Smooth distaff thistle (Carthamus baeticus)
Smooth distaff thistle (Carthamus baeticus)
Smooth distaff thistle (Carthamus baeticus)

Description
Winter annual, flowers July to August. Grows up to four feet tall. Very similar to Woolly distaff thistle, except that it has a smaller pappus, is less hairy, and the seedling leaves are more deeply lobed.

Impacts
Plants are highly competitive with cereal crops and desirable rangeland species. Because of their spiny nature, distaff thistles can injure the eyes and mouths of livestock forced to graze within dense populations of the weeds...

South American waterweed (Elodea/Egeria densa)
South American waterweed (Elodea/Egeria densa)

Other common names
Brazillian elodea, Brazilian waterweed, Leafy elodea, Dense waterweed

Description
Perennial, aquatic herb that grows underwater. Leaves are one to three centimeters long, up to five millimeters broad, and in whorls of four to eight. Leaves are linear, and short internodes give it a very leafy appearance. Leaves and stems are generally bright green. Stems are erect, simple or branched, and grow until they reach the...

Spanish broom (Spartium junceum)
Spanish broom (Spartium junceum)
Spanish broom (Spartium junceum)

Description
Perennial, blooms April to June. Grows 3 to 10 feet tall. Similar to Scotch broom, except stems are thicker and rougher; it has very few leaves; and flowers are larger and fewer in number.

Impacts
The least abundant of weedy brooms in Oregon, Spanish broom is adapted to dryer sites where its lack of leaves and thick waxy stems resist dessication. Like other broom species, it offers strong competition to other plants and...

Spanish heath (Erica lusitanica)
Spanish heath (Erica lusitanica)
Spanish heath (Erica lusitanica)

Other common names
Portuguese heath, Spanish heather

Description
A woody, upright, perennial, evergreen shrub, growing up to 10 feet tall. Leaves are light-green, needle-like, 3 to 7 mm long, and arranged around the stem in groups (whorls) of three to four. Blooms are a showy mass of small, white to pink, bell-shaped (tubular) flowers, with three bracts, four sepals and a corolla. Plants begin flowering in December and continue until...

Spikeweed (Hemizonia pungens)
Spikeweed (Hemizonia pungens)
Spikeweed (Hemizonia pungens)

Other common names
Common tarweed, Maritime spikeweed

Description
Annual that blooms July to September. This plant grows 1 1/2 to 3 feet tall and has basal leaves that are pale, straw-colored, stiff, several inches long, with narrow lobes. Stem leaves are 1/2 inch long or less and sharp-pointed, bearing dwarf stems in their axils. Flower heads are yellow, 1/3 inch broad, and borne at the tips of short leafy branches.

...

Spiny cocklebur (Xanthium spinosum)
Spiny cocklebur (Xanthium spinosum)
Spiny cocklebur (Xanthium spinosum)
Spiny cocklebur (Xanthium spinosum)

Description
Annual, blooms in summer. Grows 4 to 5 feet tall. Leaves are covered with short white hairs and are white-veined above, one to three inches, have two short lobes and a three-forked spine at the junction with the stem. Male flowers are uppermost and clustered. Female flowers below male flowers and form a bur that bears a beak with hooked bristles.

Impacts
Most frequently found in highly disturbed waste areas and barnyards...

Spotted knapweed (Centaurea maculosa)
Spotted knapweed (Centaurea maculosa)
Spotted knapweed (Centaurea maculosa)
Spotted knapweed (Centaurea maculosa)

Description
Biennial or short-lived perennial, blooms midsummer to fall. Grows up to 3 feet tall. Multi-stemmed plant with several stems arising from crown. Flowers are purple or rarely cream-colored. Tips of flower head bracts are usually black; thus the name "spotted." Seeds are dispersed by wind, animals, and people.

Impacts
Spotted knapweed is one of the most dominant weed species in the western United States. Millions...

Spurge laurel (Daphne laureola)
Spurge laurel (Daphne laureola)
Spurge laurel (Daphne laureola)
Spurge laurel (Daphne laureola)

Other common names
Daphne, Daphne-spurge, Daphne-laurel

Description
Attractive ornamental plant known for its spiraling evergreen leaves and greenish-yellow, bitter-fragrant flowers. Larger patches of this species emit a strong unpleasant odor. Flowering occurs in late winter to early spring, producing clusters of blue berries during the spring. The one-seeded drupes are eaten and dispersed widely by birds and small mammals. Shrubs...

Squarrose knapweed (Centaurea virgata)
Squarrose knapweed (Centaurea virgata)
Squarrose knapweed (Centaurea virgata)
Squarrose knapweed (Centaurea virgata)

Description
Perennial plant that grows 1 1/2 to 3 feet tall and is multi-branched and erect, with many small pink- to rose-colored flowers. Flowers fall readily from plant stems after flowering, and are more slender than most knapweeds, with recurved bract tips. Individual seed heads are deciduous, disloging from the main plant and aiding dispersal by attaching to fur and clothing.

Impacts
Squarrose knapweed is a rangeland and...

St. Johnswort (Hypericum perforatum)
St. Johnswort (Hypericum perforatum)
St. Johnswort (Hypericum perforatum)
St. Johnswort (Hypericum perforatum)

Other common names
Klamath weed, Goat weed, Common St. Johnswort

Description
Perennial, blooms June to July. Grows 1 to 3 feet tall. Stems are erect, numerous-branched, somewhat two-ridged, rust-colored and woody at their base. Leaves are opposite, oblong, not over 1 inch long, and covered in transparent dots. Flowers are 3/4 inch in diameter, bright yellow, numerous, in flat-topped cymes with five petals, with occasional small black...