Klamath weed is an erect, semiwoody, perennial, rhizomatous herb. The
stems are often winged. The leaves are simple, sessile, opposite, and
net-veined. The blade has pinnate major veins and numerous punctate dots
that can be viewed when the blade is held up to the light. The base of
the blade is obtuse and clasps the stem. The flowers are perfect and produced
in clusters at the tips of the branches. The petals are bright yellow
and often black dotted. The fruit is a capsule with many, small, brown
to black cylindrical seeds.
Plants grow in cultivated fields, pastures, waste areas, and often in
the edges of forests. It reproduces by seeds and by rhizomes.
The rhizomatous perennial habit of this species makes it a difficult
weed to eradicate and allows it to compete with and replace natives. The
plant is usually avoided by grazing animals, which gives it a competitive
edge in pasture competition. Plants contain compounds that when ingested
by light-colored animals cause the skin to be photosensitive and produce
severe sunburn conditions. The leaf feeding beetle (Chrysolina quadrigemina
(Suffr.) has been used to control klamath weed along the Pacific Coast.