellow, which after pollination, produces woody burs that can puncture bike tires and injure animals and people. Puncturevine only reproduces via seeds. 

This plant grows from a singular woody tap root and stays low and creeps along the ground often forming mats. Stems are reddish-brown spreading from a central crown. 

Seeds can remain in the seed bank for up to 3-6 years and sprout from early spring to late summer. Puncturevine will flush after a heavy rain. 

Puncturevine is often found in disturbed habitats such as cultivated crops, orchards, vineyards, and rights-of-way.

Originally this plant is native to Europe’s Mediterranean region and is classified as a noxious weed in the western United States and Australia. 

SOURCES

Colorado Dept. of Agriculture 

Washington Noxious Weed Control Board 

Photo Credits

Eric Coombs, D. Walters and C. Southwick, and Karan A. Rawlins

Plumeless thistle

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June 2023

Plumeless thistle 

(Carduus acanthoides) 

Plumeless thistle (Carduus acanthoides) is a List B member of the Sunflower family (Asteraceae).  It is a biennial or sometimes a short-lived perennial.

Flowers are reddish-purple, ½-1 inch wide.  The bracts are narrow and tipped with a spine.  Flowers are present June to September and grow singly or in small groups at the tips of branched stems.

Plumeless thistle reproduces by seed.  The seeds lack a pappus.

Plants have taproots and can grow to 8 feet tall.  Stems are winged and branch towards the ends. Leaves are dark green with a lighter midrib.  The undersides of the leaves are hairy.

Rosettes form in the first year and bolt early in May to June of the next year.  The rosette leaves  are deeply lobed and spiney.

Plumeless thistle is found in disturbed areas, such as overgrazed pastures, rangelands, roadsides, and rights-of-way.

Originally from Eurasia, it has been known in the northeast US since 1837 and in Colorado since the mid-1950s.

Photo Credits

Rosette - Loke T. Kok, Virginia Polytechnic Institute and State University, Bugwood.org

Stem - Steven Katovich, Bugwood.org

All others: Jeffco Invasive Species Management


Poison hemlock

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May 2023

Poison Hemlock 

Conium maculatum

Poison hemlock is a List C biennial species and belongs to the parsley family (Apiaceae).

It is an escaped ornamental that is found through the contiguous United States. Poison hemlock invades riparian corridors, irrigation ditches, pastures, and waste places. 

Plants grow up to a height of six feet tall and have umbelliferous inflorescences (umbrella shaped) at the end of branches. The individual flowers are white and less than a ¼ inch wide. Leaves are alternate and finely divided, giving it a fern-like appearance. Purple splotches along the stem differentiate poison hemlock from the native water hemlock. Poison hemlock is a lookalike to many edible plants such as wild carrot, oshá, and Indian celery. 

Poison hemlock is toxic to humans and livestock (cattle, swine, goats, horses, and sheep). The primary toxin in coniine. Poison hemlock’s claim to fame is it’s use in the execution of Socrates.

PHOTO CREDITS

Jeffco Invasive Species Management

Sulfur cinquefoil

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April 2023

Sulfur cinquefoil

Potentilla recta

Sulfur cinquefoil is a herbaceous perennial that regrows from a root crown every spring.  This List B species has palm-shaped (palmate) leaves with 5 to 9 toothed leaflets that are 1¼ -3½ inch long and ¼-¾ inch wide. The majority of the leaves grow on the stem with leaves growing smaller as they go up the stem. The leaf surfaces are green above and below and are sparsely hairy.

Flowers are light yellow with five petals that are notched. The center of the flowers is usually a darker yellow with the stamens and pistils very evident.  The flowers are in branched groups at the end of the stems.  Flowering occurs May-July.

Stems are slender, light green to reddish-green and sometimes whitish.  The stems branch towards the upper third of their length. There are hairs along the stems that tend to be at right angles.  The hairs are less evident on older stems. Plants can reach three+ feet tall but usually are less than two feet tall.  Plants can last 10+ years with some as long as 20 years.

The roots are a taproot with fibrous horizontal roots.

Reproduction is from seed.  The seeds have a net-patterned covering.

Sulfur cinquefoil contains tannins and is unpalatable to livestock and wildlife.  It outcompetes forage plants and can invade native areas.  It is found in pastures, rangelands, roadsides, grasslands, shrubby areas, open forest, and other disturbed sites.

This member of the Rose family was introduced from Eurasia.

PHOTO CREDITS

Alicia Doran, Jeffco Invasive Species Management

Dame's Rocket

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March 2023

Dames Rocket 

Hesperis matronalis

Dames Rocket is a List B biennial species and belongs to the mustard family (Brassicaceae).

It is an escaped ornamental that can be found in rangelands, pastures, gardens, forests, ditches, and rights-of-way. It can be found in many wildflower seed mixes. It is a prolific seeder and escapes ornamentation quickly. It may be mistaken for some species of phlox. Phlox have five petals, compared to the four that Dames Rocket sports. Dames Rocket was introduced around the same time as European settlement.

Plants grow up to a height of four feet tall and have white or purple, four-petaled flowers. Flowers are clustered on terminal stalks. Seed pods are narrow and up to 1½ inches long, as with all mustard species, these pods are called siliques. Leaves are alternate, lanceolate (lance-like), 2-4 inches long, and have toothed margins.

Dames Rocket can spread quickly and displaces many of our native plant species.


PHOTO CREDITS

Jamie Jones

Alicia Doran 

and Jeffco Invasive Species Management


Cypress spurge

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February 2023

Cypress Spurge

Euphorbia cyparissias

Cypress spurge is a List A perennial that is an escaped ornamental.  Originally from Eurasia, it was first reported in the US in 1858 and in Colorado in 1895.  It is not very common in Colorado but can be found in landscaped areas and has been found escaping into natural areas.

The plants usually grow to less than 16 inches tall but may reach up to 26 inches tall.  It forms dense patches of multiple slender stemmed plants. The stems may branch in the upper half of the plant. Leaves are narrow, less than 1/8 inch wide and about 1 inch long.  Flowers are reduced and have a pair of yellowish-green bracts beneath.   Flowers occur in spring through early fall and become reddish later in the season.  Roots can be 15 feet deep and woody rhizomes spread horizontally up to 35 feet.  

There are both seed and non-seed producing populations.  The seeded varieties form seed pods that contain one to three seeds. The sticky seeds explode when the pods ripen.  The seed producing varieties will sometimes hybridize with Leafy spurge (Euphorbia virgata formerly known as Euphorbia esula).  The genetics of Cypress spurge are complicated including both diploid and tetraploid types.

The plants contain toxic latex that is irritating to skin and eyes and causes rashes and blisters on lips and skin.  Most native animals and livestock do not feed on it.

Common mullein

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January 2023

Common mullein

Verbascum thapsus

Common mullein is a List C biennial that reproduces by seed.  Each plant can produce over 200,000 seeds that can last in the soil for 70 to 100 years.

Common mullein is an early invader in disturbed sites and can move into natural areas and form large monocultures.

Each plant grows from a thick taproot and sends up a central stalk that forms long flowerheads with yellow flowers.  The hairy leaves are smaller as they form up the stalk and are mainly at the base of the plant.  First year plants germinate in the fall or spring and spend their first year as a rosette.  Leaves are blue-gray and 1 to 5 inches wide and up to 12+ inches long.  Plants can grow to 6+ feet tall.

Originally from Eurasia and northern Africa, Common mullein arrived in North America in the 1700s and was dispersed intentionally and unintentionally by settlers as they moved across the continent.  

Common mullein can serve as a reservoir for agricultural pests and has been used as a piscicide to kill fish.  It has also been used as an herbal remedy.

Control includes mechanical or chemical options.  Common mullein can easily be controlled by removal of the rosette before the flower stem starts to form.  If removing after the flowers form, the stalks will need to be bagged and placed in the trash.  It is not recommended to remove old, dried stems because most seed have already dropped.  If using herbicides, it is best to include a good surfactant to ensure good contact with the leaf.