| Walla
Walla County Noxious Weed Control Board
Weed
of the Month
Diffuse &
Spotted Knapweed
Diffuse Knapweed
|
Spotted
Knapweed
|
Diffuse knapweed (Centaurea
diffusa Lam.) and spotted knapweed (Centaurea maculosa
Lam.) are members of the sunflower family (Asteraceae). These
aggressive, exotic invaders from Europe were accidentally introduced
as crop seed contaminants. Knapweed thrives on any disturbed soil
in gravel pits, roadside right-of-ways, vacant lots, trails, and
heavily grazed pasture or rangeland. It outcompetes native plants
species, decreases forage production for livestock and wildlife,
and is a wildfire hazard.
Diffuse knapweed is an
annual or short-lived perennial plant with a stout taproot and a
single rough stem, 1 to 2 feet tall, branched toward the top. Grazed
plants may produce multiple stems. Rosette and lower shoot leaves
are finely divided. Spotted knapweed is a short-lived, perennial
that reproduces primarily from seed and forms a new shoot each year
from a taproot. The weed produces one or more shoots that are branched
and 1 to 3 feet tall. Basal leaves can be 6 inches long and deeply
lobed. Upper leaves are similar to diffuse knapweed.
Spotted knapweed flowers
are usually pinkish-purple, sometimes white. Flower bracts have
dark margins with short fringe. Diffuse knapweed flowers are often
white but may be purplish. Bracts under the flowers have yellow
spines with comb-like teeth on the spine margins. Sometimes the
bracts are dark-tipped or spotted like spotted knapweed. The long
terminal spine differentiates diffuse from spotted knapweed.
Knapweed germinates in
spring or fall and flower continuously from early summer into the
fall, as long as moisture and temperatures permit. Many narrow flowering
heads occur on shoot tips singly or in clusters of two or three.
They are about 1/8 inch in diameter and 1/2 to 2/3 inch long. A
single flower stalk can produce 1,200 seeds and seeds may remain
viable in the soil up to 5-8 years. The seeds are dispersed when
the plant breaks off at the base becomes a tumbleweed, often spread
by vehicles. The seeds are also transported by shoes and clothing,
by feeding rodents and other wildlife, and in contaminated hay and
crop seed.
The best knapweed control
is prevention. Avoid travel through infested areas, clean clothing
and shoes after walking in infested areas, don’t graze livestock
when ripe seeds are present, and use weed-free hay. The plant is
usually easy to control with herbicides but an area must be monitored
for several years and treated as necessary for seedling control.
Biocontrol agents used
in Walla Walla County include three seed eating beetles, Larinus
minutus, Larinus obtusus, Eustenopus villosus, and a bud weevil,
Bangasternus orientalis.
Existing plants may be
killed with 2,4-D or glyphosate but yearly reapplication is needed
to control new plants germinating from seed stored in the soil.
Products that contain clopyralid such as Transline, Curtain (clopyralid
plus 2,4-D), and ReDeem (clopyralid plus triclopyr) should be applied
in fall or early spring at rosette stage. Follow-up treatment the
following year may be necessary to control seedlings. Banvel (dicamba)
and Tordon provide longer control, perhaps for two or more seasons
varying with soil type and precipitation patterns. Seeding suitable
perennial grasses helps prevent weed reinvasion.
Please refer to the annually
updated Pacific Northwest Weed Management Handbook at http://weeds.ippc.orst.edu/pnw/weeds
for specific control recommendations. ALWAYS FOLLOW PESTICIDE
LABEL INSTRUCTIONS.
|
