Noxious Weeds are non-native plants that have become established or that may be introduced in the state.  These Noxious Weeds may render land unfit for agriculture, forestry, livestock, wildlife, or other beneficial uses.  These plants are designated as “Noxious” by rule of the Montana Department of Agriculture or as a District Noxious Weed by the Yellowstone County Weed Board.

Weed Management Criteria – The following criteria apply to management of noxious weeds in Yellowstone County.  Noxious Weeds in Yellowstone County are those declared “noxious” by the Weed Board and are prioritized accordingly.  These weeds will be managed to achieve two effects:  1.)  limit and prevent the spread of noxious weeds from existing infestations to uninfested sites and 2.)  to eventually reduce the overall size of the existing infestations.  The following weeds exist in the county and have been declared “noxious” in Yellowstone County by the Yellowstone County Weed Board:

Canada Thistle (Cirsium arvense)  
Leafy Spurge (Euphorbia esula)
Dalmation Toadflax (Linaria dalmatica) 
Field Bindweed (Convolvulus arvensis)
Whitetop or Hoary Cress (Cardaria draba) 
Sulfur Cinquefoil (Potentilla recta)
Russian Knapweed (Centaurea repens) 
Spotted Knapweed (Centaurea maculosa)
Diffuse Knapweed (Centaurea diffusa) 
St. Johnswort (Hypericum perforatum)
Common Tansy (Tanacetum vulgare) 
Houndstongue (Cynoglossum officinale L.)
Oxeye Daisy (Chrysanthemum leucanthemum L.)

Category 1 noxious weeds are weeds that are currently established and generally widespread in many counties of the state.  Management criteria includes awareness and education, containment and suppression of existin infestations and prevention of new infestation.  These weeds are capable of rapid spread and render land unfit or greatly limit beneficial uses.

Dyers Woad (Isatis tinctoria)  
Tall Buttercup (Ranunculus acris L.) 
Tansy Ragwort (Senecio jacobea L.) 
Salt Cedar (Tamarix ramosissima Ledeb.)
Orange Hawkweed (Hieracium aurantiacum L.)
Meadow Hawkweed Complex (Hieracium pratense, H. floribundum, H. piloselloides)
Purple Loosestrife or Lythrum (Lythrum salicaria, L. virgatum) and any hybrid crosses

Category 2 noxious weeds have recently been introduced into the state or are rapidly spreading from their current infestation sites.  These weeds are capable of rapid spread and invasion of lands, rendering lands unfit for beneficial uses.  Management criteria includes awareness and education, monitoring and containment of known infestations and eradication where possible.

Yellow Starthistle (Centaurea solstitialis) 
Common Crupina (Crupina vulgaris)
Rush Skeletonweed (Chondrilla juncea)

Category 3 noxious weeds have not been detected in the state or may be found only in small, scattered, localized infestatjions.  Management criteria includes awareness and education, early detectijon and immediate action to eradicate infestations.  These weeds are known pest in nearby states and are capable of rapid spread and render land unfit for beneficial uses.

Poison Hemlock (Conium maculatum L.) 
Western Water Hemlock (Cicuta douglasii)
Puncturevine (Tribulus terrestris L.)  
Common Teasel (Dipsacus fullonum L.)
Common Mullein (Verbascum thapsus L.)
Blue Mustard (Chorispora tenella)
Showy Milkweed (Asclepias speciosa Torr.) 

In addition to the State Declared Noxious Weed list, each county weed district can declare additional non-native plants to be noxious within the county.  The above weeds are declared noxious by the Weed Board in Yellowstone County.