List of Key Exotic Plant Species in Oregon

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Download the List as a zipped Excel Spreadsheet file by clicking here.

Click on the scientific name of the exotic species in the table below to see the explanation for the ranking of that species. Invasiveness Ranking Forms for key invasive plants were used for analysis in ODFW's Comprehensive Wildlife Conservation Plan (CWCP) This data was created in 2005.

Scientific Name Common Name I-Rank
Aegilops ovata Ovate goatgrass N/A
Aegilops truncialis Barbed goatgrass High/Medium
Alhagi pseudalhagi Camelthorn N/A
Ambrosia tomentosa Skeletonleaf bursage N/A
Carduus acanthoides Plumeless thistle Medium/Insignificant
Carthamus baeticus Smooth distaff thistle N/A
Carthamus lanatus Woolly distaff thistle Low/Insignificant
Centaurea calcitrapa Purple starthistle Medium/Low
Centaurea iberica Iberian starthistle Low
Centaurea maculosa Spotted knapweed High/Medium
Centaurea solstitialis Yellow starthistle High
Centaurea virgata Squarrose knapweed Medium/Insignificant
Chondrilla juncea Rush skeletonweed Medium
Cyperus rotundus Purple nutsedge N/A
Cytisus striatus Portuguese broom Low
Euphorbia esula Leafy spurge High
Helianthus ciliaris Texas blueweed N/A
Heracleum mantegazzianum Giant hogweed Medium/Low
Hieracium aurantiacum Orange hawkweed Medium/Insignificant
Hieracium floribundum Yellow hawkweed N/A
Hieracium pilosella Mouse ear hawkweed Medium/Low
Hieracium piloselloides King devil hawkweed N/A
Hieracium pratense Meadow knapweed Medium/Insignificant
Nardus stricta Matgrass Medium/Insignificant
Peganum harmala African rue Insignificant
Pueraria lobata Kudzu Low
Senecio jacobaea Tansy ragwort Medium/Low
Solanum elaegnifolium Silverleaf nightshade Low/Insignificant
Tussilago farara Coltsfoot N/A
Ulex europaeus Gorse High/Medium
Zygophyllum fabago Syrian bean caper N/A

I-Rank Definitions

High -- Species represent a severe threat to native species and ecological communities.

High/Medium -- Species represents a severe to moderate threat to native species and ecological communities.

Medium -- Species represents moderate threat to native species and ecological communities.

Medium/Low -- Species represents a moderate to relatively low threat to native species and ecological communities.

Medium/Insignificant -- Species represents moderate threat to insignificant threat to native species and ecological communities.

Low -- Species represents a significant but relatively low threat to native species and ecological communities.

Low/Insignificant -- Species represents a relatively low to insignificant threat to native species and ecological communities.

Insignificant -- Species represents an insignificant threat to native species and ecological communities.

N/A -- Not yet documented from Oregon.

Rank Reasons

Aegilops ovata - Not yet documented from Oregon

Aegilops truncialis - One of the few weeds capable of invading serpentine habitat, and the only report from the state is in the region with serpentine soils. If indeed this occurrence was eradicated then the state I-rank would be on the lower end or even lowered all together.

Alhagi pseudalhagi - Not yet documented from Oregon

Ambrosia tomentosa - Not yet documented from Oregon

Carduus acanthoides - Only known from one site; not considered an invasion threat to conservation areas in other states.

Carthamus baeticus - Not yet documented from Oregon

Carthamus lanatus - Primarily a weed of croplands and adjacent disturbed areas; unlikely to invade conservation areas.

Centaurea calcitrapa - Known from three disparate sites in Oregon; two have been been eradicated and one is undergoing treatment. Sites appear to be limited to pastureland or lands under agricultural improvement although the species appears to have the ability to outcompete native perennial grasses since it has invaded a grassland preserve in California. Treatment for this species is limited to hand pulling for smaller infestations or herbicide application for larger populations.

Centaurea iberica - Few sites in Oregon, limited to roadsides and fields; not known to have invaded any type of wildlands in Oregon or elsewhere in US

Centaurea maculosa - Has invaded 70% of Oregon lands with the potential of increasing to 100%. Can infest multiple types of habitat (grassland, savanna, ponderosa pine forests, Douglas fir forests, and sagebrush). Capable of producing copious amounts of seeds which stay viable in the soil for 5 years or more. Can invade wildlands--first in areas of natural or manmade disturbance then gradually increasing into non-disturbed or relatively good condition habitats.

Centaurea solstitialis - Often creates artificial drought conditions even after average precipitation years, increases erosion from the switch from perennial to annual system, and negatively impacts or eliminates microbiotic crust. It creates a more uniform density in grassland layer and displaces native plants and animals by reducing forage and habitat, as well as fragmenting plant and animal habitat. It can form impenetrable stands and threatens native bunchgrass communities and rare plants associated with them. Seedlings monopolize soil moisture and are highly competitive for soil nutrients and space. It is threatening an area, Agate Desert, that contains 2 federally listed plants and 1 listed animal. In California, it is threatening an endangered grassland species, Chlorogalum purpureum var. purpureum. It is very reproductively aggressive. An integrated management approach can control this species. It has increased 11 fold in the past 12 years and has now heavily infested the northeastern and southwestern portion of the state.

Centaurea virgata - Especially able to withstand droughty conditions; can remain in basal rosette stage until conditions are favorable; seedheads can easily stick to clothing and fur and other moving objects and disperse long distances. Can be confused with the more widespread diffuse knapweed so this weed may actually be more common than reported.

Chondrilla juncea - Generalized state range is 33%, can dominate an area where established, fairly aggressive but tends to grow in roadsides and fallow fields or other disturbed areas. Does not usually invade healthy natural areas.

Cyperus rotundus - Not yet documented from Oregon

Cytisus striatus - Only documented from two occurrences within the state but this species may be underreported since it is very similar in appearance to the more widespread Scotch broom (Cystisus scoparius). Can form dense cover which may increases fire frequency and intensity. Has invaded a variety of habitats from coastal prairies to interior oak woodlands and conifer forests. Seeds are long-lived. Plants are difficult to remove and in California, control efforts have taken as long as 6 years. If people are confusing this species with Scotch broom and there truly are more populations of this species, then the I-Rank needs to be upgraded.

Euphorbia esula - Considered to be a pest because of the speed that it invaded (and continues to invade) as well as the extreme difficulty to control and manage the infestation.

Helianthus ciliaris - Not yet documented from Oregon

Hieracium aurantiacum - Has been reported from 3 counties and 3 ecoregion. Prefers the milder and wetter regions of the state and would probably not invade the drier parts of the state. Repoductively aggressive, produces a lot of seed which can live up to 7 years, can re-grow from root fragments, and can germinate immediately upon reaching the ground. Not much specific information is available for the state populations.

Hieracium floribundum - Not yet documented from Oregon

Hieracium pilosella - Hieracium pilosella is found in grasslands, primarily disturbed sites, where it can form an exclusive monoculture. Given the highly disturbed (and sometimes cultivated) nature of these locations, impacts on native species may be low.

Hieracium piloselloides - Not yet documented from Oregon

Hieracium pratense - In Oregon, it appears to be primarily a weed of disturbed areas although other states have found it in more remote mountainous areas although it is unknown if these remote locations were spread naturally or with the help of man. The US Forest Service was using it as part of their re-seeding mix in clearcuts and maybe in other restoration areas. It is reproductively aggressive.

Nardus stricta - Slow growing but difficult to control. Grows in very dense clumps and is hard to detect until it has become established. No good control methods. Of the two sites in Oregon, one is in a state park with conservations values and near the locations for a listed federal plant species

Peganum harmala - Information on this weeds biology is scarce. Most websites refer to its pschychotropic and hallucinogenic properties. There is also a lack of information from the other states where this weed is reported--WA south to CA then eastward along the border states to TX, and also in MT. California considers it primarily a problem in degraded rangeland habitats or waste areas along roadsides. It scored as Medium/Insignificant because of this lack of information but the final rank for Oregon was downgraded to Insignificant.

Senecio jacobaea - Primarily a weed of western grasslands (highest impacts are west of the Cascades). This is a species that primarily invades very disturbed, low quality habitats such as roadsides, clearcuts, abandoned fields, pastures; primarily a rangeland pest, with significant impacts because it is toxic and can poison livestock, but with fairly low biodiversity impacts.

Solanum elaegnifolium - Questionable whether this weed is established in Oregon. If it is, then probably limited to croplands, pastures, and roadsides.

Tussilago farara - Not yet documented from Oregon

Ulex europaeus - An aggressive plant that can outcompete native species, can produce copius amounts of seed which can survive in the soil for a long time (up to 30 years), and can form dense almost impenetrable stands. Its growth habit and production of oils allows for intense fires. Once established it is almost impossible to eradicate.

Zygophyllum fabago - Not yet documented from Oregon

Photo of sagebrush steppe in Eastern Oregon
© Ellen Morris Bishop

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