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A noxious weed is a non-native plant that is generally highly aggressive, and which can cause problems for natural areas, agriculture, and/or recreation. "Noxious" is a legal designation, and not simply a matter of personal annoyance. In the US, noxious weeds are usually regulated on a State and County basis, depending on how serious the threat is to a particular area and how widespread the weed is already. If a weed is under quarantine, it is illegal to propagate, transport, or offer to buy or sell any part of the plant. One sometimes needs a permit even to kill or remove the weed, because for some species, such as purple loosestrife, careless removal can spread seeds or other plant parts further, causing more of a problem than if they were left alone.
There are several important things to notice about noxious weed lists other than the particular species on them:
Many species that are known to be harmful to natural areas are not on the list! Decisions about whether to classify a non-native plant as noxious tend to be made more on the basis of harm to agriculture or recreation than of harm to the environment. So just because you don't see a plant on this list, doesn't necessarily mean that it cannot escape and cause harm in your nearby woods and meadows.
Beautiful is as beautiful does. About 80% of noxious weeds across the country are horticultural escapees. Purple loosestrife, ivy, oxeye daisy, Queen Anne's lace, Scots broom, and many others are quite beautiful to look at. But they all form such dense monocultures that no other plant can grow through them; they crowd out the native plants and leave nothing for most native animals to eat. Scots broom can alter the soil enough that even after it's removed, it is difficult to get the original natives to live there again for decades. So please don't make personal exceptions 'just this once, because it's so pretty'.
Non-native plants that are favored by wildlife can be more aggressive and harmful than non-natives that aren't favored by wildlife. For example, English holly and English ivy, which are beloved by many birds, spread not only rapidly but quite widely in our area, and cannot be contained to your yard. So be careful when following the planting suggestions in many wildlife gardening books-- don't choose plants that are or could be a problem where you live, no matter how much the birds may enjoy it. A good source of advice is your local Native Plant Society, whose members spend enough time surveying wild areas to know which non-native plants are becoming a problem, even if they aren't on any official lists yet.
In Washington state, noxious weeds are put into three classes, A, B or C, depending on how widespread they already are. The classes are not necessarily based on the severity of threat they pose, so it's possible for some Class C weeds to be more harmful to habitat than some Class A weeds.
Class A weeds are not yet significantly present in the state, and are fully under state control to prevent their introduction or spread.
Class B weeds are already present in some areas. Where they are not yet present, state laws apply. Where they are already present, control decisions are at a local level (usually county?)
Class C weeds are already present in most areas of the state, and control is always at the local level.
[Benton] [Chelan] [Clallam] [Clark] [Cowlitz] [Grays Habor] [Island] [Jefferson] [King] [Kitsap] [Kittitas] [Klickitat] [Lewis] [Mason] [Okanogan] [Pacific] [Pierce] [San Juan] [Skagit] [Skamania] [Snohomish] [Thurston] [Wahkiakum] [Watcom] [Yakima]
Volunteers are needed to help fill in all of the blank Western WA county information. Links to official sites are just as useful as plant lists, because they're more likely to stay up to date. If your county is still blank, you're elected! Check with your county Cooperative Extension to see if they can send you the list, or else can refer you someone such as your Surface Water Management department who might have a list. If your county isn't listed here, either you are in Eastern WA, outside of the ecological region that I can support, or else I goofed and missed a county...
Thanks to John R. Farmer for the following list:
1996 BENTON COUNTY NOXIOUS WEED LIST Benton County Noxious Weed Control Board PO Box 311, Prosser, WA 99350 (509) 786-6988 *found in Benton County CLASS A bean-caper, Syrian (Zygophyllum fabago) blue weed, Texas (Helianthus ciliaris) buffalobur (Solanum rostratum) cordgrass, salt meadow (Spartina patens) crupina, common (Crupina vulgaris) four o'clock, wild (Mirabilis nyctaginea) hawkweed, mouseear (Hieracium pilosella) hogweed, giant (Heracleum mantegazzianum) hydrilla (Hydrilla verticulata) *johnsongrass (Sorghum halepense) knapweed, bighead (Centaurea macrocephala) knapweed, Vochin (Centaurea nigrescens) mallow, venice (Hibiscus trionum) nightshade,silverleaf (Solanum elaeagnifolium) peganum (Peganum harmala) sage, Mediterranean (Salvia aethiopis) starthistle, purple (Centaurea calcitrapa) thistle, Italian (Carduus pycnocephalus) thistle, milk (Silybum marianum) thistle, slenderflower (Carduus tenuiflorus) unicorn-plant (Proboscidea louisianica) woad, dyers (Isatis tinctoria) *velvetleaf (Abutilon theophrasti) CLASS B-DESIGNATE blackgrass (Alopecurus myosuroides) blueweed (Echium vulgare) broom, Scotch (Cytisus scoparius) bryony, white (Bryonia alba) bugloss, annual (Anchusa arvensis) bugloss, common (Anchusa officinalis) camelthorn (Alhagi maurorum) catsear, common (Hypochaeris radicata) cinquefoil, sulfur (Potentilla recta) cordgrass, smooth (Spartina alterniflora) cordgrass, common (Spartina anglica) daisy, oxeye (Chrysanthemum vulgare) deadnettle, hybrid (Lamium hybridum) elodea, Brazilian (Egeria densa) fieldcress, Austrian (Rorippa austriaca) gorse (Ulex europaeus) hawkweed, orange (Hieracium aurantiacum) hawkweed, yellow (Hieracium caespitosum) hedgeparsley (Torilis arvensis) *indigobush (Amorpha fruticosa) knapweed, meadow (Centaurea jacea x nigra) *knapweed, spotted (Centaurea biebersteinii) knapweed, brown (Centaurea jacea) knapweed, black (Centaurea nigra) lepyrodiclis (Lepyrodiclis holosteoides) loosestrife, garden (Lysimachia vulgaris) *nutsedge, yellow (Cyperus esculentus) oxtounge, hawkweed (Picric hieracioides) parrotfeather (Myriophyllum brasiliense) ragwort, tansy (Senecio jacobaea) *skeletonweed, rush (Chondrilla juncea) sowthistle, perennial (Sonchus arvensis arvensis) spurge, leafy (Euphorbia esula) *starthistle, yellow (Centaurea solstitalis) *thistle, Scotch (Onopordum acanthium) *thistle, musk (Carduus nutans) thistle, plumeless (Carduus acanthoides) *toadflax, Dalmatian (Linaria dalmatica spp. dalmatica) *watermilfoil, Eurasian (Myriophyllum spicatum) CLASS C *saltcedar (Tamarix species) EDUCATION LIST *bull thistle (Cirsium vulgare) *canada thistle (Cirsium arvense) *kochia (Kochia scoparia) *puncturevine (Tribulus terrestris) *purple loosestrife (Lythrum salicaria)
There's now an official King County Noxious Weeds site, with a current list of weeds, and pictures of many to help identify them.
Thanks to Eric Schild, email@example.com for the following list:
Here is a list of weed species classified as noxious Province-wide (others, which I haven't listed, are classified as noxious in certain areas of the Province.
Annual Sowthistle (Sonchus oleraceus) Canada Thistle (Cirsium arvense) Crupina (Crupina vulgaris) (not yet known to be present here) Dalmatian Toadflax (Linaria dalmatica) Diffuse Knapweed (Centaurea diffusa) Dodder (Cuscuta spp.) Gorse (Ulex europaeus) Hound's-tongue (Cynoglossum officinale) Jointed Goatgrass (Aegilops cylindrica) (not yet known to be present here) Leafy Spurge (Euphorbia esula) Perennial Sowthistle (Sonchus arvensis) Purple Nutsedge (Cyperus rotundus) (presence not confirmed) Rush Skeletonweed (Chondrilla juncea) Scentless Chamomile (Matricaria maritima) Spotted Knapweed (Centaurea maculosa) Tansy Ragwort (Senecio jacobaea) Velvetleaf (Abutilon theophrasti) Wild Oats (Avena fatua) Yellow Nutsedge (Cyperus esculentus) Yellow Starthistle (Centaurea solstitialis) (not yet known to be present) Yellow Toadflax (Linaria vulgaris)
Volunteers needed for all empty sections above! Links to official sites are even more useful than plant lists, because they're more likely to stay up to date.