Weed Descriptions | CSU Extension

Summer Annuals

Common Mallow, Malva neglecta

Most frequent in cultivated ground, gardens, newly seeded lawns, or stressed lawns that lack density; found at 4,500 to 7,000 feet in elevation.
Prostrate, low-spreading annual, biennial, or perennial; deep taproot; foliage similar to geranium, pinkish-white flowers, fruits look like small round wheels of cheese.
Increase turf density. 
Pull plants from moist soil. 
Pre-emergent herbicides are effective. 
Post-emergent herbicides can be effective.

Common Purslane, Portulaca oleracea

Summer annual, found in newly seeded or thinning, non-vigorous lawns and also in cultivated garden sites; up to 8,500 feet in elevation. 
Smooth, thick, succulent, alternate (to sub-opposite) edible leaves; small yellow flowers in leaf axils; stems are smooth and reddish; plant is sprawling, prostrate, forming dense vegetative mats from shallow fibrous root system. 
Increase turf density may reduce purslane populations or prevent purslane establishment in lawns. 
Pulls easily when soil is moist; easily re-roots after cultivation—remove and dispose of plant.
Pre-emergent herbicides may be helpful. 
Post-emergent herbicide use is more effective when plants are young; difficult to kill with an herbicide when larger.

Crabgrass, Digitaria sanguinalis

Low-growing, prostrate, summer annual grass; leaf blades wider and lighter green color than Kentucky bluegrass with leaf sheaths with long stiff hairs. 
Base of stems are often reddish-purple in color; plant spreads by rooting at the lower stem nodes as well as by seed; forms seedheads below mowing height; seedheads are composed of slender, finger-like spikes. 
Crabgrass is less prevalent when turf has good density; mowing too low promotes crabgrass seed germination; maintain mowing height at 2.5 to 3 inches. 
A pre-emergent herbicide applied correctly and at the proper time should provide control; do not use a pre-emergent herbicide on a newly seeded or sodded lawn or when overseeding a lawn. 
Post-emergent “crabgrass killer” sprays are not effective unless crabgrass plants are at the young seedling stage.

Green Foxtail, Setaria viridis

A summer annual grass with wider blades and a lighter green color than Kentucky bluegrass. 
Faster growing than Kentucky bluegrass; seedheads (known as spikes) have bristles that give it a fuzzy appearance; may form a seedhead despite regular mowing. 
Foxtail is much less prevalent when turf has good density; resod or reseed bare spots. 
A pre-emergent herbicide applied correctly and at the proper time should provide control; do not use a pre-emergent herbicide on a newly seeded or sodded lawn or when overseeding a lawn.
Post-emergent herbicides will kill foxtail seedlings (but not mature plants).

Kochia, Kochia scoparia

Very prevalent in disturbed soils, cultivated fields, gardens. 
In spring, seedlings have alternate leaves; lower leaves often wider than upper leaves; underside of leaves hairy, margins hairy. 
Flowers are yellow, inconspicuous; seed production occurs from July to October. 
Stems are 1 to 6 feet tall. 
In fall, entire plant first becomes reddish-brown, then brown, becomes “tumbleweed.”
Germinates early; use pre-emergent herbicides before soil temperatures reach 38°F. 
Post-emergent herbicides can be effective. 
Mulch inhibits seedling development.

Netseed Lambsquarters, Chenopodium berlandieri

Summer annual prevalent in disturbed soils, gardens, cultivated fields. 
Extremely variable in appearance; stems 1 to 6 feet tall, grooved, often reddish tinged; undersides of leaves whitish, mealy (mottled, granular appearance). 
Flowers inconspicuous, greenish, at tips of stems and leaf axils; seed production occurs from July to September. 
Edible when plant is young and tender. 
Competitive weed with rapid growth and high-water use. 
Can be hoed or pulled when young. 
Pre-emergent herbicides applied at the right time in spring can provide good control. 
Post-emergent herbicides can be effective. 
Mulch inhibits seedling development.

Prostrate Knotweed, Polygonum aviculare

Prostrate summer annual with a thin taproot; tough, durable plant common along sidewalks, in turf that is stressed and less vigorous, and in gardens; found to 9,500 feet in elevation. 
Thrives in dry, compacted soils or wherever there is excessive foot traffic. 
Forms a tough, wiry mat of stems that are enlarged at each joint as well as a papery sheath at each leaf node; to differentiate from spurge, broken stem does not produce a milky sap; leaves and stems are not hairy, and leaves are alternate.
Flowers small, white, inconspicuous; found where leaf meets stem; produces many seeds. 
Annual core aeration spring and/or fall will reduce knotweed infestation in lawns. 
Apply pre-emergent herbicides in late fall/winter (knotweed can germinate in February or March). 
Post-emergent herbicides are mostly ineffective after plants become larger.

Prostrate Spurge, Chamaesyce maculate

Prostrate summer annual forming dense mats; found in thinning, less vigorous turf. 
Leaves are opposite and each leaf has a reddish-purple spot in the center; small pinkish flowers in leaf axils; stems and leaves are both hairy; sap is milky latex; some people develop a rash after skin contact with sap. 
Increase turf density. 
Plants can be pulled and bagged if soil is moist, wear gloves because of the sap. 
Post-emergent herbicides can be effective.

Redroot Pigweed, Amaranthus retroflexus

Coarse, summer annual; fast growing to 12 to 36 or more inches tall; dependent on moisture received. 
Alternate leaves vary in appearance but have prominent veins and midrib. 
Lower stem reddish or red striped; pinkish red roots even down the taproot. 
Flowers/seedheads at top of plant; prickly; produces many small black seeds. 
Very toxic to cattle and swine. 
Found in waste areas, gardens, disturbed soils, and in turf if thin and patchy in quality. 
Hoe or pull from moist soil before seedheads mature; bag plants if pulled later. 
Easy to kill with most herbicides but apply according to label directions well before seedheads mature; herbicides suggested only where large numbers of plants exist or where large areas are infested.

Scentless Chamomile, Matricaria perforata

Noxious weed in Colorado List B. 
Annual forb that can persist as a biennial or short-lived perennial. 
Stems of the plant are green, erect, often branched, glabrous, or slightly pubescent, and can range in height from 6 to 20 inches tall. 
Leaves are alternate, 1 to 2 inches long, slightly pubescent or glabrous, and are finely divided into several short thread-like segments. 
Terminal flowers are 0.75 to 1.25 inch in diameter, with a daisy-like appearance consisting of white petals surrounding a central yellow core. 
Key to control is reducing seed production; hand pulling is effective but may not be practical in larger patches; mowing conducted early in the growing season before plants flower and prior to seed production will reduce populations. 
Maintaining healthy stands of desirable vegetation can also be an effective control measure because scentless chamomile seedlings cannot tolerate intense competition.
Post-emergent herbicides can be effective.

Winter Annuals

Downy Brome/Cheat Grass, Bromus tectorum

Noxious weed in Colorado (List C). 
Winter annual, extremely abundant in intermountain west; after maturity can become a fire hazard, especially when dry; found at 4,000 to 9,000 feet in elevation. 
Leaf sheaths and blades are covered by dense soft hairs. 
Droopy seedheads develop in spring; long awns; prolific seed producer; plants turn reddish brown in early summer (mid to late June) and then fade to a blond color. 
Competes vigorously with other perennial grasses for moisture because of its winter and early spring growth habit; root growth during winter can occur until soil temperature goes below 37°F.
Hand-pulling effective for small infestations—repeat pulling over the season is necessary, as seeds will germinate irregularly; extract as much root as possible to prevent re-growth. 
Infrequent in mowed turf.
Post-emergent herbicides work well in early spring prior to seedhead appearance; best when non-target species are dormant.

Shepherd’s Purse, Capsella bursa-pastoris

Small winter annual with small white flowers early in spring; common in cultivated gardens and roadsides; common up to 9,000 feet in elevation. 
Slender stems from basal rosettes; leaves are hairy below, smooth above, and often deeply lobed; seed pods are heart-shaped (or purse-shaped); seed production from April to September. 
Hand-pulling or hoeing before seed set is very effective. 
Post-emergent herbicides should be labeled for use in turfgrass.

Biennials

Dame’s Rocket, Hesperis matronalis

Noxious weed in Colorado (List B). 
Can be a short-lived perennial. 
Was introduced as an ornamental. 
Flowers have four petals, are purple or white, clustered in loose stalks, and fragrant. 
Mature plants range from 1 to 3 feet tall. 
Can be aggressive in the landscape. 
Pulling or cutting flower heads before seed set will control the plant, but this will need to be repeated for several years to exhaust seed bank. 
For larger infestations, post-emergent herbicides can be effective. 
Do not buy seed mixes that contain this plant.

Diffuse Knapweed, Centaurea diffusa

Noxious weed in Colorado (List B). 
A biennial, short-lived perennial, or occasionally an annual. 
The plant develops a single shoot (stem), 1 to 2 feet tall that is branched toward the top; first year rosette leaves and lower shoot leaves are finely divided; leaves become smaller toward the top of the shoot and have smooth margins. 
Many solitary flowering heads occur on shoot tips; they are about one-eighth inch in diameter and 0.5 to 0.66 inch long; flowers usually are white but may be purplish; involucre bracts are divided like teeth on a comb and tipped with a slender spine that makes them sharp to the touch; sometimes the bracts are dark-tipped or spotted like spotted knapweed; the long terminal spine differentiates diffuse from spotted knapweed. 
It reproduces and spreads from seed—keep from going to seed; hoeing or hand pulling before the plant goes to seed can accomplish this. 
For larger areas, post-emergent herbicides can be effective. 
Cultural controls include revegetating with desirable grasses. 
Biological controls include the seedhead flies Urophora affinis and U. quadrifasciata and root-feeding insects such as the diffuse knapweed root beetle (Sphenoptera jugoslavica), the yellow-winged knapweed moth (Agapeta zoegana), and the knapweed root weevil (Cyphocleonus achates).

Musk Thistle, Carduus nutans

Noxious weed in Colorado (List B). 
Musk thistle is a biennial or winter annual that can grow up to 8 feet tall. 
Leaves are up to 10 inches long, dark green with a light green midrib, spiny, and deeply lobed; often have a white margin. 
Solitary, lightly spiny, and nodding flower heads develop at the stem tips in midsummer and grow to a diameter of 1.5 to 3 inches and are deep rose to violet. 
The key to control is not to let the plant go to seed; herbicides and hand pulling the rosette are both effective. 
Herbicide applications should be made in late spring/early summer and again in the fall for optimum control.

Prickly Lettuce, Lactuca serriola

Biennial or winter annual to 48 inches tall from a large taproot; invades disturbed garden soils. 
Cut stems/leaves exude a “milky juice;” more common in areas from 4,500 to 6,000 feet. 
Upper leaves lobed like oak leaves and are often twisted to lie in a vertical plane, also known as “compassplant,” because leaves may “point” to north and south; lower leaves often not as lobed; leaves have prominent spines on back side of midrib. 
Small yellow daisy-like flowers on elongated stems; seedheads are like those of dandelion. 
Hoe or pull from moist soil before yellow flowers mature. 
Easy to kill with many herbicides, especially when younger; apply according to label directions well before seedheads mature; herbicides suggested only where large numbers of plants exist or where large areas are infested.

Yellow Sweet Clover, Melilotus officinalis

Biennial herbaceous plants: second year plants grow 3 to 5 feet high and are bush-like; sweet clovers are very fragrant. 
Leaves are alternate, divided into three finely toothed leaflets; middle leaflet grows on a short stalk. 
Flowers are crowded densely at the top 4 inches along a central stem; each flower is attached by a minute stalk. 
There are one or two hard small seeds per flower; they stay viable in the soil for 30 years. 
Strong taproot. 
Can be good forage; however, moldy hay made from yellow sweet clover (or hay made from drought stressed or frost-damaged plants) is toxic to livestock (contains coumarin which converts to Dicoumarin, a blood thinner). 
The key to controlling sweet clovers is to keep them from flowering and then concentrate on depleting viable seeds in the soil. 
Hoe, hand pull, or spray with post-emergent herbicide when young.

Simple Perennials

Curly Dock, Rumex crispus

Leaves emerge from stout taproot in spring. 
Elongated leaves have wavy (curly) margins.; leaves mostly basal, with long petioles. 
Stems 2 to 4 feet tall, reddish, ridged; nodes sheathed with clear membrane. 
Flowers greenish, May.
Winged fruits on flowering stems, reddish-brown. 
Habitat—Fields, roadsides, railroads, waste ground, disturbed sites, turf/landscape. 
Dig taproot, must remove at least 75% of the taproot to control. 
Post-emergent herbicides can be effective.

Myrtle Spurge, Euphorbia myrsinites

Noxious weed in Colorado (List A). 
Mat-forming perennial to 9 inches tall. 
Escaped ornamental; formerly sold as a drought-tolerant ground cover. 
Blue-green succulent leaves form a “donkey tail;” has chartreuse bracts (“flowers”). 
For small infestations, dig or pull out clumps with caution; white latex sap from stems and leaves can cause severe dermal reactions—always wear gloves if hand pulling. 
For larger infestations, use an herbicide; the best time to treat myrtle spurge with herbicide is during late fall. 
Eradication of all plants is required throughout Colorado. If you see it, contact your county weed supervisor or the state weed coordinator.

Spotted Knapweed, Centaurea maculosa

Noxious weed in Colorado (List B). 
A short-lived, non-creeping perennial that reproduces from seed (primary means of spread). 
Produces one or more shoots that are branched and 1 to 3 feet tall; rosette leaves can be 6 inches long and deeply lobed.
Leaves are similar to diffuse knapweed. 
Lavender to purple flowers are solitary on shoot tips and about the same size as diffuse knapweed flowers; involucre bracts are stiff and black-tipped; the tip and upper bract margin have a soft, spine-like fringe and the center spine is shorter than others. 
For control measures, see diffuse knapweed.

Creeping Perennials

Bouncing Bet, Saponaria officinalis

Noxious weed in Colorado (List B). 
An escaped ornamental, aggressive in landscapes and wild areas. 
Spreads aggressively through rhizomes and seeds. 
White to pink five-petaled flowers are clustered at the ends of branches. 
Leaves are opposite, smooth, and have three veins from base. 
Mature plants are up to 3 feet tall. 
Toxic to livestock. 
Can be controlled by mowing or pulling several times a year—before seed production. 
Post emergent herbicides can be effective.

Canada Thistle, Cirsium arvense

Noxious weed in Colorado (List B). 
Colony-forming creeping perennial spreading primarily by horizontal roots (can grow as much as 18 feet in one season) and to a lesser degree by seed; found from 4,000 to 9,500 feet in elevation. 
Flowers are purple and are borne in clusters; spiny foliage with variable leaf shapes; when mowed in a lawn, will not develop full height and flower. 
Highly invasive species; control is difficult because of its extensive root system; pulling generally is not effective due to the tremendous reserves in the root system; regular, persistent pulling may gradually starve root system; shoots should be pulled as they are noticed, as all shoots (leaves) are producing food reserves. 
Increase density and competitiveness of turf. 
Post-emergent, systemic herbicides can be effective. 
Biocontrol insects include a seed head weevil, a stem-mining weevil, and a gall-forming fly; these may not be significantly effective alone but can provide good results when combined with other control methods; biocontrol insect releases are best suited to large acreage infestations; backyard releases are generally impractical.

Common Tansy, Tanacetum vulgare

Noxious weed in Colorado (List B). 
Introduced from Europe as an ornamental and medicinal herb. 
Found in yards, along roadsides, stream banks, and in waste places. 
Spread by rhizomes, can reach 3 to 4 feet tall. 
Flowers are button-shaped and yellow in flat-topped clusters. 
Leaves are deeply divided into narrow leaflets and rank smelling. 
Toxic to livestock, although unpalatable. 
Mowing before seed production can limit spread, although it may have to be repeated several times in a season to prevent regrowth from rootstocks. 
Hand pulling in damp soil can remove small infestations; wear gloves; will readily regrow from fragments in soils. 
For larger infestations, post-emergent herbicides can be effective.

Creeping Wood Sorrel/Oxalis, Oxalis corniculata

Prostrate, creeping perennial from slender taproot; stems root where they touch the ground. 
Leaves have a shamrock appearance; plants often mistaken for a clover; leaves may “fold up” at night or on cloudy days; leaves turn purplish with the arrival of cooler weather in fall; some plants may have purplish leaves year-round. 
Small yellow flowers. 
Fruits “explode” when mature, scattering seed often more than 10 feet. 
More common in thin, less vigorous turf given too frequent, light irrigation; increase turf density. 
Pre-emergent herbicides may be helpful. 
Post emergent herbicides can be effective.

Field Bindweed, Convolvulus arvensis

Noxious weed in Colorado (List C). 
Creeping perennial; found as high as 10,000 feet in elevation; general range 4,000 to 8,000 feet. 
Vining, sprawling, prostrate growth habit; may climb by twining around fence wire or around stems of other plants; not shade tolerant but drought tolerant due to extensive roots; leaves are arrowhead-shaped; attractive, white or pink bell-shaped flowers that resemble morning glory from late June until frost. 
Increase density and competitiveness of turf.
Control is difficult because of its extensive root system, which can penetrate the soil profile to a depth of 20 feet; seeds also can remain viable for 20 to 50 years; pulling generally is not effective due to the tremendous reserves in the root system; regular, persistent pulling may gradually starve root system; shoots should be pulled as they are noticed, as all shoots (leaves) produce food reserves. 
Post-emergent herbicides can be effective. 
The bindweed mite has been used as a biological control with some success; initial impact is reduction of growth and limited flower and seed production; mowing moves mites around and stimulates plant growth for mites to feed on; survival is better in drier settings; excessive moisture may limit establishment.

Hoary Cress (White Top), Cardaria draba

Noxious weed in Colorado (List B). 
A creeping perennial that reproduces by seed and creeping roots; one of the earliest perennial weeds to emerge in the spring. 
It grows erect from 10 to 18 inches high and has a white color. 
The alternate leaves clasp the stem and are oval or oblong with toothed or almost smooth margins; the leaves are often covered with very fine white hairs; each leaf is 0.5 to 2 inches long with blunt ends. 
The flowers are white, one-eighth inch across, and numerous in compact flat-top clusters, which give the plant its name; each heart-shaped seed pod contains two oval, finely pitted, red-brown seeds each about one-twelfth inch long.
Due to the rhizomes of this perennial weed, mechanical control provides minimal control; diligent digging can provide control of very small infestations; hand pulling of above-ground
plant parts is ineffective; successful digging requires complete plant removal within 10 days after weed emergence throughout the growing season for 2 to 4 years; cultivation 6 inches deep must be repeated within 10 days of weed emergence throughout the growing season for 2 to 4 years. 
Revegetate with desirable vegetation. 
Post-emergent herbicides can be effective.

Leafy Spurge, Euphorbia esula

Noxious weed in Colorado (List B). 
An erect plant that grows 1 to 3 feet tall. 
Leaves are bluish green with smooth margins, 0.25 inch to 0.5 inch wide, and 1 inch to 4 inches long. 
Umbel flowers are surrounded by heart-shaped, showy, yellow-green bracts (an umbel looks like the stays of an umbrella if it is held upside down); flowers occur in many clusters toward the top of the plant; seeds are round to oblong, about one-twelfth inch long, gray or mottled brown with a dark line on one side. 
Leafy spurge contains a white milky latex in all plant parts; latex distinguishes leafy spurge from some other weeds (e.g., yellow toadflax), particularly when plants are in a vegetative growth stage. 
Leafy spurge has an extensive root system that is abundant in the top foot of soil, and it may grow 15 feet deep or more; roots contain substantial nutrient reserves that allow the weed to recover from stress, including control efforts; many vegetative buds along roots grow into new shoots. 
Use a combination of methods to control leafy spurge; vigorous grass helps weaken leafy spurge through competition. 
Post-emergent herbicides can be effective.

Orange Hawkweed, Hieracium aurantiacum

Noxious weed in Colorado (List A). 
Shallow, fibrous roots. 
Leaves are hairy, spatula shaped, up to 5 inches long, and basal.
Extensive stolons create a dense mat that practically eliminates other vegetation—makes mechanical control very difficult once established. 
Stems and leaves exude a milky latex when cut or broken. 
Up to 30 half-inch red to orange flowers appear in late May or June. 
Post-emergent herbicides can be effective. 
Eradication of all plants is required throughout Colorado. If you see it, contact your county weed supervisor or the state weed coordinator.

Oxeye Daisy, Chrysanthemum leucanthemum or Leucanthemum vulgare

Noxious weed in Colorado (List B). 
A perennial from rhizomes with characteristic “daisy-like” flowers.
Plants initially develop as a basal rosette; lower rosette leaves occur on petioles and are from 1.5 to 6 inches long; leaves are lobed. 
Flowers are white with a yellow center and range from 1.25 to 2 inches. 
Oxeye daisy should be mowed as soon as flowers appear to reduce seed production; root systems are shallow, and the plant can be dug up and removed; hand removal will have to be continued for several years because seeds may remain viable in the soil for a long time 
Post-emergent herbicides can be effective. 
Native daisies are a good, non-invasive garden alternative.

Purple Loosestrife, Lythrum salicaria

Noxious weed in Colorado (List A). 
Aggressive in riparian areas. 
Square stem, whorled leaves. 
Purple-magenta flowers with five to seven petals in long racemes. 
If left unchecked, a wetland may become a monoculture of loosestrife.
Control of small infestations can be managed through digging all the plants and roots—this will need to be monitored for a few years. 
Large infestations should be controlled with an aquatic-labeled herbicide. 
Eradication of all plants is required throughout Colorado. If you see it, contact your county weed supervisor or the state weed coordinator.

Quackgrass, Elytrigia repens

Noxious weed in Colorado (List B). 
Very aggressive creeping perennial grass especially in moist soils; found from 4,500 to 9,000 feet in elevation; spreads by seeds and invasive rhizomes (underground stems). 
Rhizomes are yellow white, with brown sections; rhizome ends are sharp-pointed and can penetrate hard soils; base of leaf blade with claw-like appendage that clasps the stem. 
Believed to be allelopathic (release of a chemical that inhibits growth of nearby plants). 
Mechanical control is difficult as any rhizome segment produces new plants.
A few quackgrass plants can be spot sprayed with glyphosate, or individual blades can be painted with glyphosate; note that glyphosate will kill any bluegrass it contacts; repeat applications will likely be needed. 
Fluazifop is a selective herbicide targeting grassy weeds and can provide effective control of quackgrass in non-turf areas, groundcovers, flower beds, and around shrubs.
Renovate severely infested lawn areas—spray area with glyphosate; repeat applications will likely be needed; ensure that quackgrass is killed before areas are resodded or reseeded.

Russian Knapweed, Centaurea maculosa

Noxious weed in Colorado (List B). 
Creeping perennial that reproduces from seed and vegetative root buds. 
Emerges in early spring, bolts in May to June, and flowers through the summer into fall. 
Shoots or stems are erect, 18 to 36 inches tall, with many branches; lower leaves are 2 to 4 inches long and deeply lobed; upper leaves are smaller, generally with smooth margins, but can be slightly lobed; shoots and leaves are covered with dense gray hairs. 
The solitary, urn-shaped flower heads occur on shoot tips and generally are 0.25 to 0.5 inch in diameter with smooth papery bracts; flowers can be pink, lavender, or white. 
Has vertical and horizontal roots that have a brown to black, scaly appearance, especially apparent near the crown. 
Toxic to horses; allelopathic to other plants. 
The key to Russian knapweed control is to stress the weed and cause it to expend nutrient stores in its root system. 
An herbicide alone will usually not effectively manage Russian knapweed; combine treatment with perennial grasses sown in late fall; tillage is necessary to overcome the residual allelopathic effects of Russian knapweed.

White Clover, Trifolium repens

Creeping perennial that forms runners that root at nodes. 
Many people like clover in lawns, while others find white flowers and the bees they attract objectionable. 
A legume that fixes nitrogen, so it is often found in lawns having low fertility. 
Increase turf density with proper watering, mowing, and fertilization. 
Post-emergent herbicides can be effective.

Wild Violet, Viola spp.

Heart-shaped leaves on long petioles, purple flowers in spring; may also spread by rhizomes. 
Difficult to control due to resistance to many herbicides. 
Improve light penetration to shaded areas by pruning trees and shrubs. 
Mow lawn higher to increase competition from grass. 
Best control may be to pull plants when ground is moist.

Yellow Toadflax, Linaria vulgaris

Noxious weed in Colorado (List B). 
Yellow toadflax is a perennial that spreads sideways by underground rhizomes and by seeds. 
Flowers are small, yellow, look like snapdragons, and bloom in mid-late summer; leaves are linear. 
Some people confuse a native plant, golden banner, with toadflax, but golden banner blooms very early and has three leaves, like a clover.
Yellow toadflax is difficult to control; its extensive root system lets it recover from control attempts. 
Yellow toadflax is variable, genetically; therefore the effectiveness of herbicides is also variable. 
Hand pulling can be effective on small patches, especially in gravelly soils when you can pull a large part of the root; it will need to be pulled for several years; pull before it goes to seed. 
Post-emergent herbicides can be effective.

Woody Plants

Russian Olive, Elaeagnus angustifolia

Noxious weed in Colorado (List B). 
Small tree 10 to 25 feet tall; originally planted as an ornamental and for windbreaks. 
Leaves are narrow and appear silvery. 
Branches have long thorns 1 to 2 inches in length. 
Small, sweet smelling yellow flowers are followed by a berry-like fruit which is spread by birds. 
Has become a serious weed in low-lying pastures, meadows, and waterways. 
The most effective control is to cut the tree and immediately paint the stump with an herbicide. 
Silver buffalo berry is an excellent native alternative plant.

Tamarisk, Tamarix ramosissima

Noxious weed in Colorado (List B). 
Tamarisk was sold as an ornamental plant for gardens during the 1800 and 1900s; tamarisk has now spread to most of the western United States, displacing the native cottonwoods and other plants. 
Plants can grow to 6 inches tall during the first 2 months and can grow over 18 feet tall; the taproot can reach 100 feet down with a root spread of up to 150 feet; adventitious roots can produce new trees when buried. 
Mature tamarisk trees can produce millions of pollen-size seeds dispersed through wind and water; seeds can germinate while floating and establish themselves on wet banks within 2 weeks; newly formed sand banks are particularly susceptible; trees may reproduce in the first year, but typically they reproduce during the second year. 
It is very “thirsty”—one tree can use up to 300 gallons per day, and it alters hydrologic conditions in riparian areas. 
Salt glands on the leaves release salt, increasing salinity of soil. 
Tamarisk is difficult to control; single treatment approaches to control tamarisk have not proven feasible because no method completely eliminates tamarisk or its regeneration; use revegetation in conjunction with other methods. 
The salt cedar leaf beetle, Diorhabda elongaa, has been released on some stands, and has shown to be fairly effective.

This publication, reference GardenNotes #352, is developed as part of the Colorado State University Extension Master Gardener Program.