2022 Weed of the Month

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.

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.  



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.

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.

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.  

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.