By Michelle Le Blanc, Coordinator
San Luis Valley Weed Management Association
The extra moisture this past season pulled a lot of weeds out of dormancy. Weeds don’t stay in town, because their seeds are easily dispersed by wind, water, shoes, pets, and birds to surrounding farms, ranches, and waterways.
In the San Luis Valley, it is estimated that invasive weeds cost private landowners, the public, and federal lands at least $1.2 million in agricultural production, wildlife habitat and recreation. Therefore, it’s important to stem the tide of invasive weeds’ spread.
While you have to abide by your county or town ordinance for weed management, here are a few tips that can help make each year progressively easier.
First, don’t let weeds flower and go to seed before you treat or pull them. You will save yourself tons of time next year if you spend some time this year getting rid of them before they can reproduce.
Second, I find that watering the area where I’m going to pull weeds the night before helps loosens the roots (and saves my back). Water for about an hour, let the soil soak up the water overnight, then loosen the soil with a shovel, and pull. Remember to wear gloves, bag up the weeds, and toss in the trash.
Third, ask for help. I’ve had my share of weed problems over the years and have been lucky to have friends help me. The promise of shared time, a home cooked meal, and a tasty beverage will go a long way in getting some help and making the task less onerous.
Fourth, not all treatments work for all weeds, so do some research. Some weeds really do require chemical herbicide, because pulling or mowing simply creates new plants. Most likely, though, consistent effort to keep weeds from seeding works best if done throughout the season and over many years. If you don’t know what a weed is or how to get rid of it, go to www.slvnoxiousweeds.org, www.colorado.gov/ag/weeds or www.cwma.org to see if you can find a match. These websites also have treatment recommendations. You can also call or visit the Colorado State University Cooperative Extension service.
If you don’t want to use chemicals, you can try a vinegar spray. Mix a gallon of white vinegar, one tablespoon of dish soap, and one tablespoon of cooking oil, and spray the weeds when it’s hot and sunny. This will kill the leaves, stress the plant, and possibly kill it, but the roots may not die. I’ve used it and it (mostly) works, but I’ve found that I have to spray several times a season.
Here are some of the most common weeds found in towns around the San Luis Valley.
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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.
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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.
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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.
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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”.
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Perennial Pepperweed can grow to a height of six foot tall and reproduces via both seed and root. Each plant produces over 4,800 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 inch in length are capable of generating new shoots. The Perennial Pepperweed is also known to have Allelopathic properties which prevent propagation of desired plant life.
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Tamarisk or Salt Cedar is beautiful, but it uses about 200 gallons of water a day which is very dangerous in our arid climate. A bulldozer or prescribed fire can be used to open up large stands of salt cedar. These methods must be followed up with an herbicide treatment of the re-sprouts when they are 1 to 2 meters tall. Chainsaws, or loppers for smaller plants, are effective for cut-stump treatments to smaller infestations or in environmentally-sensitive management areas.
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The Common Tansy is often found in flower beds, this perennial plant reproduces by both seed and creeping rootstocks. Control this plant through repeated mowing or cutting. Tilling will cause common tansy to spread. Wear gloves and goggles, because tansy can be toxic.
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The showy snapdragon-like flowers are bright yellow with a deep orange center. It develops an extensive root system, making control options varied. Pulling is not recommended due to the extensive root system, but several chemical options are available.
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With silvery-green leaves and a scent like heaven, these trees are beautiful but demand a lot of water. Replace Russian olive trees with native trees and remove seedlings and saplings before they mature. Stumps respond well to a long, hot burning fire or herbicide treatment.
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Diffuse Knapweed is a non-native biennial forb that reproduces solely by seed. A biennial is a plant that completes its life cycle within two years. During the first year of growth, Diffuse knapweed appears as a rosette in spring or fall. During the second year in mid to late spring – the stem bolts, flowers, sets seed, and the plant dies. Once the plant dries up, it breaks off at ground level and becomes a tumbleweed which disperses the still viable seeds over long distances. A prolific seed producer, Diffuse knapweed can produce up to 18,000 seeds per plant. Therefore, the key to managing this plant is to prevent seed production. Diffuse knapweed can grow 1 to 3 feet tall, and is diffusely branched above ground. This gives the plant a ball-shaped appearance and tumble-weed mobility when broken off. Leaves are small, and are reduced in size near the flowering heads. Flowers are mostly white, sometimes purple, urn-shaped, and are located on each branch tip. Bracts that enclose the flowerheads are divided like the teeth of a comb, and are tipped with a distinct slender spine. Upon drying, the bracts become rough, rendering them injurious to the touch. Flowers bloom July through August. Seed set usually occurs by mid-August.
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Houndstongue is a short-lived perennial or biennial forb. It produces rosettes in the first year and bolts a stout, erect stem, that is 1 to 4 feet tall by mid-summer of the second year. Then it flowers and produces fruit. Flowers are reddish-purple (occasionally white) and droop slightly from densely clustered panicles. The five rounded petals are cupped by five sepals covered with long, soft white hairs. Flowering occurs from May to July. The simple leaves are lance or oblong shaped, with a smooth edge and no teeth or lobes. Leaves are alternate, 1 to 12 inches long and 1 to 3 inches wide. The leaf tip is sharply pointed, like a hound’s tongue, yet are covered with long-soft white hairs. Leaves often appear dusty and insect-ridden. A thick, dark, woody taproot can reach 3 to 4 feet deep.
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Oxeye Daisy was introduced from Europe as a seed contaminant and as an ornamental. It is a rhizomatous, creeping, short-lived perennial that grows 10 inches to 2 feet tall. The basal and lower leaves are spoon-shaped, toothed, and with long petioles (leaf stem). The upper leaves are narrow, toothed, and clasp the stem. Flowers bloom between June and August. The flowers are 1 to 3 inches in diameter, with 15 to 30 white ray flowers, and mostly solitary. The phyllaries beneath the flower head are green with a dark brown margin. One flower head can produce up to 200 seeds. Oxeye daisy spread vegetatively from roots, root fragments, or by seed. Seeds may be viable up to 38 years or more. Infestation sites need to be monitored for at least 10 years after the last flowering plant has been eliminated and treatments repeated when necessary. Ornamental Shasta daisy (Leucantheum x superbum) is not an aggressive invader and looks similar to oxeye daisy, but it is 6 to 12 inches taller and has larger flowers.
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Cypress Spurge is a low growing perennial that overwinters as root and crown tissue. Cypress spurge reproduces by seed and lateral root buds. Leaves are linear, approximately 1/2 to 1 1/4 inches long and 1 to 2 mm wide. Upper stem leaves that occur near the inflorescence are yellow or yellowish-green in color. Leaves are stalkless, alternate, narrow and linear to lance-shaped. Stems are 4 to 32 inches high, hairless, green to yellowish green in color and branch in the upper portions. The leaves and stems emit a milky, toxic sap when broken. Flowers are yellowish-green usually turning reddish green towards maturity and are clustered in bunches at the ends of stems.
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Leafy Spurge is a non-native deep-rooted perennial that spreads by seed and extensive, creeping roots. The roots can extend as deep as 30 feet into the soil and are extremely wide-spreading. The roots are brown and contain numerous pink buds that generally produce new shoots or roots. Leafy spurge can grow from 1 to 3 feet in height. The stems are smooth, pale green, and thickly clustered. Leaves are alternate, narrow, linear, and 1 to 4 inches long. The flowers are very small and yellowish-green. They are enclosed by very visible yellowish-green, heart-shaped bracts. The entire plant contains white, milky sap that exudes readily upon a stem or leaf breakage. This sap can damage eyes and sensitive skin. Leafy spurge is one of the earliest plants to emerge in the spring. Flower clusters develop 1 to 2 weeks after stem emergence which is from mid-April to late May. One large leafy spurge plant can produce up to 130,000 seeds. Three-sided seed capsules explode when ripe and project the seeds up to 15 feet away from the parent plant.
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Spotted and hybrid knapweed is a non-native, short-lived perennial forb that reproduces mainly by seed. Spotted knapweed can grow up to 900 seeds per plant annually that are viable for up to 8 years. The key to distinguishing spotted from other knapweeds is the black-tipped, spiny, involucral bracts (phyllaries) at the base of the flower. Unlike diffuse knapweed, it does not have a long, distinct terminal spine at the tip of the bracts. Spotted & hybrid knapweed can grow up to 3 feet tall on rigid stems that are openly branched on the upper half of the plant. Urn-shaped flowers are solitary on the tip of each branch. Flowers are pink to purple and rarely white. Leaves on the stem are alternate, deeply lobed, and become smaller and simple near the tips of the stem. Basal rosette leaves are deeply lobed and up to 6 inches long. Flowers bloom from June to October and seed-set usually occurs by mid-August. Spotted knapweed can also reproduce vegetatively from lateral roots. Spotted knapweed can cross with diffuse knapweed to form the hybrid knapweed.
Canadian Thistle
Hoary Cress
Russian Knapweed
Black Henbane
Perennial Pepperweed
Salt Cedar
Common Tansy
Yellow Toadflax
Russian Olive
Diffuse Knapweed
Houndstongue
Oxeye Daisy
Cypress Spurge
Leafy Spurge
Spotted Knapweed
Other San Luis Valley Weeds:
Dalmatian toadflax – looks like yellow snapdragons, but shouldn’t be planted in your flower beds. Pulling is effective on small infestations and if done every year for 5-6 years.
Quackgrass – grows from underground rhizomes to a height of 1 to 4 feet with erect stems. Till to the middle of the stand and prevent it from going to seed. Like other weeds, it is wise to find something to out-compete it like a desired grass or ground cover.
Bindweed – has arrowhead shaped leave that grow on a long vine close to the ground. Trumpet-like flowers range from white to pink. Persistent stress to this plant as well as healthy grasses will help manage this weed.
Common mullein – this one hurts me because it’s so beautiful and I love it stalk of sulfur-yellow flowers with my fuchsia hollyhocks, but again, mullein produces so many seeds that it can take over an area in a short time. Hand pull or dig when soil is moist, prior to flowering and seed production can be effective.
Cheat grass aka downy brome – is a major fire hazard. This grass grows in the understory of the forest and fuels very hot and hard to fight fires. You can mow it, graze it, till it to control it as long as the seed heads are purple; when the seeds turn green or yellow, it’s too late. To eradicate, plant desirable grasses and maintain appropriately.
If you have questions about weeds or weed management, contact the SLV Weed Management Association Coordinator at 719-588-3268 or via email at This email address is being protected from spambots. You need JavaScript enabled to view it.. Our website is www.slvnoxiousweeds.org. Information is also available through the Colorado Weed Management Association’s website www.cwma.org and the Colorado Department of Agriculture’s website www.colorado.gov/ag/weeds. Photos courtesy of Colorado Department of Agriculture.
The SLV Weed Management Association is a public and private partnership created to promote awareness and management of noxious weeds through local and regional initiatives in the SLV area with the vision to cooperatively manage and/or control noxious weeds throughout the San Luis Valley Management Area regardless of geographic or political boundaries to promote ecological and economic values.
