List A Weeds in Larimer County

The Colorado Noxious Weed Act requires mandatory eradication of weeds categorized as list A species. These are species with the potential to significantly impact the environment and economy of Colorado, but exist in numbers small enough that eradication is possible. There are currently 20 weeds in this category, and seven are known to be in Larimer County. Methods of control required for these weeds are those that result in eradication, or prevention of seed production. Management techniques must be digging, pulling, or herbicide application. Management methods that result in suppression, such as mowing, grazing, and insect biocontrol, are not acceptable.

Yellow starthistle (Centaurea solstitialis)

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Yellow starthistle

Yellow starthistle is potentially Colorado's worst weed. Though less than 100 acres statewide are currently infested with yellow starthistle, this invasive species could rapidly dominate rangeland and natural areas if aggressive steps are not taken. Other western states face insurmountable infestations of this weed. California alone reports estimated infestation levels as high as 20 million acres. Locally, several patches are known to exist on about 5 acres west of Berthoud in Larimer County. Private landowners and the Weed District are working cooperatively on these sites to eradicate yellow starthistle before it can become a problem in the area.

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An example taken from Hell's Canyon, ID
of how yellow starthistle can spread through
rangelands and hillsides.

Yellow starthistle is an annual in the sunflower family. In June, and after every summer rainstorm, the plants emerge into a rosette with deeply lobed leaves. The plants produce branched stems that are about 2 feet tall, winged and covered with cottony hairs. It has a disk only flower that is bright yellow, with 1-2 inches straw-colored spines below, creating a star-like appearance on the end of short branches. Though not palatable, inadvertent consumption of hay contaminated with yellow starthistle can cause chewing disease in horses.

The most effective herbicides for control of yellow starthistle are Milestone, Transline and Tordon. Herbicides need to be applied before the plants are flowering to prevent seed production.

Digging and hand pulling are effective methods for controlling this annual plant. If the plants are in the flowering stage they need to be bagged and properly disposed. See the article "Yellow Starthistle Saga."

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Mediterranean sage (Salvia aethiopis)

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Mediterranean sage

Mediterranean sage was introduced into this country as an ornamental plant and has since escaped and become a noxious weed in rangeland areas of Idaho, Oregon and Washington. Scattered infestations of this invader can be found over several hundred acres of pastureland west of Berthoud in southern Larimer County near Campion and the Blue Mountain area. Local information on Mediterranean sage can be found in the article "Mediterranean Sage in Larimer County." Private landowners and the Weed District are working cooperatively to eradicate Mediterranean sage from the area. Only a couple of other counties in Colorado are known to have this weed, and the Colorado Department of Agriculture hopes to see Mediterranean sage eradicated from the state in the next ten years.

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Mediterranean sage infestation

Mediterranean sage is a biennial member of the mint family that reproduces only from seed. Plants germinate in spring/summer/fall, over-winter as rosettes, flower in spring, and become tumbleweeds in the fall. No data are available on seed longevity in the soil, but it appears to be 10 years or more.

The most effective herbicides for control of Mediterranean sage are Telar, Escort, Tordon, and 2,4-D. Herbicide applications at the rosette stage provide best control.

Digging and hand pulling are effective methods for controlling this biennial plant. The top 2-4 inches of tap-root need to be removed to prevent re-growth. If the plants are in the flowering stage they need to be bagged and properly disposed.

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Myrtle spurge (Euphorbia myrsinites)

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Myrtle spurge

A garden plant long used as a xeriscape (drought tolerant) species in flower beds and rock gardens throughout Colorado and other western states. Myrtle spurge has escaped into natural areas and with its competitive characteristics has become an ornamental weed.

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Example of milky latex
Myrtle spurge is a tap-rooted perennial that flowers in the early spring. Seed longevity is estimated to be eight years. The plant contains a milky latex that is quite caustic and irritating to skin, and if in contact with an eye can cause temporary blindness. Myrtle spurge could still be purchased from plant nurseries five years ago until it was placed on the prohibited sales list. Plants can still be found in many flower gardens throughout Larimer County. Hopefully homeowners will recognize the importance of removing this invasive species and eliminate the threat to our natural areas

The most effective herbicides for controlling myrtle spurge are 2,4-D and dicamba along with a good surfactant. Fall applications provide better control than spring.

Hand pulling and digging are effective management methods. Precautions must be taken to protect skin and eyes. If plants have set seed they must be bagged to prevent seed dispersal. For more information about myrtle spurge and why it poses such a problem, please see the article "Myrtle Spurge - Ornamental Garden Plant or Noxious Weed?"

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Cypress spurge (Euphorbia cyparissias)

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Cypress spurge
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Cypress spurge infesting a garden

Much like myrtle spurge, this ornamental can be found in many flower beds, and has escaped into natural areas and become an invasive weed. Cypress spurge is a simple perennial that reproduces only by seed. Flowering occurs in April and May. The latex sap is irritating to the skin and can cause eye damage. Seed longevity is estimated to be 8 years in the soil.

The most effective herbicides for controlling Cypress spurge are 2,4-D and dicamba. Fall applications provide better control than spring.

Hand pulling and digging are effective management methods. Precautions must be taken to protect skin and eyes. If plants have set seed they must be bagged to prevent dispersal.

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Orange hawkweed (Hieracium aurantiacum)

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Orange hawkweed

A perennial forb that grows to 12 inches tall with a very showy flower that appears mid-June through August. Leaves occur primarily at the base, the stem has prominent hairs, and the plant contain a milky juice.

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An orange hawkweed infestation at a roadside
At maturity, these plants produce a seed head much like that of a dandelion that is readily dispersed in the wind. Seed longevity is unknown. Orange hawkweed is another escaped ornamental that has become a weed in many natural areas, particularly in the Pacific Northwest. Few sites with orange hawkweed are known to exist in Larimer County, what has been found was located around Rocky Mountain National Park and off Waltonia road in the Big Thompson Canyon.

The most effective herbicides for control of orange hawkweed are Milestone, Transline, Tordon, or 2,4-D. Apply prior to seed production.

Hand pulling or digging can reduce seed production and stress plants, but unless enough of the root is removed, this perennial will readily grow back.

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Purple loosestrife (Lythrum salicaria)

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Purple loosestrife

A rhizomatous perennial that reproduces by rootstalks and seed. Plants grow 6 - 8 feet tall. Purple loosestrife flowers are very showy, lavender to purple, and arranged along a vertical spike. Seed longevity is 20 years. Purple loosestrife is an escaped ornamental plant that thrives in a wet environment and readily invades riparian sites. If unchecked, this invasive species can dominate riparian areas, displacing native plants and reducing wildlife habitat. The Colorado Division of Wildlife has led the effort to eradicate purple loosestrife from the state. With enough cooperation and diligence the effort could succeed within the next 10 years.

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A prime example of how damaging an infestation
of purple loosestrife can become.

Due to this plant's proximity to water, the choice of herbicides is limited to those that have an aquatic label. Rodeo (glyphosate), Garlon A (triclopyr), Habitat (imazapyr) and several formulations of 2,4-D effectively control purple loosestrife and are registered for use in and around water.

Hand pulling or digging is only effective if all the rootstalk is removed. A common management practice is to cut and bag seed heads, then spot-spray the remaining portion of the plant with an herbicide.

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Knotweeds (Polygonum spp.)

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Japanese Knotweed

Knotweeds (Polygonum spp.) - knotweeds are shrubby, non-native perennial plants that can grow to 9 feet tall. Knotweed was introduced into this country as an ornamental, but has escaped to become a major weed problem in natural areas of the Pacific Northwest and other parts of the United States. Not yet common in Larimer County, some plants have been identified and landowners are encouraged to eradicate this plant before it becomes a problem here.

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An example of a knotweed infestation

Due to this plant's proximity to water, the choice of herbicides is limited to those that have an aquatic label. Rodeo (glyphosate), Garlon A (triclopyr), Habitat (imazapyr) and several formulations of 2,4-D effectively control knotweeds and are registered for use in and around water.

Mechanical methods of control are ineffective and may exacerbate the problem.

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