Crooked River Weed Management Area

Central Oregon

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B-Listed Species ... Crooked River Weed Management Area

Canada Thistle
Cirsium arvense

Canada thistle is a colony forming perennial herb. Stems are 1 to 4 feet tall and spineless. Flowering occurs from June to August producing purple to pink disk flowers ½ to ¾ inches in diameter. The flowers form in bunches at the end of stems. The upper surface of mature leaves is dark green and hairless while the lower surface is lighter green and can be either hairy or hairless.
What to look for:
            Clumps of mature plants
            Multiple pink to purple flowers per stem
When & where to find Canada thistle:

Look for Canada thistle starting in May when plants begin to emerge and later in June through August when it’s flowering. Canada thistle will most likely be found on roadsides, irrigation ditches, in open moist meadows, croplands or even your garden.

The best medicine is to maintain healthy plant communities able to withstand Canada thistle invasion. Newly established plants can be dug up before they become well established. For chemical control Milestone is the best product spraying from full bloom until the first killing frost in the fall. There are also biocontrol agents available from the Oregon Department of Agriculture for very large patches.
Diffuse knapweed
Centaurea diffusa
Diffuse knapweed is an erect biennial. Its stems typically have many branches, with short, stiff hairs on its branch axils. Diffuse knapweed has both long, deciduous basal leaves that are stalked and divided into narrow, hairy segments, and smaller stem leaves which are alternate, less divided, stalkless, and become bract-like near the flower clusters. Flower heads are urn-shaped
and grow singularly or in clusters of two to three at the ends of the branches. The bracts surrounding the flower heads are yellowish with a brownish margin, sometimes spotted, fringed on the sides, and terminate into a slender bristle or spine. Flowers can be white, rose-purple, to  lavender in color.
When to find diffuse knapweed

In fall and winter, it will be found as basal a rosette. It bolts in May, and flowers from June until August.
Where to find diffuse knapweed

Like all knapweed species, it prefers open habitats such as plains, rangelands, riparian areas, sandy river shores, gravel banks, rock outcrops, roadsides and forested benchlands.

Hand pulling or grubbing this plant can be done, especially if the infestation is small and you are certain of its identification. Bag and remove material from the site, being careful not to disperse seeds. If populations are too large for hand pulling herbicides are useful. Milestone and Weedmaster are both effective herbicides and your local chemical dealer can give you
Cynoglossum officinale

Houndstongue is an attractive plant that grows from 1 to 4 feet tall depending on site conditions. The leaves are alternate, 4 to 12 inches long, have entire margins, and are covered with soft hairs. The leaves clasp the stem and decrease in size towards the top of the plant. The small flowers are red to purple in color with 5 petals. Plants generally flower between late June and September. Seeds at maturity are light brown to gray-brown. The erect stems remain upright through the winter, and often still have seeds attached through the following year.
What to look for:
            - Distinctive reddish-purple flowers originating from the upper part of the stem.
            - Groups of four round to triangular shaped nutlets (seeds) covered with small hooked prickles.
When to find Houndstongue:

Houndstongue is most easily found when flowering which helps distinguish it from common mullein and forget-me-not. It is also recognizable from the previous years’ tan colored stems and seeds which are present all year long.
     Where to find Houndstongue:
            Houndstongue is found in a wide range of site conditions, including rangelands, edges of cropland, open woodlands, roadsides, riparian corridors and disturbed areas. It prefers disturbed sites in full sunlight with loamy to sandy soil.

Small populations can be hand pulled or grubbed from the ground with a hoe. Severing the root below ground is necessary to stop bolting. Preventing seed production is critical for successful control of this plant. Biocontrol agents are not available. Chemical control is effective. Refer to Crook County noxious weed treatment recommendations for chemicals, rates and timing. After control, establishment of perennial grass cover is usually necessary.
Medusahead Rye
Taeniatherum caput-medusae


Medusahead rye is a slender, annual grass that grows six to 18 inches tall. The leaf blades are rolled and slightly hairy. Each blade is topped by a bristly and densely crowded spikelet, with each bristle growing from one to four inches long. When the bristles are green, they are straight and compressed; they become twisted and erratically spread upon drying, resembling the hair of the mythical Medusa and turning the color of straw.


It can be found in late spring and throughout the summer and fall. In May as it begins to bloom it will be green in color and as it reaches full maturity by mid- June it turns straw colored.

It grows best in clay soils, and can be found on disturbed sites (e.g. overgrazed land or land with sparse vegetation), grasslands, oak woodlands, and agronomic fields.

Because of the high silica content of Medusahead a dense thatch will begin to form if left untreated. Therefore, in many cases fire is used in the fall to burn of the thatch and then a residual chemical is applied. Treatment in the spring can also be effective. Glyphosate can be used in early spring to treat
Medusahead before perennial grasses begin to emerge. Multiple years of treatment is needed to deplete the seed bank in the soil.
Myrtle Spurge
Euphorbia mysintes

Myrtle spurge (Euphorbia myrsinites) is a low growing perennial with trailing fleshy stems. The leaves are fleshy, blue-green and alternate. Flowers are inconspicuous with yellow-green, petal-like bracts that appear from March to May. Myrtle spurge spreads by seed and plants are capable of projecting seeds up to 15 feet. The plant grows from a taproot, with new stems emerging in early spring and dying back in the winter. Plants can grow up to 8-12 inches high and 12-18 inches in width.

Myrtle spurge contains a toxic, milky sap which can cause severe skin irritations, including blistering. This plant is poisonous if ingested; causing nausea, vomiting and diarrhea. Wearing gloves, long sleeves, shoes, and eye protection is highly recommended when in contact with myrtle spurge, as all plant parts are considered poisonous.

Myrtle spurge is an invasive ornamental that is native to Eurasia. It is popular with rock gardens, preferring sunny to partly sunny areas and well drained soils. Myrtle spurge rapidly escapes gardens and invades sensitive ecosystems, out competing native vegetation and reducing wildlife forage. Alternatives to planting myrtle spurge include native plants such as sulphur flower (Erigonum umbellatum), Kinnikinnick (artcostaphylos uvursi), or creeping mahonia (Mahonia repens). The soil seed reserve of myrtle spurge is estimated to be eight years. The site must be monitored for at least nine years after the last flowering adult plants have been eliminated and treatments repeated when necessary.

The key to effective control of myrtle spurge is to remove plants prior to seed set and to detect and remove new populations in natural areas early on. Small areas can be easily removed by mechanical means but should be done early to prevent triggering seed launching. Details on the back of this sheet can help to create a management plan compatible with your site ecology.
Poison Hemlock
Conium maculatum


Poison hemlock is a perennial member of the parsley family. They can grow up to 6 feet tall with smooth, hollow stems covered with purple spots. Flowers are stalkless with 5 white petals and develop numerous umbrella-shaped clusters. Leaves are finely divided, resembling those of parsley or carrots. Crushed leaves have a mouse like odor. The plant is sometimes confused with wild carrot or Queen Anne's lace. It also has a large white to pale yellow taproot.

What to look for:

Purple spotted stems & parsley-like leaves


Cultivation can prevent these species from persisting. Pre-emergent herbicide products
containing chlorsulfuron and metsulfuron can be successful. Post-emergent phenoxy herbicides and
glyphosate products also work well.
Puncture Vine
Tribulus terrestris


Puncture vine is a prostrate annual herb that grows from a simple woody taproot. It produces numerous green to reddish-brown stems that can grow up to six feet long. These stems have many branches that arise from the crown to produce a dense mat. The leaves are opposite with short, oblong petioles that are divided into four to eight pinnate leaflets and are covered in silky to bristly silver hairs. Small, yellow, five-petaled flowers are borne on short stalks at the leaf nodes. The fruit is a woody burr with sharp, rigid spines that are strong enough to puncture bike tires or shoe soles.
What to look for:
            Woody burrs (fruits)
            Leaves with 4-8 opposite leaflets
            Silver hairs on the foliage

When & where to find puncture vine:

This plant prefers warm conditions and light textured soils, but is highly adaptable to a range of growing conditions. It is commonly found in pastures, roadsides, orchards, vineyards, waste places, parks, railway yards, walk ways, and agricultural areas.

Small populations of puncture vine can be hoed or pulled at the top of the tap root before or during flowering. If the seeds have begun to develop, remove the pulled material from the site. During active growth puncture vine can be sprayed with 2,4-D.
 Russian Knapweed
Acroptilon repens

Russian knapweed is a bushy, branched perennial growing 1 to 3 feet tall. It is rhizomatous, forming colonies through its vigorous root system and alleopathy (biochemicals it produces that inhibit the growth of other plants). Young stems of the plant may be whitish and woolly while older stems will be
more woody and black or brown in color. Leaves are gray-green in color. Basal leaves are highly notched and upper leaves are smaller and more linear with
broken edges. Pink to purple flowers grow in solitary heads at the tips of leafy branches. Bracts are green to straw colored and have broad papery
What to look for

Black roots distinguish Russian knapweed from other knapweeds When to find Russian knapweed July to September, when in bloom.
Where to find Russian knapweed

It can be found in bottomlands, sub irrigated slopes and flats. Also in hayfields, pastures, along roads or irrigation ditches. It will commonly invade Basin Wild rye sites.

A bio control in the form of a nematode called Subanguina picridis is available; however, the best form of control is through integrated management. The key is to stress the plant making it expend its nutrient reserves. A fall application of herbicide can be very effective as the plant will take the chemical down with winter reserves to its vigorous roots.
Spotted Knapweed
Centaurea stroebe

Spotted knapweed is a biennial or short-lived perennial. Growing up to three feet tall, it is a multi-stemmed plant, with the majority of its slender stems branching from the upper half of the plant. In its first year, it forms a basal rosette of leaves; in subsequent years it produces flowers. The rosette leaves are roughly 8 inches long, are borne on short stalks, and are lobed once or twice on each side. Flowering stems are hairy and bear tubular, pink to purple (or rarely, cream colored) flowers. The bracts of the flower heads are
fringed at the margins and black tipped, hence the name “spotted.”

            • Urn-shaped flower heads
            • Brown fringed bracts below the flower heads


Spotted Knapweed is most easily found during the months it is in bloom (July through August).


Pastureland and open ground or disturbed sites like roadways.


Several herbicides are effective on spotted knapweed. Usually digging will not control this plant because it has a stout taproot; deep tillage may have a limited effect.
St. John’s Wart
Hypericum perforatum L.

St. John’s Wart is a perennial growing from 1 to 3 feet tall. Flowers are ¾ of an inch wide and yellow with 5 petals, with each petal having tiny black dots on the margins. The flowers occur in clusters at the ends of branches with 50 to 100 flowers per cluster. Stems are single to multiple and reddish. They are also smooth and slightly 2-edged with a woody base. Leaves are opposite elliptic to oblong and have oil glands covering the surface. The fruit is a 3-sectioned pod that contains numerous dark brown seeds. One plant can produce up to 100,000 seeds per plant that are viable for up to ten years.
What to look for:

Yellow 5-petaled flowers, and the distinctive oil glands covering the leaves which look like tiny holes when held up to the light. When to find St. John’s Wart:
When in bloom June to September
Where to find St. John’s Wart:

Poor, sandy, dry soils with full sun. It can be in wastelands & disturbed areas or healthy range.

. John’s Wart can be pulled and left on site if flowers aren’t present or removed from the site if they are. Herbicides can be effective and there
are numerous combinations that may be used.
Cardaria draba

Whitetop is a long-lived rhizomatous perennial with two distinct leaf types. Its basal leaves are blue-green and lance shaped; the upper leaves
have two lobes clasping the stem. Leaves are covered with fine white hairs that give the plant a grayish green appearance. White flowers are
born in clusters at the top of slender stalks that grow from 10 to 24 inches tall. Individual flowers bare less than ¼ inch wide. They have four petals
and six stamens. The flat-topped flower clusters may be several inches wide. Hoary cress has a ½ninch long heart-shaped seedpod, which is reddish
brown and tipped with a small beak.
What to look for:
            • white flowers with four petals, giving plant a white, flat-topped appearance
            • leaves covered with fine white hair, giving a grayish green appearance
When to find whitetop

Early May when flowering.
Where to find whitetop:

You can find whitetop along roadsides, ditch banks, and in many other unshaded, disturbed areas.

Because of the aggressive nature of whitetop chemical treatment is preferred. The most common chemical used by the public are Cimmaron Plus and Cimmaron Max. Spraying can be done at full bloom. Please contact your local chemical dealer for details.

Perennial Pepperweed

Lepidium latifolium





Perennial pepperweed is a long lived perennial which thrives in seasonally wet areas that have a high water table. It reproduces from perennial roots or seed. In early spring, new shoots emerge from root buds forming low growing rosettes. Plants are multiple stemmed and grow in stiffly erect masses up to 5 feet tall. The roots form a woody crown and are creeping forming new plants from root sections as small as two inches. Basal leaves are up to 1 foot long and 3 inches wide, while stem leaves are smaller, 3 to 10 inches in length and tapered at the base, entire to weakly serrate. Abundant small white 4-petaled flowers are borne in dense clusters near the stem tips. The fruits are small, flattened pods, each containing two seeds. These pods will irregularly drop throughout the winter.


Perennial pepperweed has been used as an ornamental in landscapes so it’s important to not use it in landscaping. If found in your home landscape pulling can be done taking care to remove as much of the root as possible and treating with herbicide.



Perennial pepperweed rarely produces seedlings, reproducing mainly through the vegetative roots. Therefore, seed viability is not an issue. Short of drowning out control of this plant is difficult and takes a myriad of techniques. Spring grazing or mowing followed by an application of herbicide at the flower bud stage can be effective. If populations are small herbicide alone at the bud stage are also effective. In wetland areas mowing may be used followed by the appropriate herbicide. Mechanical treatment alone is not recommended due to its ability to resprout from the crown or small root sections.
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