Quick Facts...
- Diffuse knapweed is a short-lived, non creeping perennial, a biennial,
or occasionally an annual that reproduces and spreads solely from seed.
- Spotted knapweed is a short-lived, noncreeping perennial that reproduces
from seed and forms a new shoot each year from a taproot.
- Diffuse and spotted knapweed are readily controlled with herbicides.
Unless cultural techniques are used, however, the weeds will reinvade.
Diffuse knapweed (Centaurea diffusa) is a short-lived perennial,
a biennial, or occasionally an annual. It reproduces and spreads from
seed. The plant develops a single shoot (stem), 1 to 2 feet tall, that
is branched toward the top. Grazed plants may produce multiple stems.
Rosette and lower shoot leaves are finely divided. Leaves become smaller
toward the top of the shoot and have smooth margins.
Many solitary flowering heads occur on shoot tips. They are about 1/8 inch in diameter and 1/2 to 2/3 inch long. Flowers usually are white but may be purplish. Involucre bracts are divided like teeth on a comb and tipped with a slender spine that makes them sharp to the touch. Sometimes the bracts are dark-tipped or spotted like spotted knapweed. The long terminal spine differentiates diffuse from spotted knapweed.
Spotted knapweed (Centaurea maculosa) looks like diffuse knapweed with some notable exceptions. Spotted knapweed is a short-lived, noncreeping perennial that reproduces from seed (primary means of spread) 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. Rosette leaves can be 6 inches long and deeply lobed. Leaves are similar to diffuse knapweed. Lavender to purple flowers are solitary on shoot tips and about the same size as diffuse knapweed flowers. Involucre bracts are stiff and black-tipped. The tip and upper bract margin have a soft, spine-like fringe and the center spine is shorter than others.
Phenology, Biology and Occurrence
Diffuse knapweed seeds germinate in spring or fall or anytime during the growing
season following a disturbance, if adequate soil moisture is present.
Seedlings develop into rosettes and diffuse knapweed remains as a rosette
until it grows to a critical size, then it bolts, flowers, and sets seed.
It may take from one to several years for diffuse knapweed to reach the
critical size necessary to reproduce by seed.
Diffuse knapweed is native to degraded noncropland (waste places) and seashores from southern Europe to north-central Ukraine. It generally is found on dry, light, porous soils in Europe. Diffuse knapweed appears to occupy similar areas in the United States. Diffuse knapweed will not tolerate flooding or shade and thrives in the semiarid west (generally in 9- to 16-inch precipitation zones). Environmental disturbance (e.g., overgrazed pastures or rangeland, roadsides, rights-of-way, gravel piles, etc.) promotes its invasion.
In Colorado, the worst infestations occur along the Front Range in Larimer,
Boulder, Douglas and El Paso counties. Severe infestations also occur
in Archuleta and La Plata counties. A 2002 survey conducted by the Colorado
Department of Agriculture found 145,148 infested acres of diffuse knapweed
and 1,093 infested acres of spotted knapweed.
Spotted knapweed germinates in spring or fall. Perennial plants resume growth in early spring and bolt at approximately the same time as diffuse knapweed. Flowering occurs through the summer into fall.
Spotted knapweed is native to central Europe, where it is found in light, porous, fertile, well-drained and often calcareous soils in warm areas. It occupies dry meadows, pastureland, stony hills, roadsides, and the sandy or gravelly floodplains of streams and rivers. Spotted knapweed tolerates dry conditions, similar to diffuse knapweed, but survives in higher moisture areas as well (e.g., it thrives in the wetter conditions of the western Montana mountains). Spotted and diffuse knapweed infestations often occur together in Colorado.
Spotted knapweed infestations are not as severe in Colorado as diffuse knapweed. However, this weed spreads rapidly. For example, spotted knapweed was first observed in Gallatin County, Montana, in the 1920s, but is now found in all Montana counties. Today, over 4.7 million acres are infested.
Management
Diffuse and spotted knapweed can be managed similarly. They are readily controlled
with herbicides. However, the weeds will reinvade unless cultural techniques
are used.
Chemical control. Research conducted at Colorado State University
indicates that Tordon 22K (picloram) at 1 to 2 pt/A, Transline (clopyralid)
at 0.67 to 1 pt/A, Curtail (clopyralid + 2,4-D) at 4 to 6 pt/A, or Banvel/Vanquish/Clarity
(dicamba) at 1 to 2 pt/A control diffuse knapweed. Tank mixes of Banvel/Vanquish/Clarity
plus 2,4-D at 1 pt + 2 pt/A or Banvel/Vanquish/Clarity plus Tordon 22K
at 1 to 2 pt + 0.5 to 1 pt/A or Tordon plus 2,4-D at 0.75 pt + 2 pt/A
all control diffuse knapweed. These tank-mixes may save money and reduce
grass injury resulting from higher use rates of a single herbicide.
Spotted knapweed and diffuse knapweed generally occupy the same areas in Colorado, so the same herbicide treatments can be applied. Weed scientists at Montana State University indicate that 1 pt/A of Tordon (0.25 lb) controls spotted knapweed for two to three years, but the weed will reinvade the area unless other management techniques are used.
Cultural control. If desirable grass competition is evident
in diffuse or spotted knapweed stands, judicious herbicide application
that does not injure grasses may allow them to compete effectively with
the weeds. Irrigation (where possible) may help stimulate grass competition
in these cases. However, infested rangeland or pastures often are degraded,
allowing knapweed invasion, and herbicides alone will not restore the
land to a productive state. Seeding suitable perennial grasses is necessary
to prevent weed reinvasion.
Biological control. Many insects are being evaluated
for biological control of diffuse and spotted knapweeds. Researchers at
Montana State University believe it will take a complex of insects (perhaps
12) to reduce diffuse and spotted knapweed populations.
Several insects are available in Colorado, from the Colorado Department
of Agriculture. The seedhead flies Urophora affinis and U.
quadrifasciata have been released in many Front Range counties. These
insects cause plants to produce fewer viable seeds and abort terminal
or lateral flowers.
Root-feeding insects may have a more detrimental effect on knapweed populations
than seed-feeding ones. Larvae of the diffuse knapweed root beetle (Sphenoptera
jugoslavica) feed in the roots of diffuse knapweed. Larvae of the
yellow-winged knapweed moth (Agapeta zoegana) feed and the knapweed
root weevil (Cyphocleonus achates) in the roots of both knapweed
species.
Livestock (sheep, goats, cattle) will eat diffuse and spotted knapweed.
Recent research completed by Colorado State University shows that cattle
grazing diffuse knapweed twice in spring decreased seed set by 50 percent
and tumbling off-site over winter by 15 percent. Cattle were managed to
achieve 50 percent utilization of pasture and were allowed to graze at
two 10-day intervals when diffuse knapweed was bolting and about 6 to
12 inches tall.
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