Noxious Weeds on Open Space & Mountain Parks
Learn about Weed Control on OSMP.
Please follow the links below to see photos and learn about these invasive weeds:
The "A" List
Some of the plants listed below are illegal to grow, cultivate and sell and are mandated for eradication. Follow this link to learn more about the state's A List and Colorado landowners' and gardeners' legal responsibilities. You can help by using native plants in your garden. The first step to eliminating noxious weeds is to know them when you see them. This allows you to eliminate them from your own land and gardens, and to avoid buying them as ornamentals. You may also spot them while hiking on OSMP: please report where you find a patch, since it may be new to our records. Once identified, the patch can be eliminated using integrated pest management methods described below. Remember, the plants pictured below are one of the greatest threats facing OSMP's ecosystems!
Myrtle Spurge grows in xeriscape gardens all over Boulder. You've seen it in the spring, when it creates carpets of yellow flowers. This tap-rooted perennial produces a poisonous milky latex which is caustic to human skin and terrible when accidentally rubbed into the eyes. Wear gloves when you pull it! It produces wiry light green stems roughly 12- 18 inches in length that form conspicuous yellow flowers. Myrtle Spurge is an A-listed Colorado Noxious Weed, and its cultivation and sale is illegal. Download the city's Myrtle Spurge brochure (Adobe PDF, 260 Kb) and eradicate this bad one today.
Purple Loosestrife is a perennial that grows 6 to 8 feet tall. The flowers are purple and grow vertically up the stem near the apex of the stems. You're most likely to see it in a wet, marshy area. Still cultivated in some gardens, Purple Loosestrife is an escaped ornamental that poses a serious threat to wetland areas. If you see this plant in other locations please contact your local area weed management organization. Purple Loosestrife is an A-listed Colorado Noxious Weed, and its cultivation and sale is illegal. Note: nurseries sell hybrids that are advertised as sterile, but they do in fact produce some fertile seed.
Mediterranean Sage is an aromatic biennial which grows 2 to 3 feet tall. In the first season it develops a rosette of large grayish woolly leaves. In the second season the plant bolts, producing multi-branched stems with white to blue-green felt-like leaves. The flowers are yellowish-white and are borne in clusters on profusely branched stems. One plant may produce thousands of seeds which spread easily because the mature plant forms a tumble weed. If you see this plant anywhere else make sure to report it to your local weed management agency. Mediterranean Sage is an A-listed Colorado Noxious Weed, and its cultivation and sale is illegal.
Dame's Rocket: Don't be fooled by this one's lovely lavender flowers, which bloom in late spring. This attractive invader takes over along streams, in canyons and moist hillsides. It grows to about 2 to 3 feet tall. Since it's a biennial, it blooms in its second year and then dies. You may control it by eliminating all flowers and thus preventing the plant from leaving seeds.
Perennial Sweetpea is beautiful but deadly to our native ecology. This vegetative thug sprays its bean-like seeds for great distances, helping it rapidly take over and dominate an area. The large, deep tap root makes it difficult to control. Blooming in mid summer, it is still widely cultivated for its beguiling beauty. It is found in various parts of Open Space and Mountain Parks, most notably in Chautauqua Meadow where the patch grows larger every year.
Oxeye Daisy (Chrysanthemum leuchnthemum) has a deceptively pretty flower masking the invasive brute within. It easily escapes from flower beds and gardens and will out compete our native plants. Look for white ray and yellow disk flowers on an erect stalk, surrounded by lobed basal leaves. This Eurasian perennial produces many flowering stalks each year. It will usually stay along ditches or streams, but can move into pastures when there is enough moisture. The best way to control them is to remove them from your garden. Beware! Some seed mixtures still contain Oxeye Daisy seeds.
Jointed Goatgrass (Aegilops cylindrica) is a nasty Turkish weed that was introduced in the 1800s. It can reach 30 inches in height, mostly made up of cylindrical jointed seeds. In Boulder, Jointed Goatgrass is becoming an increasingly serious problem along trails, since the seeds are likely spread by hikers and dogs.
Soapwort or Bouncing Betis another escaped garden ornamental that quickly spreads into wild areas, where it wreaks havoc. You may have this flower hiding out in your garden. It stands about a foot tall, bearing clusters of whitish or pale pinkish flowers, each about an inch across. Seed capsules form in clumps, looking like bushy-topped bottles. Please do not cultivate this plant!
Dalmatian Toadflax is an escaped garden ornamental that was historically used in flower gardens. It looks like a tall yellow and orange snapdragon. It is a perennial that grows 1 to 2 feet high and bears rows of yellow orange-lipped flowers. Dalmatian Toadflax has a very extensive root system which makes it very difficult to control.
Bladder Senna is an invasive shrub with an attitude. It produces yellow pea-like flowers in summer, which quickly mature into unmistakable inflated pea pods, like little balloons. Each leaf bears multiple rounded leaflets, suggestive of a fern or locust tree. Left unchecked, bladder senna may turn into an 8 to 10 foot tall bush. When chopped off, it will quickly re-sprout from its roots.
Russian Olive trees were originally planted to promote soil conservation. We now know that Russian olives degrade wildlife habitat and compete with native tree species in riparian areas. While wildlife will eat the fruits, they are low in food value compared to those of other native species. Furthermore, the dense wood does not form hollow cavities, limiting habitat for cavity-nesting birds like flickers, tree swallows, wood ducks and owls. Russian olive trunks and branches are armed with 1 to 2 inch thorns. The leaves are narrow and covered with minute scales which give the foliage it's distinctive silvery appearance. Some nurseries still sell this tree, although we encourage you to purchase a less invasive alternative.
Canada Thistle is not really from Canada, but from Europe. This perennial plant has sharp, prickly leaves that can grow up to 4 ft. in height. Its flowers appear in small light lavender bristly clusters, 3/8 to 5/8 inch in diameter, in late summer. This one is difficult to control by hand-pulling, since it it quickly re-sprouts from any root stock left in the ground. Canada Thistle can be found all over our area.
Houndstongue is a biennial weed which grows 1 to 4 feet tall and is especially common in disturbed areas like trail borders. Its flowers are reddish-purple and produce an annoying prickly "hitch-hiking" seed that you've probably seen attached to your socks after hiking on OSMP lands. Houndstongue is toxic and causes liver cells to stop reproducing. It has been known to kill cattle and horses on occasion. Houndstongue is prevalent throughout OSMP.
Cheatgrass or Downey Brome also sticks in your socks, with long sharp needle-like seeds. Although it's an annual (it only lives one season), this noxious grass produces huge quantities of seed, allowing it to spread quickly and dominate an area. It is especially fond of disturbed sites, and once established it is very hard to control.
Diffuse Knapweed, perhaps the King of Noxiousness, is one of the most aggressive weeds found on OSMP. It can be an annual or biennial plant that grows up to 2 feet tall. The small unattractive flowers are single and usually white. The seed head bracts end as sharp, ridged spines and may cause skin irritation. Knapweed is difficult to control because once the seeds are mature the plant breaks at the base of the stem and tumbles across any open ground, dispersing hundreds of seeds. It also produces a compound that inhibits the growth or germination of nearby plants and seeds that may have been able to compete with the plant for resources. OSMP is utilizing several biological controls on this weed with a goal that the insects and weeds will reach an equilibrium that results in a much lower density of diffuse knapweed.
Musk Thistle is a biennial which grows up to 6 feet tall. The leaves are lobed with large spikes at each point. Flowers are large and deep rose, violet , or purple. It spreads rapidly, forming extremely dense stands which crowd out desirable forages. An introduced biological control species, the musk thistle weevil, feeds on the seeds and can limit the spread of this plant.
Scotch Thistle grows up to 13 feet tall making it the giant of all thistles. It's a biennial that produces large spiny leaves covered in fine dense hair and a flower that is purple in color. Scotch thistle can produce stands so dense they are impenetrable to humans, cattle, and wildlife.
Common Teasel is a stout, tap-rooted biennial which grows up to 6 feet tall. Flowers are purple and surrounded by large spike-like bracts. Common Teasel is found in areas that are poorly drained where they are typically competing with cattails.
Learn about how OSMP manages trees: When, Where, Why and How OSMP Removes Trees 503.79 Kb
Noxious weeds are a very serious threat to our native ecosystems and have been described as a "Biological Wildfire." Each year noxious weeds attempt to further invade OSMP ecosystems on both the plains and foothills. What makes certain plant species "noxious" is that they are non-natives which can grow in a variety of ecosystems. Visit our Noxious Weed Gallery above to learn how to identify these plants and how to eliminate them from your land or garden!
What are we doing?
OSMP has an Integrated Pest Management Program which uses many different control techniques on different weed species. Grazing, burning, mowing, hand-pulling, flooding, insect introduction, reclamation and prevention are a few control techniques that are used on Open Space. Herbicides, applied at the lowest rates possible, are used as a last resort.
An example of Integrated Pest Management at its best can be found along South Boulder Creek, where the threatened Ute Ladies' Tresses orchid, is competing with Canada thistle, an aggressive "noxious" weed. OSMP has been controlling Canada thistle and helping the orchid flourish by combining spring cattle grazing, mowing and historical haying practices, with the release of thistle eating insects.
Many people are responsible for weed management on open space. A summer crew maps weed infestations; cuts, hand pulls and digs weeds; applies herbicides; and cuts down Russian olive trees. Over 1,000 acres of weeds are mowed every summer. Spring cattle grazing and releasing insects distributed by the Colorado Department of Agriculture also keep certain weeds under control, reducing the need for herbicides. The key to Integrated Pest Management is to map each infested area and monitor the successes and failures of different control techniques.
Why not let nature (and evolution) take its course?
Protecting native ecosystems is central to OSMP's mission. We cannot stop the spread of noxious weeds everywhere, but we can preserve areas where native plants thrive as they have for millions of years before we introduced exotic species to Colorado.
What advantages do noxious weeds have over the natives?
A noxious weed is an introduced plant species that spreads very quickly, displacing native plants and altering local ecology.
How can you slow the spread of noxious weeds?
The public plays a crucial role in slowing the spread of noxious weeds. The following are some simple guidelines everyone should follow.
Last Updated on Wednesday, 08 August 2012 10:04