In colorado, diffuse knapweed cover declined from 8. Seed germination characteristics of centaurea diffusa and c. In montana, viable seeds were recovered from soil that had no seed inputs for eight years davis et al. Growth habit perennial, spotted is biennial or short lived perennial. Spotted knapweed and diffuse knapweed are both shortlived perennials that sometimes behave like annuals. Diffuse knapweed will not tolerate flooding and shade. The plant has a white flower, grows to a height of 24 inches, and has a short taproot. The problematic reinfestation of chemicallytreated sites by diffuse and spotted knapweed centaurea diffusa and c. Therefore, the key to managing this plant is to prevent seed production. Diffuse knapweed north dakota department of agriculture. The enemy diffuse knapweed centaurea diffusa this annual and sometimes biennial noxious weed is a native of eurasia and spread into the u. Diffuse knapweed was introduced from europe and is a biennial or shortlived perennial forb, which reproduces only by seed.
Methods to identify and control each weed are discussed and why the plant is a concern in the state is explained. This species is common throughout western north america but is not actually native to the north american continent, but to the eastern mediterranean. Knapweed seed head moth stevens county, washington. They have been found to reduce seed production by 50%.
Therefore knapweed control sites will need to be revisited for many. Recent research completed by colorado state university shows that cattle grazing diffuse knapweed twice in spring decreased seed set by 50 percent. Seed crops have a high percent viability and will germinate in the fall of the year produced, the following. Diffuse knapweed reproduces primarily by seed but may also regenerate from the crown. Diffuse knapweed was found in one north dakota county in 1996 and infested approximately 20 acres. This publication includes photos of all north dakota state and county listed noxious weeds as well as troublesome plants such as poison ivy. Diffuse knapweed grows from a slender, deep taproot. Effect of rh during a 30 d afterripening period on the percentages of nonviable diffuse knapweed seeds.
United states department of agriculture natural resources. It is a highly competitive plant which threatens to exclude many desirable species from pastures and wildlands. Seeds can be moved if the seed head is mature and broken off by the wind or by vehicles. Diffuse knapweed is a biennial or shortlived perennial with a deep taproot in the aster family that is similar to the more common spotted knapweed, but is generally shorter and its upper stems have numerous spreading branches giving the plant a ball shape. Larval feeding in the seedhead destroys the knapweed seed before it matures. Knapweed removal information how to get rid of knapweed.
Provide experience using the rolled paper towel and tetrazolium chloride tests. Thus, they may behave as annuals, biennials or shortlived perennials. Spotted knapweed, though, can also resprout from roots. During the first year of growth, diffuse knapweed appears as a rosette in spring or fall.
Both are biennial or shortlived perennials that reproduce solely by seed. There is a wide environmental range of where this weed can grow. The flower heads of spotted knapweed are larger than those of diffuse knapweed. Diffuse and spotted knapweed both have a taproot and grow to around 1 metre in height. Sheep and mule deer can ingest, transport, and disseminate viable seeds of spotted hnapweed in their feces.
Diffuse knapweed was likely introduced to north america in the late 1800s and is now widespread and problematic in many areas of the western united states and. A biennial is a plant that completes its life cycle within two years. Knapweed seeds have an extremely hard seed coat and can be viable for at least 510 years. Diffuse knapweed plants first form low rosettes a ring of basal leaves and may remain in this form for one to several years. King county best management practices for diffuse knapweed. Spotted knapweed is distinguished from the closely related species, diffuse knapweed c. Plants form dense colonies and readily reproduce by seeds, which can travel long distances. Identification and control of invasive and troublesome weeds. Diffuse knapweed department of agriculture conservation. Annual seed production of diffuse centurea diffusa and spotted knapweed centurea maculosa is reduced in dry years by a reduction in the number of viable seeds per seed head and increases when. Some seeds have short pappus tufts of hairs on top, enabling them to catch on fur and feathers. A weed report from the book weed control in natural areas in the western united states spotted knapweed. Diffuse knapweed can grow 1 to 3 feet tall, and is diffusely branched above ground.
Asteraceae usda nrcs, 2004 is the source for all scientific nomenclature is a designated noxious weed in 14 western states usda nrcs, 2004 and four canadian provinces. Diffuse knapweed uc weed research and information center. There are about 500 species of centaurea around the world, including the harmless annual corn flower or bachelors button c. Both diffuse and spottted knapweed are typically annual, sometimes biennial or shortlived perennial, thus they reproduce mostly by seed. United states biological control of spotted and diffuse. Spotted knapweed and diffuse knapweed are closely related noxious weeds infesting over 7 million acres in 17 western states duncan and jachetta 2005. The wind does not blow the seed far, but in the fall the wind breaks the plant off at the ground and blows it like a tumble weed dropping seeds as it rolls. Wilson1 and carol bell randall2 1 department of plant, soil and entomological sciences, university of idaho, p. When the plant is broken off at the base, it can be blown around like a tumbleweed and disperse its seed.
Seed crops have a high percent viability and will germinate in the fall of the year produced, the following spring, or will remain. Livestock sheep, goats, cattle will eat diffuse knapweed. Most states have a seed testing laboratory, which determines the quality of each seed lot to be sold in that state. Because germination occurs throughout the growing season, seedling. The lifespan and viability of seeds mother earth news. Diffuse knapweed is a shortlived perennial or sometimes biennial plant reproducing solely by seed. Spotted knapweed, centaurea stoebe, is perhaps the most widespread species, followed in abundance by diffuse knapweed, c. This plant may be known by one or more common names in. Diffuse knapweed was first introduced to the pacific northwest at the turn of the century as a contaminant in alfalfa seed imported from turkestan, turkmenistan or hybrid alfalfa seed from germany. A single diffuse knapweed plant can produce up to 18,000 seeds harris and cranston 1979 and a stand of diffuse knapweed can produce up to 40,000 seeds per square meter watson and renney 1974. Figure 9 the seed, tipped with a tuft of bristles, measures about 18 inch. United states biological control of spotted and diffuse knapweeds.
Spotted and diffuse knapweed centaurea maculosa, c. If the seed is below a certain depth 3 cm, it will not grow. It is one of the only knapweeds that spread by tumbling by the wind like russian thistle. However both species of weevils feed readily on both species of knapweed. Effect of biocontrol insects on diffuse knapweed centaurea.
Knapweed plants produce both nondormant and dormant. Diffuse knapweed identification and management diffuse knapweed. Spotted knapweed seed viability after passing through sheep and mule deer. This plant may be known by one or more common names in different places, and some are listed above. One larva can consume 100% of the seed in a single seedhead. Diffuse knapweed identification and management diffuse. The bracts of spotted knapweed seed heads have dark, comblike tips that give the head a spotted appearance.
Diffuse is a member of a large genus of over 400 species, most originating in the mediterranean region. Diffuse knapweed, and spotted knapweed scnwcb february 2006 genealogy original source for u. Reductions of 95% or greater in diffuse knapweed stand density have been. This gives the plant a ballshaped appearance and tumbleweed mobility when broken off. Box 442339, moscow, id 838442339 2 forest health protection, usda forest service, 3815 schreiber way, coeur dalene, id 838158363 for additional copies of this publication, contact. Ecology and management of diffuse knapweed centaurea diffusa. Currently distributed throughout southern bc, both spotted and diffuse knapweed are a major concern in the omineca, peace river, kootenay. Diffuse knapweed is a member of the aster family, thistle tribe. Diffuse knapweed national invasive species information. It is not known how long seeds remain viable in the soil, but it is assumed that survival would be similar to other centaurea species, 2 to. The seeds can germinate from spring through early fall. These weeds are well adapted to a wide range of habitats including open forests, rangeland, roadsides, pastureland, riparian areas, and ditch banks.
Diffuse and spotted knapweed biocontrol department of. These herbicides are selective for broadleaf weeds and generally do not harm grasses when applied at recommended rates. Tillage can be used to remove plants and release the seed bank. When seed production was controlled with herbicide treatments at 2 other heavily infested spotted knapweed sites in montana, the spotted knapweed seed bank decreased by 72% to 81% after 15 months. However, mowing will reduce the number of viable seed produced by diffuse knapweed if mowed at early flowering stages. Over 50% of a population of spotted knapweed seeds remained viable after 5 years of burial. The beetle feeds on the seed heads but is also a foliage feeder as an adult. This study confirmed that reserves of dormant seeds are present in the soil of infested sites, although greater numbers of seeds were recovered from senesced plants. Minutus prefers diffuse knapweed while obtusus prefers spotted knapweed. Cutting or mowing the aboveground portion of the plant, before seed set may be an effective way to reduce seed production, but it will not eliminate the infestation. In a natural seedbank, spotted knapweed seed populations declined. Forest service forest health technology enterprise team.
Now established in idaho, montana, oregon, and washington. The plant will spread from these sites into rangelands, meadows and other open. Defoliation timing effects on spotted knapweed seed. Seed remains viable in the soil five years or more, so infestations may occur a number of years after vegetative plants have been eliminated. Along the colorado front range, seed production of 5001500 seeds pre plant is more typical beck et al.
Rootfeeding insects may have a more detrimental effect on knapweed populations than seed feeding ones. Roche and roche 1990 reported that 22 percent of diffuse knapweed plants mowed to a twoinch height each month of the growing season of april through october were still growing after four years. Rootfeeding insects may have a more detrimental effect on knapweed populations than seedfeeding ones. Seed germination characteristics of centaurea diffusa and. Spotted knapweed is a biennial or shortlived perennial. Seeds of both diffuse and spotted knapweeds spread through rivers and along watercourses, and are transported in crop seed and hay. Not listed on any labels except for diffuse knapweed on the banvel label. Spotted knapweed is a prolific seed producer, with individual plants producing up to 140,000 seeds per square metre. Diffuse knapweed can be controlled by various integrated weed management techniques. One plant can give rise to 1 to 20 slender, upright stems 24 feet in height with most branching occurring in the upper half. Po box 315, twin bridges, mt 59754 406 6845590 888 6644153 fax. A prolific seed producer, diffuse knapweed can produce up to 18,000 seeds per plant.
This is a major revision since the first publication in 2010. Pdf the decline of diffuse knapweed in british columbia. It is often found in open, disturbed areas where it grows in tufted clumps. Mowing usually increases diffuse knapweed density, due to increased germination from the soil seed bank. Diffuse knapweed is a nonnative biennial forb that reproduces solely by seed.
Biology and biological control of knapweeds pdf 45. Identification and control of invasive and troublesome weeds in north dakota w1411, revised sept. Clancyb a northern arizona university, school of forestry, box 15018, flagstaff, az 86011, usa busda forest service research and development, rocky mountain research station, 2500 s. Diffuse and spotted knapweed csu extension colorado state. Picloram tordon, clopyralid stinger, 2,4d and dicamba banvel will control spotted and diffuse knapweed on range land.
Effects of postfire conditions on germination and seedling success of diffuse knapweed in northern arizona. Diffuse knapweed is generally a shortlived perennial or biennial in north dakota and invades habitats similar to spotted knapweed. Executive office montana weed control association, inc. Identification and control of invasive and troublesome. Spotted centaurea biebersteinii and diffuse knapweed centaurea diffusa are biennial to shortlived perennials, and considered regionally noxious under the bc weed control act. Flowerhead lower leaves have a blacktipped fringe that gives a spotted appearance. The most successful agent for use against diffuse knapweed is the seed head feeding weevil, larinus minutus. Yellow starthistle is another less robust annual type. Larinus minutus in particular is an aggressive and effective biocontrol insect for diffuse and spotted knapweed.
On the other hand, spotted knapweed can be the first species to recover from a burn. Diffuse knapweed cannot tolerate cultivation or excessive moisture and is therefore uncommon on cultivated land. Diffuse knapweed the enemy diffuse knapweed centaurea diffusa this annual and sometimes biennial noxious weed is a native of eurasia and spread into the u. After 7 years, only 5% of the original seed bank remained, leaving about 160,000 viable seeds per acre 400,000 seedsha, or about 4 viable seeds.
However, mowing at the late bud to early bloom stage will reduce seed production. Russian knapweed is the most difficult to handle, since this perennial knapweed digs in for the long haul it can set roots as deep as 20 feet below the surface of the soil. Centaurea diffusa, also known as diffuse knapweed, white knapweed or tumble knapweed, is a member of the genus centaurea in the family asteraceae. 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 11 the flower of the diffuse knapweed plant is generally white but can be lavender or purple. These two species of insects are nearly indistinguishable. Diffuse knapweed is biennial, can survive for a period of two years. Biology and biological control of knapweed linda m. Diffuse knapweed spreads along heavily disturbed areas such as road ditches, agricultural field margins, railroad beds, pipelines, and recently installed utility lines. Native to the mediterranean region, diffuse knapweed was first recorded in montana. Diffuse knapweed had spread to four counties by 2017 and infested nearly 750 acres. Diffuse knapweed was first introduced to the pacific northwest at the turn of the century as a contaminant in alfalfa seed imported from turkestan, turkmenistan or. Larvae of the diffuse knapweed root beetle sphenoptera jugoslavica feed in the roots of diffuse knapweed.
Seed crops have a high percent viability and will germinate in the fall of the year produced, the following spring, or will remain dormant and viable at a significant percentage for eight or more years davis et al. Diffuse knapweed centaurea diffusa shortlived perennial, annual, or monocarpic perennial knapweed with a long fibrous taproot and single upright stem covered in fine hairs. To help identify diffuse knapweed, look at the shape of the plant, the shape of the buds, the bracts beneath, and the color of the flowers. Diffuse knapweed infests roadsides, waste areas, dry rangelands and disturbed sites. Largescale control against diffuse knapweed has been seen at a number of locations along the front range. Compared to spotted knapweed, the leaves of diffuse knapweed are small, and the numerous flower heads are narrow with spiny bracts. The seed is commonly spread by vehicles or by the sale and movement of contaminated sand and gravel. In stevens county 10,600 adults were released between 1986 and 1990. Po box 315, twin bridges, mt 59754 406 6845590 888 6644153 fax becky l.
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