Know Your Knapweeds
W-1146 (Revised), January 2004
Rodney G. Lym, Professor, Department of Plant Sciences, North Dakota State
University
Click here for an Adobe Acrobat PDF file suitable for printing. (154KB)
North Dakota is being threatened by three noxious weeds that could infest more
acreage in the state and at a faster rate than leafy spurge. Members of this
trio include spotted, diffuse, and Russian knapweed. These three knapweeds
already infest more acreage than leafy spurge in Montana and Minnesota, and
have been found in over 25 counties in North Dakota. Knapweeds are related to
thistles and can spread even faster. For instance, spotted knapweed infested
approximately 25 acres in eight North Dakota counties in 1984; by 1997 it had
spread to more than 1,000 acres in 14 counties, and was found in 22 counties
in 2002. Diffuse knapweed can spread as quickly as spotted knapweed, but Russian
knapweed spreads much slower.
Knapweed infestations in North Dakota have been found primarily along highways,
waterways, railroad tracks, pipelines and recently constructed utility lines.
Knapweed often spreads to a new site by seed in hay, on vehicles, or in contaminated
seed (Figure 1). Watch for new infestations along any publicly traveled route,
where livestock are fed, or in disturbed areas. Control methods vary depending
on the knapweed species and proper identification is the first step towards
stopping the spread of these noxious weeds.
Figure 1 . People are the major
cause of knapweed spread from one location to another. Inspect vehicles, hay, gravel, etc.
carefully if they have come from a knapweed infested area. (70KB
color photo)
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Identification
SPOTTED KNAPWEED |
DIFFUSE KNAPWEED |
RUSSIAN KNAPWEED |
Short-lived perennial or biennial;
tap-root |
Short-lived perennial or biennial;
tap-root |
Perennial with black, spreading roots that
form new shoots |
Black-tipped bracts |
Spiny or "crab-like" bracts |
Rounded bracts with transparent tips |
Pink flowers, rarely cream colored |
White to rose, or sometimes purple |
Pink to lavender flowers |
Spotted knapweed [Centaurea stoebe L. spp. micranthos (Gugler) Hayek]. Spotted knapweed is an aggressive, introduced weed species
that rapidly invades pasture, rangeland, and fallow land and causes a serious
decline in forage and crop production. Spotted knapweed has few natural enemies
and is not preferred by livestock as forage.
Spotted knapweed infestations in North Dakota can largely be traced to seed
or hay brought in from neighboring states. Researchers in Montana have observed
that spotted knapweed may remain in a confined location for several years and
then spread rapidly to adjacent areas. It is important to control spotted knapweed
plants when they are first observed and monitor the site for several years to
prevent reinfestation from seed.
Spotted knapweed generally is a short-lived perennial, reproducing solely by
seeds. It is a prolific seed producer with 1,000 or more seeds per plant. Seed
remains viable in the soil five years or more, so infestations may occur a number
of years after vegetative plants have been eliminated. The seeds can germinate
from spring through early fall. Seedlings emerging in the fall often overwinter
as a rosette of leaves, resuming growth again in the spring (Figure 2). Rosette
leaves are deeply lobed. The plant grows 2 to 4 feet tall and can have one or
more stems with pale green leaves which are 1 to 3 inches long. The surfaces
of the upper leaves and stems are rough. Flower heads are solitary and produced
from early July through August. Flowers are pink to light purple (rarely cream
colored) and the petals are surrounded by stiff, black-tipped bracts, giving
the flower head a spotted appearance (Figures 3 and 4). The black tipped bracts
found below the flower petals are the key feature to distinguish spotted knapweed
from other knapweed species.
Figure 2. It is very difficult to distinguish
knapweed species based on the rosette alone. Spotted knapweed is deeply divided with wide
lobes (top), diffuse knapweed has more finely divided lobes similar to carrots (middle),
and Russian knapweed is seldom divided, has a "rabbit ears" appearance, and is a
perennial (bottom). (76KB color photo)
Figure 3. Spotted knapweed has stiff black tip bracts
with purple flowers (left), diffuse knapweed has both purple and white flowers and bracts
with rigid sharp spines (center), Russian knapweed has pink to purple flowers with opaque
bracts and the flower heads are generally larger than the other two species (right). (24KB color photo)
Figure 4. The most reliable way to distinguish the
knapweed species is by the bracts. Black-tipped bracts of spotted knapweed (left), spiny
crab-like bracts of diffuse knapweed (center), and transparent tips of Russian knapweed. (21KB color photo)
Diffuse knapweed (Centaurea
diffusa Lam.). Diffuse knapweed is generally a short-lived perennial
or biennial in North Dakota and invades habitats similar to spotted knapweed.
Diffuse knapweed was found in one North Dakota county in 1996 and infested approximately
20 acres. The physical appearance of diffuse knapweed is similar to spotted
knapweed, except diffuse knapweed is generally shorter and more highly branched.
Also, rosettes of diffuse knapweed have more finely divided leaves than those
of spotted knapweed (Figure 2). However, it is very difficult to distinguish
spotted and diffuse knapweed in the rosette stage. If the plant is not flowering,
search for last seasons flower stalk and identify the plant based on the flower
bracts.
Flower bracts are the key distinguishing feature between spotted and diffuse
knapweed. Diffuse knapweed bracts have a rigid terminal spine about one-third
of an inch long with four to five pairs of shorter, lateral spines (Figures
3 and 4). The spiny bracts resemble a crab in appearance, are very sharp, and
can puncture skin if touched. The flowers can be white or purple, so flower
color is not a distinguishing feature between knapweed species (Figure 5). Diffuse
knapweed flowers from July to September.
Figure 5. An example of why flower color cannot be
used to distinguish knapweed species. Note the spiny bracts on both the purple and white
flowering plants. They are both diffuse knapweed. (51KB color
photo)
Russian knapweed [Acroptilon repens (L.) DC.]. Russian knapweed
is the most wide spread of the knapweeds in North Dakota and infested approximately
3500 acres in 1997. It also is the only perennial of the noxious knapweeds and
is the most difficult to control. The largest infestations generally are found
in southwestern North Dakota. Russian knapweed is adapted to poorly drained
and saline/alkaline soils. It is often found in areas with a supplemental water
source such as the Little Missouri and Heart Rivers in North Dakota. Russian
knapweed will also infest roadsides, pasture, and rangeland and is the only
knapweed in the state that causes significant losses in cropland.
Russian knapweed is a long-lived, deep-rooted perennial with growth characteristics
similar to Canada thistle. The weed emerges in the spring from roots and grows
1 to 3 feet tall (Figure 2). Once established, Russian knapweed spreads mainly
by underground root stocks as seed production is limited compared to other knapweed
species. Two key characteristics distinguish Russian knapweed from spotted and
diffuse knapweed. First, the flowers have rounded bracts with transparent tips
(Figures 3 and 4). Second, the root of this perennial is dark brown to black
in color, scaley as if the plant had been burned, and can grow to depths of
greater than 20 feet (Figure 6). The flowers of Russian knapweed vary from light
pink to lavender (Figure 3). Flowering occurs from June to September.
Figure 6. The roots of spotted and diffuse knapweed
are taproots similar to dandelion and off-white in color (left). Russian knapweed roots
are brown to black in color with a scaley, bark-like appearance and because it is
perennial have root buds (right). (36KB color photo)
Prevention
People are the major cause of knapweed spread. Knapweeds are spread readily
in hay and on vehicle undercarriages. Producers should exercise caution when
using hay from road ditches and when purchasing hay from known infested areas
in neighboring states and provinces.
Land managers must learn to identify knapweed on their own and neighboring
land, especially on disturbed sites, pastures bordering roads and streams, and
where hay is fed. Timely control of a few plants will be very cost-effective
compared to treating larger acreage later. The public can assist county weed
officials in controlling knapweeds by reporting all suspected infestations.
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Control
Spotted and diffuse knapweed
Small infestations. Spotted and diffuse knapweed confined to small, well-defined
areas should be pulled by hand or treated with a herbicide as soon as detected
to avoid spread of the weed. First, all visible knapweed plants should be removed
and destroyed by burning or mulching. Then the areas should be treated with
a herbicide to prevent reinfestation from seedlings.
Tordon (picloram), Banvel (dicamba) or products that contain clopyralid such
as Transline Curtail, or Redeem can be used to control small infestations. One
to 2 ounces of one of the herbicides per gallon of water should be applied until
runoff using a hand-held single nozzle sprayer. Treat an extra 10 to 15 feet
around the knapweed patches to control roots and seedlings. A careful follow-up
program is necessary to control missed plants and seedlings. Many attempts to
control knapweed have failed because follow-up treatments were not applied.
Pasture and rangeland. Tordon (picloram) at 1 to 2 pints (0.25 to 0.5 pounds)
per acre will control spotted and diffuse knapweed plants and seedlings
for two to three years. The residual control period may be shorter on gravelly
soils and where soil organic matter is high. The optimum application time is
when the plant is in the rosette growth stage in the fall or in the bolt to
bloom stage in the spring. Consult the herbicide for use and grazing restrictions.
Banvel (dicamba) at 1 to 2 quarts (1 to 2 pounds) per acre or the combination
of Banvel at 1 quart per acre plus 2,4-D at 1 quart (1 pound of a 4-pound-per-gallon
concentration) per acre will give good spotted and diffuse knapweed control,
but residual control of seedlings is shorter than with Tordon. The optimum application
timing for Banvel plus 2,4-D is when the knapweed is in the bud to bloom stage.
An annual follow-up treatment of 2,4-D at 1 quart per acre for a minimum of
two years when the plants are in the rosette to early bolt growth stage may
be needed to prevent reinfestation by seedlings. Consult the herbicide label
for grazing restrictions.
Products that contain clopyralid such as Transline, Curtail (clopyralid plus
2,4-D), and Redeem (clopyralid plus triclopyr) will provide good control of
spotted and diffuse knapweed with less soil residual than Tordon or Banvel.
Control is greatest when the herbicides are fall-applied or in the early spring
when the plants are still in the rosette stage. Apply Transline at 2/3 to 1
pint (4 to 6 oz clopyralid) per acre. Curtail at 4 pints (0.19 pounds clopyralid
plus 1 pound 2,4-D) per acre, or Redeem at 2 pints (0.19 pounds clopyralid plus
0.6 pounds triclopyr) per acre. A follow-up treatment the following year may
be necessary to control seedlings.
The herbicide 2,4-D can be used to control spotted or diffuse knapweed. The
herbicide should be applied when the plants are in the rosette to early bolt
growth stage at 1 to 2 quarts (1 to 2 pounds of a 4-pound-per-gallon concentration)
per acre. Application of 2,4-D after stem elongation is not very effective.
No residual control is provided by 2,4-D, and annual spraying is necessary until
no seedlings are detected. This may require several years of annual treatment.
Spotted an diffuse knapweed are generally easy to control with herbicides.
However, a treated area must be monitored for several years and retreated as
necessary for seedling control.
Russian knapweed
Russian knapweed is one of the most difficult perennial weeds to control. If
the plant is found in cropland, then a combination of cultivation and herbicide
treatments will suppress the plant. However, herbicides at labeled rates for
cropland use will not control Russian knapweed.
Small patches in pasture and rangeland. Russian knapweed is best controlled
when herbicides are applied following several hard frosts (usually mid-October).
The plants will look dormant with grey stems and no leaves, but control the
following growing season has been excellent with herbicides applied very late
in the growing season. Application in mid-September or during flowering in mid-summer
generally provides shorter-term control.
The same herbicides, except 2,4-D and Banvel, used for spotted and diffuse
knapweed will control Russian knapweed when applied at higher rates very late
in the season. Tordon (picloram) should be applied at 3 to 4 pints (0.75 to
0.1 pound) per acre, Transline (clopyralid) at 2/3 pint (8 oz.) per acre, Curtail
at 2 quarts (0.38 pounds clopyralid plus 2 pounds 2,4-D) per acre, and Redeem
at 4 pints (0.38 pound clopyralid plus 1.1 pounds triclopyr) per acre. These
treatments have provided 90 to 100 percent Russian knapweed control when applied
in mid-October.
In addition, Escort (metsulfuron) plus 2,4-D is labeled for Russian knapweed
control at 1 oz Escort plus 1 to 2 pints 2,4-D (0.6 ounces metsulfuron plus
0.5 to 1 pound of a 4-pound-per-gallon concentrate 2,4-D) per acre and should
be applied with a non-ionic surfactant. Metsulfuron has no grazing restrictions.
The optimum application time is when Russian knapweed is in the bud to early
bloom growth stage or in the fall following a frost.
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Biological Control
In general, the knapweed infestations are small enough that herbicide and hand
removal are the best and most cost-effective treatments in North Dakota. Biological
control of various knapweed species in neighboring states has not been successful
in reducing established knapweed stands. Biocontrol agents have not been introduced
into North Dakota and their use is not recommended.
The best knapweed control is prevention, and to keep an infestation from
becoming established you must correctly identify the plant. Small and young
infestations of knapweed are generally easy to control with herbicides. However,
an area must be monitored for several years and retreated as necessary for seedling
control.
Consult Extension Circular W-253 "North Dakota Weed Control Guide"
for additional information concerning herbicides for knapweed control.
Photos 1, 2, 5 and 6 by Rodney G. Lym.
Photos 3 and 4 courtesy of Dr. Steve Dewey, Utah State Univ., Logan.
For more information on this and other topics, see www.ag.ndsu.nodak.edu
W-1146 (Revised), January 2004
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