Houndstongue
This plant will produce only a rosette during the first year, so control during that time is critical. In its second year of growth this plant will produce many seeds which stick to animals, people, and vehicles. Houndstongue contains tumorigenic pyrrolizidine alkaloids which are toxic and can kill livestock.
Canada Thistle
Canada thistle is found in every county in Montana. It can reach heights of up to four feet and has extensive root systems, which make it very difficult to control. The flowers resemble those of spotted knapweed. Both female and male flowers must be present for the plant to go to seed. One plant can produce over 3,000 seeds annually, but it spreads primarily from the roots.
Leafy Spurge
Leafy spurge is a common noxious weed that is very difficult to control because of its deep root structure. Hand-pulling this weed releases a toxic, milky substance, which can cause skin irritation. It can even cause blindness. Each plant can produce 30 to 130 seeds, and most of those seeds will germinate within 2 years. The plants can reproduce by root, as well.
Spotted Knapweed
This weed has allelochemicals or allelopathies that act as a natural herbicide to inhibit other plant species from growing within their vicinity. Spotted knapweed is a highly adaptable plant. It can be found nearly everywhere in Montana.
Here is a list of the weeds which are classified as noxious by the state of Montana:
Canada thistle (Cirsium arvense)
Field bindweed (Convolvulus arvensis)
Whitetop or hoary cress (Cardaria draba)
Leafy spurge (Euphorbia esula)
Russian Knapweed (Centaurea repens)
Spotted knapweed (Centaurea maculosa)
Diffuse knapweed (Centaurea diffusa)
Dalmatian toadflax (Linaria dalmatica)
St. Johnswort (Hypericum perforatum)
Sulfur (Erect) cinquefoil (Potentilla recta)
Common tansy (Tanacetum vulgare)
Ox-eye daisy (Chrysanthemum leucanthemum L.)
Houndstongue (Cynoglossum officinale L.)
Yellow toadflax (Linaria vulgaris)
Hoary alyssum (Berteroa incana)
Rush skeletonweed (Chondrilla juncea)
Purple loosestrife or Lythrum (Lythrum salicaria, L. virgatum)
Tansy ragwort (Senecio jacobea L.)
Meadow Hawkweed Complex (Hieracium pratense, H. floribundum, H. piloselloides)
Orange hawkweed (Hieracium aurantiacum L.)
Tall buttercup (Ranunculus acris L.)
Tamarisk [Saltcedar] (Tamarix spp.)
Perennial pepperweed (Lepidium latifolium)
Blueweed (Echium vulgare)
Yellow flag iris (Iris pseudacoru)
Yellow starthistle (Centaurea solstitialis)
Common crupina (Crupina vulgaris)
Eurasian watermilfoil (Myriophyllum spicatum)
Dyers woad (Isatis tinctoria)
Knotweed (Japanese knotweed, Polygonum cuspidatum; giant knotweed P. sachaliness; and Bohemian knotweed, P. bohemica)
Flowering rush (Butomus umbellatus)
Scotch broom (Cytisus scoparius)