Invasive Plants
ABOUT EXOTIC INVASIVES
Invasion by exotic species is second only to direct habitat loss in reducing and eliminating native biodiversity. Not only do exotic invasives occupy space needed by native plants, but when native plant species are locally replaced by exotics, the native invertebrates, fungi or other species that specifically may depend on them also may disappear. This disappearance is termed "extirpation," and it can further impact species "higher" on the food chain. "Alien, introduced and non-native" all are considered synonyms of "exotic" within this document.

Many of the invasives in our area are intentional introductions brought in for landscaping or other uses. These species escape from cultivation and may become dominant, causing immense damage to natural biotic communities. As a result, they are a contributing cause of many local extinctions of native plants and the animals (both vertebrates and invertebrates) that are dependent on them. Invasive species also are a contributing factor in native species listings under the state and federal Endangered Species Acts. Additionally, many choke waterways, degrade recreational and agricultural areas, lower land values, and some are poisonous to people and/or livestock.

Typically, a small subset of exotic species is considered to be invasive, and although some of them are widely known to be already, others may become so in the future. Predicting future invasiveness is often not possible; especially considering invasiveness of any species may change over time. Useful clues as to whether or not a plant species may become invasive here include whether it is a known invasive elsewhere (particularly in similar climates), if it has close relatives that are invasive, and how it is reproducing here (e.g., producing fertile seed, producing progeny in landscaped areas, etc.). Aspects that are more difficult to predict are if a plant will mutate genetically in a way that increases survival in our present climate, and/or if a plant will change behavior as the climate changes.


ABOUT THE INVASIVE ORNAMENTALS LIST
The purpose of this list is to identify those exotic plants introduced deliberately for landscaping or gardening use which escape and become problematic in wildland habitats. They are spread primarily by birds (which eat the seeds and deposit them elsewhere), wind or water, and after establishment, they often continue to spread vegetatively as well as by producing more seed. The primary list does not include plants that are not introduced deliberately for landscaping or gardening such as agricultural crops, crop weeds or the wide array of accidentally introduced lawn and pasture weeds such as dandelion and wild carrot. Although they may cause large impacts to native habitats, they are not ranked here. A partial list of those species is included as a supplement at the end.

The plants on this list have been ranked in three categories based on the collective assessment of professionals in botany, native nursery/horticulture, public natural area management, and related fields who have field familiarity with the vegetation of the southern Willamette Valley. This list is not a result of a specific academic study or search of published literature applicable to other areas, although some reviewers here utilize published research as supplemental information. It is based on collective, on-the-ground field observation in wildland areas of the southern Willamette Valley.

The complete list document (available in HTML format below) consists of two primary lists (High and medium impact species and Suspected/watch species) and two supplemental lists (Outplanted flowers and Common weeds). A printable PDF file is also available.


ABOUT PLANT USE
Because many of these plants are traditionally sold by nurseries, the NPSO's Emerald Chapter strongly urges gardeners, landscapers and landscape architects to start new traditions: avoid buying these species and inform nurseries of their invasive qualities and negative impacts on our native biodiversity. Use of these plants, and their subsequent escape and need for control, is costly, and the costs often are not seen where they are planted initially. When invasive plants escape to neighboring private property and public sites, not only may native plants - and the species that depend on them - be lost to invasive exotics, but extensive volunteer or paid public staff time may be invested and use of herbicides may be needed to control spread. All these impacts may be unknown to the owner of the source plants - where an invasive tendency may not be evident. And unfortunately, many of these species simply are impossible to control once they escape. Some of these species are listed noxious weeds by the Oregon Department of Agriculture and are prohibited to sell .

We hope that gardeners and landscape professionals will honor the integrity of our remaining natural habitats above tradition and desire for a wider variety and use locally native species wherever possible. When exotic species must be used, we ask that species on this list, and closely related species, be avoided. There are numerous native species and benign ornamental species which can be used as alternatives to nearly every plant on this list. Better still, use only locally native and propagated species in your landscaping if possible. The native plant gardening and landscaping plant booklets produced by the Emerald Chapter NPSO are very helpful in this regard.

We urge you to copy and distribute this list freely, especially to nurseries, horticulturists, permaculturists, gardeners, landscape contractors, and landscape architects and designers.


ABOUT THE RANKINGS
The following three categories comprise the simple ranking assigned to each plant on the master table.

R = Rank:

  • H - These species repeatedly have been observed to be very invasive locally (or nearby) in native, wildland areas. They often form near-monocultures, becoming the only dominant member of a plant community. They often severely modify native habitats, likely causing local plant extirpations (and their dependent invertebrate and/or fungi extirpations) and significantly alter ecological functions and processes (such as disrupting invertebrate life cycles or arresting plant succession). Examples of high impact species are English and Irish Ivy and Reed Canarygrass, which grow vigorously and regularly form large near-monocultures.
  • M - These species are moderately invasive but may not disperse widely from a source (e.g., planting or dumping/disposal site). May form small near-monocultures, or be one of two or more dominant members of a plant community. They moderately impact native habitats, likely not causing native plant or invertebrate extirpations. They may be in the initial lag period that some exotic plants go through before they become high impact species. Examples of moderate impact species are English Holly and Sweetbriar Rose.
  • S - May be invasive but local observations are limited. May be combined on the list with either of the above abbreviations if a plant is known to be M or H in similar regions, but is not yet well-documented here. Or, if a plant may be a low impact species at present, but local experts suspect it could become M or H impact based on research, it may be combined with M or H.

Rank

Latin name

Common name

Notes

HIGH AND MEDIUM IMPACT SPECIES

MH

Acer platanoides

Norway Maple

Escaping in the Portland area.  Documented in Eugene at Skinner Butte Park (in large populations), Alton Baker Park and Hendricks Park.

M

Aesculus hippocastanum

Horsechestnut

Present in several places in south portion of Hendricks Park, known to be a problem in other cities.

M

Ailanthus altissima

Tree-Of-Heaven

Very invasive, especially in more disturbed sites.  Coming into prairie on west side of Skinner Butte, spreading rapidly around Beltline/I-5 area.

SH

Alliaria petiolata

Garlic Mustard

One of the most invasive forest understory plants in the East and Midwest; now established in the Portland area and up the Columbia Gorge.  A Working Group has formed to address this species in the Willamette Valley.

H

Brachypodium sylvaticum

False Brome

Highly invasive grass expanding very rapidly in forests and along rivers in our area in numerous places.  It may drastically change forest understories.  In the northwest Corvallis area it also invades upland and wetland prairies (threatening rare, native species).  A Working Group has formed to address this species in western Oregon.

H

Buddleja davidii

Butterfly Bush

Invading riparian zones and other areas in western Oregon and Washington.  Can outcompete native willows which are essential host plants for native butterflies. Called B. variabilis in some sources.  Deadheading is infeasible because of the long flowering and fruiting time.

H

Clematis vitalba

Traveler’s-Joy

Vigorous climber in Portland and Seattle, also in our area.  Somewhat difficult to distinguish between this and the native clematis (C. ligusticifolia).

M

Cotoneaster spp.

Cotoneasters

Escaping in native prairies, savannas and woodland edges.   (Cotoneaster lacteus, C. franchetti, C. horizontalis, C. parneyi, etc. Best to be very cautious with any red-fruited ornamentals that appeal to birds.)

H

Crataegus monogyna        

English Hawthorn

Dominates wetland and upland prairies, savannas, and understories in woodland and forest areas.  A very serious problem in the Portland area, Corvallis, Umpqua Valley, and on the increase here. 

H

Cytisus scoparius

Scot’s Broom

Serious problem in prairies and savannas, along the Willamette, at the coast, etc.  Watch also for a closely-related invasive species, Cytisus striatus (Portuguese Broom).

H

Daphne laureola

Spurge Laurel

An increasing problem in woodlands around Eugene, Corvallis, McMinnville (dominating a 52 ac. hillside there, according to CWMA coordinator), Portland, Seattle, Vancouver B.C.

M

Digitalis purpurea

Foxglove

Widely escaped, especially in foothills of Cascades and Coast Range.  Forms large stands, particularly in clearcuts and along roadsides. Also in mesic meadows.  Likely introduced originally for medicinal purposes, but now also planted for aesthetic reasons.

H

Egeria densa

Brazilian Waterweed

Established and dominant in Coyote Creek north of Fern Ridge Reservoir, Lake Creek below Triangle Lake, and several other Willamette Valley and nearby coastal sites.  A very aggressive aquatic invader.

H

Fallopia ×bohemica

F. japonica

F. sachalinensis

Bohemian Knotweed

Japanese Knotweed

Giant Knotweed

All formerly classified in the genus Polygonum.  Form monocultures in riparian or other moist habitats.  Bohemian most frequent in Lane County.  Occurs along McKenzie, Willamette, and Tenmile Creek, and at Sweet Creek Falls trailhead.  See the related Persicaria wallachii (Himalayan knotweed), also.  A Working Group has formed to address these species in the Willamette Valley.

MH

Foeniculum vulgare

Fennel

S. side Skinner Butte and north side Willamette just upstream from Valley River Center shopping center (both Eugene), Hwy. 58 roadside at Lookout Point Reservoir, I-5 shoulders (Eugene), etc.  Occasional in other places locally, but expanding rapidly.   Extensive in coastal southern Oregon and northern California.

M

Genista monspessulana

French Broom

Spreading rapidly by seed in Coburg Hills, invading meadows with rare species.  Serious problem in CA and OR south coast; now appearing in Eugene.

H

Geranium lucidum

Shining Crane’s-Bill

In the last 10 years has become a dominant in forest and oak woodland understories in the central and southern Willamette Valley.  Especially thrives in riparian areas, and other shady sites.

H

Geranium robertianum

Herb Robert (Stinky Bob)

Dominates forest understories all over the greater Portland area and Columbia Gorge, and is now becoming regular at the Coast, in Eugene and Corvallis.  Up the McKenzie River at least as far as lower delta of Horse Creek.

H

Hedera helix

H. hibernica

English Ivy

Irish Ivy

Completely overruns forest understories and riparian areas.  An immense and expensive problem.  H. hibernica may be more common as an escapee in the Eugene area than H. helix.  Additional species may be present in limited areas.

H

Heracleum mantegazzianum

Giant Hogweed

ODA has worked hard to control two populations of this in Lane County.  It likely will show up again.

M

Hypericum perforatum

St. John’s Wort

Planted for medicinal use, but has spread widely into meadows and roadsides in the Valley and Cascades.

MH

Ilex aquifolium

English Holly

Appears regularly in forest understories. Spread by birds.

M

Impatiens glandulifera

Policeman’s Helmet

Four locations in Wolf Creek Drainage.   Also in Alsea drainage, and near Portland.

H

Iris pseudacorus

Yellow Flag Iris

Forms monocultures in wetlands.  Has established itself in Bertelsen Slough, Amazon Creek, other west Eugene wetland areas, and now is regular along the Willamette River.  Also, Kelly Creek in upper Siuslaw watershed, Leaburg Lake (reservoir) on the McKenzie River, Portland and Columbia River habitats

M

Juniperus virginiana

Eastern Redcedar

Birds eat berries and spread seeds.  Occurs in Willow Creek Preserve (TNC) and other wetlands in west Eugene.

H

Leucanthemum vulgare

Oxeye Daisy

Widely escaped in upland prairies, along roadsides.  Formerly Chrysanthemum leucanthemum.  Watch also for Chrysanthemum maximum (Shasta daisy) which occasionally appears as an escapee.

M

Lamiastrum galeobdolon

Yellow Archangel

Primarily moved by humans.  Very aggressive, primarily moving out from landscaped areas.  Escaped in Eugene, Springfield, Corvallis, and Seattle (where it “covers hillsides,” according to a botanist there).

M

Lathyrus latifolius

Sweet, Perennial Or Everlasting Pea

Well established, primarily along roadsides and hedgerows.  Now moving higher into the Cascades.

M

Ligustrum vulgare

Common Privet

Naturalizing on slopes of Skinner Butte in partial shade to shade.  Occasional elsewhere in the greater Eugene area.

M

Linaria genistifolia ssp. dalmatica

 L.  vulgaris

Dalmatian Toadflax

Butter And Eggs

Both move in along roadsides, then invade native meadows in the Cascades.  A few clumps are present along BPA corridor in S. Eugene, and in Kirk Park at Fern Ridge.

H

Lotus corniculatus

L. uliginosus

Birdsfoot Trefoil

Greater Birdsfoot Trefoil

L. corniculatus (and possibly L. uliginosus) sold in pasture and rough lawn mixes.  Invade and dominate wet and moist prairies.

M-H

Ludwigia hexapetala

L. peploides

Water Primrose; Floating Primrose Willow; others

Pests in both flowing and ponded water.  One or both are known from Smith and Bybee Lakes and along the lower Columbia Slough, the Corvallis area, and in the Eugene area in Delta Ponds, Spring Creek, Golden Gardens Ponds and along the lower Amazon.

M

Lunaria annua

Honesty; Money Plant

Somewhat invasive in forest understories; widespread.

M

Lychnis coronaria

Rose Campion

Becoming established in lower Horse Creek watershed of upper McKenzie River (east Lane County) and along Middle Fork Willamette, where it can dominate open areas and stream banks

H

Lythrum salicaria

Purple Loosestrife

Forms near-monocultures in wetlands.  Has been found in along Amazon Creek and the Willamette River.  Immense problem across the continent.

M

Lysimachia nummularia

Moneywort

Regular dominant of riparian wetlands in our area, both in sun and shade.

H

Mentha pulegium

Pennyroyal

Widespread in emergent wetlands in west Eugene Wetlands and elsewhere.  Difficult to control.

M

Melissa officinalis

Lemon Balm

Widespread weed in native prairies and openings in woods.

H

Myriophyllum spp.

Parrot’s Feather, etc.

Includes water-milfoils.  Myriophyllum aquaticum (M. brasiliense; parrot’s feather) is the most common, and M. spicatum (Eurasian milfoil) also is impacting aquatic habitat.

M

Nasturtium officinale

Watercress

Chokes out small waterways on the valley floor.   Up the McKenzie as far as lower Horse Creek. ( = N. aquaticum; Rorippa nasturtium-aquaticum)

H

Nymphoides peltata

Yellow Floating Heart

Known from two private ponds: one in SE Springfield, one in Marcola.  Recent addition to ODA list.

H

Pentaglottis sempervirens

Evergreen Bugloss

Large escaped population on Lorane Highway along Spencer Creek; small populations in NW Lane County, Cottage Grove,  Masonic Cemetery (Eugene); likely it is frequently misidentified as Anchusa.  Widespread escapee in NW Oregon counties.

H

Persicaria wallachii

Himalayan Knotweed

Documented as an escapee below Blue River Dam, and more common on the N. Coast.  One of the “giant” knotweeds (see Fallopia also; was Polygonum polystachum).

H

Phalaris aquatica

Harding Grass

P. aquatica infests drier areas than reed canarygrass, and is newer to the area.  It is expanding very rapidly in west Eugene near and other areas in the Willamette Valley.  Seed for sale on the internet 1/2008.

H

Phalaris arundinacea

Reed Canarygrass

Phalaris arundinacea is a widespread problem in wetlands and riparian areas, forming huge monocultures.  Control is very difficult, primarily because of water-side locations.  A variegated form of this grass is forming large monocultures along the Metolius River (Jefferson Co.) and along Tenmile Creek estuary (Lane Co.) at the Coast.  That form also has been seen in the Bull Run Watershed east of Portland, but it is not yet documented in our area.  Seed for sale on the internet, 1/2008.

---

Polygonum spp.

Giant Knotweeds

See Fallopia.

SH

Potentilla recta

Sulphur Cinquefoil

Extremely invasive in far eastern OR and eastward.  Just beginning to show up here; was being sold accidentally in a native plant outlet in our area, so likely is established here.  Not easy to separate from the native Potentilla gracilis.

H

Prunus avium

Sweet Cherry

Shades out forest understories.  A widespread problem.

MH

Prunus laurocerasus

P. lusitanica

English Laurel

Portugal Laurel

Appear regularly in forest understories, sometimes common.

M

Prunus cerasifera

P. domestica

P. spinosa

Thundercloud Plum

Domestic Cherry

Sloe

Grafted species and rootstocks that sucker and flower, produce fruit spread easily by birds & raccoons.  Not as invasive as P. avium.

SH

Pueraria montana var. lobata

Kudzu

This vine has a reputation in the South of being the worst (or nearly so) invasive plant to ever escape there.  It is valued for medicinal properties.  It recently has shown up twice in the Willamette Valley.  Formerly known as P. lobata.

M

Pyracantha spp.

Firethorn

Birds eat fruits and spread plants into prairies.  P. angustifolia, P. coccinea, et al.

MH

Pyrus communis

P. calleryana

Domestic Pear

Callery Pear

Both become thorny as they revert to non-horticultural forms.  Callery includes the widely used “Bradford” and “Autumn Blaze” cultivars, as well as others.  Sterile when alone, but fertile when different introductions cross-pollinate.

H

Ranunculus ficaria

Lesser Celandine

Highly invasive in Hendricks Park, Mt. Pisgah Arboretum, and more recently, upper Amazon Creek and Tugman Creek (and many other areas in the Willamette Valley).  Very difficult to control.  A look-alike, Caltha palustris (yellow marshmarigold), does not seem to be invasive in the southern Willamette Valley area.

H

Ranunculus repens

Creeping Buttercup

Highly invasive and widespread, especially on moist riparian terraces.  It forms large monocultures, especially in moist areas.

MH

Robinia pseudoacacia

Black Locust

Widely escaped east of the Cascades, beginning to naturalize on West Side: Portland area; Benton County; Lane County, Highway 126 east and west of Eugene, lower Horse Creek Delta (McKenzie Bridge).  Has been seen in significant numbers in west Cascades timber sale areas.  Can form woodland monocultures.

M

Rosa eglanteria

Sweetbriar Rose

Invades native prairies: mostly upland, occasionally wetland.

MH

Rosa multiflora

Multiflowered Rose

Nationwide problem escapee.  Oregon Department of Transportation has planted it widely.  A major problem in West Eugene Wetlands, Fern Ridge Wildlife Area and E.E. Wilson Wildlife Area in Benton County.

H

Rubus armeniacus

R. vestitus

Armenian Blackberry

European Blackberry

Very invasive and widespread.  Expensive control measures are being widely implemented.  Identification of R. vestitus likely is lumped under R. armeniacus.  (Note:  Himalaya Blackberry, R. discolor, has not been documented here.)

M

Rubus laciniatus

Evergreen Blackberry

Not as invasive as R. armeniacus, but forms dense clumps.

M

Saponaria officinalis

Bouncing Bet

Forming dense patches along Willamette River at Elijah Bristow State Park.

M

Securigera varia

Crown Vetch

Known at sites near Hills Cr. Reservoir and Blue River Reservoir; also escaped around Corvallis. Formerly known as Coronilla varia.

M

Solanum dulcamara

Bittersweet Nightshade

Widespread in many wetlands and riparian area.

H

Ulex europaeus

Gorse

Still occasionally planted.  Very serious infestations on central and south coast, now appearing in numerous sites in Willamette and Umpqua valleys.

M

Verbascum blattaria

Mullein

Colonized sandy/gravelly river bars and banks.  Especially problematic east of the Cascades, but increasing in our area.