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.
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.
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).
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).
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.
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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.
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.
Figures
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.
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.
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.
Figure 5
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.
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.
Picloram (Tordon), dicamba (Banvel) or clopyralid plus 2,4-D (Curtail) 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. Picloram (Tordon) at 0.25 to 0.5 pounds (1 to 2 pints) 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.
Dicamba (Banvel) at 1 to 2 pounds (1 to 2 quarts) per acre or the combination of dicamba at 1 pound per acre plus 2,4-D at 1 pound (1 quart 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 picloram. The optimum application timing for dicamba 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 pound 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.
Clopyralid plus 2,4-D (Curtail) will provide good control of spotted and diffuse knapweed with less soil residual than picloram or dicamba. Control is greatest when the herbicides are fall-applied or in the early spring when the plants are still in the rosette stage. Apply clopyralid at 0.19 to 0.28 pounds plus 2,4-D at 1 to 1.5 pounds per acre (2 to 3 quarts Curtail) . 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 pounds (1 to 2 quarts 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.
Although spotted and diffuse knapweed are generally easy to control with herbicides, a treated area must be monitored for several years and retreated as necessary for seedling 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.
North Dakota State University
Photos 3 & 4 courtesy of Dr. S. Dewey, Utah State Univ., Logan.