2021 Weed of the Month

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.

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.


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.