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Species Abstracts of Highly Disruptive Exotic Plants

at Scotts Bluff National Monument

Centaurea diffusa

Park: Scotts Bluff National Monument (SCBL)
Species: Centaurea diffusa Lam.
Common Name: Diffuse knapweed
Urgency Ranking: High
Overall Ranking: 8
Significance of Impact: 51
A. Current impact: 12
B. Ability to be a pest: 39
Feasibility of Control: 40

Taxonomic Description:
Diffuse knapweed is an annual or biennial herb with an elongated taproot. It has an erect stem, 50-80 cm tall, which is diffusely branched, angled and scabrous-puberulent. Initial growth is in the form of a rosette. Lower leaves are obovate (3-8 cm long and 1-3 cm wide), deeply pinnatifid and scabrous-puberulent. Lower leaves are early deciduous. Cauline leaves are alternate, smaller, and pinnatifid, while the upper reduced leaves are entire. Flower heads are radiate (3 mm diameter and 14-16 mm high), numerous, and solitary. Involucres are ellipsoid-cylindric, (8-10 mm tall and 4-5 mm wide). Outer involucral bracts are stiff, glabrous, ovate-lanceolate with a terminal spine (1.5-8 mm tall), and with 4-6 pairs of shorter lateral spines. Corollas (9-12 mm long) are usually white or creamy to purplish. Achenes are 2-3 mm long, light brown to black, with the pappus generally absent or consisting of only a fringe of bristles less than 1 mm long.

Similar species, also exotic, include spotted knapweed (C. maculosa Lam.) and Russian knapweed (C. repens L.). The bracts of spotted knapweed have dark tips with no spine. Russian knapweed has rounded bracts with pointed papery tips.

Diffuse knapweed is a highly competitive, noxious weed native to Eurasia. The earliest record of diffuse knapweed in western North America is from an alfalfa field in Washington in 1907. It may have been introduced with Turkestan alfalfa from the Caspian Sea region. It quickly invades disturbed sites and is common along road and railroad rights-of-way, in waste places, and on overgrazed rangeland. This plant is also capable of invading relatively undisturbed native plant communities. Diffuse knapweed generally forms large, dense infestations, but solitary plants and small patches are common in recently invaded areas. Reported infestations now occur in nine states and two Canadian provinces. Over 1.2 million hectares in the western United States are infested with diffuse knapweed.

In addition to being a prolific seed producer (up to 40,000/mm2), diffuse knapweed possesses several features which contribute to its success as a weed. The rosette growth form of the first year resists mowing and grazing. Diffuse knapweed has a vigorous taproot system that penetrates the soil more deeply and rapidly than do the fibrous roots of grasses. This weed is, thus, far more aggressive than annual or perennial grasses and can quickly dominate rangelands. This plant also resists grazing because of its unpalatable and fibrous foliage and spiny heads. The resistance of knapweed to chemical control may result from pubescence sufficient to limit either retention or penetration of herbicidal sprays. Diffuse knapweed also exhibits an allelopathic effect on associated species establishing essentially single-species stands.

Diffuse knapweed readily colonizes different soils with a wide range of chemical and physical properties. It prefers open habitat, quickly invades disturbed sites, and is well adapted to semiarid areas. Reproduction is by seed, which germinates over a broad range of environmental conditions. Germination occurs in the fall or early spring when environmental conditions are favorable. Moisture appears to be the limiting factor. Germination has been found to occur over a temperature range of 7-34°C with optimum seedling emergence occurring from seeds germinating near the soil surface. Diffuse knapweed seedlings were not found to emerge from below a soil depth of 3 cm. Seedlings develop into rosettes, and maximal root growth occurs when the plants are rosettes. Plants that overwinter as a rosette bolt in early May producing one or rarely two stems. Flowering occurs from July through September. If moisture is adequate the seeds will germinate and develop into rosettes by fall. Each plant is capable of producing 400-900 seeds. Mature plants break off at ground level and tumble with the wind dispersing seeds. The seeds, held in urn-shaped heads which do not open widely, are lost gradually, giving the plant the advantage of distant distribution. Dispersal close to the parent plant is facilitated by horizontally placed involucres, which open as dehydration occurs, allowing the seeds to drop.

A few scattered populations of diffuse knapweed occur at Scotts Bluff National Monument (SCBL) combining to cover an area of less than 5 hectares. These plants were found within sites disturbed within the last three years. A few scattered plants were found in the area of the old country club as well as in the old field area in the northwest corner of the park. All plants observed during the 1993 survey were pulled up. These areas were resurveyed in 1994 and no plants were observed. Diffuse knapweed is highly competitive and has the potential to invade and modify existing native communities. It is a threat to the areas' primary resources. Although no recurrence was observed following the plants' removal in 1993, diffuse knapweed warrants close monitoring throughout the park to prevent its establishment.

Mechanical, cultural, chemical, and biological control methods have all been used on diffuse knapweed with varying levels of success. An important consideration prior to applying control is to determine if enough desired plants are present to replace the controlled species. If desired vegetation is scarce or absent control will be of little value. Control methods may harm other plants and result in a disturbance that will favor reinvasion by diffuse knapweed or other exotic species.

Research has shown that shallow plowing (7 cm) did not control diffuse knapweed, but deep plowing (18 cm) eliminated knapweed. Research has also found that populations of diffuse knapweed increase after mowing. The rosettes escape mowing, but the bolting and flowering stems can be mowed. Mowing once at bud stage, once at flowering or once at bud stage, and again at flowering significantly reduced the number of plants that produced seeds.

A number of chemical control options exist for diffuse knapweed. Many herbicides are not specific to diffuse knapweed or may not be specifically licensed for this use. It is important to read and follow all label directions.

Research has found that 2,4-D applied at a rate of 1 kg/ha completely eliminated diffuse knapweed and associated forbs. The similar spotted knapweed (C. maculosa Lam.) was found to be controlled with 2,4-D applied at a rate of 1.5 kg/ha and with dicamba at 1 kg/ha. However, control of knapweeds with 2,4-D was found to be temporary and did not prevent heavy seedling establishment in the fall in trials conducted in British Columbia. Research has also shown that Tordon (picloram) applied at a rate of 0.42-0.56 kg/ha provided selective control of knapweed in grass. However, if reseeding is desired, a waiting period of 6-12 months is necessary after treatment as picloram is detrimental to root growth in grass seedlings. Other herbicides that provide control of diffuse knapweed are Arsenal (imazapyr), Curtail (premix), and Stinger (clopyralid).

The first biological control agents developed for use against diffuse and spotted knapweed (C. maculosa Lam.) were seedhead flies. The banded seedhead fly [Urophora affinis Frauenfeld (Diptera:Tephritidae)] was released in British Columbia during 1970 and in Montana and Oregon in 1973. In addition, from 1974 to 1980 it was released in Washington, Idaho, and California. It readily established in all of these states. The UV knapweed seedhead fly [U. quadrifasciata Meigen (Diptera:Tephritidae)] was also released in British Columbia in 1972. By 1981, it had dispersed as far as Montana. It is now found throughout the Pacific Northwest. These flies oviposit into knapweed flower heads and the larvae develop within the galls that form from receptacle tissue. As well as directly reducing seed production, the seedhead flies' galls may devitalize the rest of the plant by acting as metabolic sinks for nutrients from other plant parts. The activities of these flies can cause up to 95% reduction in knapweed seed production.

The leaf galling mite, Aceria centaureae, can cause severe damage to the rosettes and shoots of diffuse knapweed. A budgalling mite, Aceria sp., causes witches broom growth of the plants. This species appears to reduce the growth and seed production of diffuse knapweed. In addition, it can be fatal to plants in the rosette stage.

Other insects being released against diffuse and spotted knapweed are the root boring beetles and moths. These insects, because of their direct damage and gall formation plus the pathogens that may enter through their tunnels, are probably the most effective bioagents to use against these biennial and perennial weeds. The diffuse knapweed root beetle, [Sphenoptera jugoslavica Obenberger (Coleoptera:Buprestidae)] was released in British Columbia in 1976 and by 1981 was infesting 25-50% of the plants at White Lake. In the United States, release began in 1980, and it is now established in Idaho, Oregon, and Washington. A European root-mining moth [Agapeta zoegana (L.) (Lepidoptera:Cochylidae)] attacks the rosette stage of diffuse knapweed. This moth was released in Idaho, Montana, Oregon, and Washington, in 1984 and is now established in Montana.

In addition, a rust, Puccinia jaceae Otth., may provide some control. The rust infection discovered in British Columbia, has since spread into diffuse knapweed infested areas of the interior. The rust infection stresses the plant, and it is expected that in combination with other control agents, diffuse knapweed will be controlled biologically.


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