Rio Grande County Weed District                                                                                                                  updated   2/14/2017         

Brianna Brannan

Noxious Weed Fact Sheet                                                                                                 

-canadian thistle

Canadian Thistle-

 Canada thistle (Cirsium arvense) is a non-native, deep-rooted perennial that spreads by seeds and aggressive, creeping, horizontal roots (rhizomes) that can grow 15 to 30+ foot in depth. Canada thistle can produce about 1,000 to 5,000 seeds per plant that can survive in the soil for 20 years or more. Typically, it reproduces vegetatively about 6 foot in a season through a creeping root system, and can quickly form dense monotypic stands. Every piece of root, from 2 cm in length, is capable of forming new plants.


Hoary Cress (short white top)-

Hoary Cress is a non-native deep rooted perennial that thrives in a wide range of soil types. They grow to two foot tall and are extremely invasive.  Hoary Cress reproduces via both seed and root, with root being the primary reproduction source. One plant can produce from 1,200 to 4,800 seeds per plant with a soil seed bank life of three or four years. Within three weeks of germination the seed produces new rosettes which emerge within 3 ½ weeks and root shoots that have been documented to grow as fast as approximately 4 ft in a season. A single plant can produce 450 new shoots in a single growing season.


Perennial Pepperweed (Tall White Top)-

 Perennial Pepperweed can grow to a height of six foot tall and reproduces via both seed and root. Each plant produces over 1,000 seeds per plant but seeds have been reported to not have the reproductive capabilities as the root. Most of the pepperweed’s root system grows within the first 2 foot of the soil but have been known to penetrate to a depth of 10 foot or greater and expand at a rate of 10 feet per year. Root segments produce adventitious buds; pieces as small as 1/2 inch in length are capable of generating new shoots. The Perennial Pepperweed is also known to have Allelopathic (secretes toxins in the soil) properties which prevent propagation of desired plant life.  

Black Henbane

Black Henbane is an annual / Biennial with a long tap root and reproduces only by seed, but produces approximately 100,000 to 500,000 seeds per plant. Seeds are known to last approximately 5 years in the soil. Black henbane is an opiate and all parts of the plant are poisonous andpotentially lethal to humans and livestock. The plant and seeds have been used as a hallucinogenic and theflowers and give off an odor that causes a kind of “high”.   


Russian Knapweed -

Russian Knapweed develops on many soil types, is very drought tolerant and is known to have allelopathic properties. Russian Knapweed is reported to have lethal effects on livestock due to its uptake of Zinc in the soil. The plant reproduces primarily through root system and fragments of one inch or less can reproduce into a new plant from depths of six inches. Russian Knapweed also reproduces via seed in small quantities, survival of seeds are from 3 to 5 years in soil bank, and new growth by seed is uncommon.




PLEASE NOTE: These are only a few of the noxious weeds that are controlled in the Rio Grande County, for additional weed information please contact the weed district at 719-852-4804 or email This email address is being protected from spambots. You need JavaScript enabled to view it.


Resources used :

Montana University Extension: http://store.msuextension.org/publications/AgandNaturalResources/MT201005AG.pdf

DiTomaso, J.M., G.B Kyser at al. 2013. Weed Control in Natural Areas in the Western United States. Weed Research and Information Center, University of California.

Colorado State University: http://extension.colostate.edu/topic-areas/agriculture/noxious-weeds-invasive-plant-species/

Colorado Department of Agriculture: https://www.colorado.gov/pacific/agconservation/canada-thistle

Whitson, T. (2006) Weeds of the west. Jackson, WY: Grande Teton Lithography

 Merrill A. Ross : /https://www.btny.purdue.edu/Pubs/WS/CanadaThistle/CanadaThistle.html