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Species Abstracts of Highly Disruptive Exotic Plants

at Pipestone National Monument

Bromus inermis

Park: Pipestone National Monument (PIPE)
Species: Bromus inermis Leyss.
Common Name: Smooth brome, Hungarian brome, Austrian brome, Russian brome, awnless brome
Urgency Ranking: Medium
Overall Ranking: 2
Significance of Impact: 85
A. Current impact: 42
B. Ability to be a pest: 43
Feasibility of Control: 18

Taxonomic Description:
Smooth brome is a strongly rhizomatous, sod-forming perennial grass. It has an erect to decumbent growth form (0.4-1.2 m tall). Leaf blades are flat (9-33 cm long and 4.5-13 mm wide) and glabrous. Ligules are membranous (0.5-2 mm long) and lacerate. Sheaths are round and glabrous. The inflorescence is a loosely contracted panicle (10-20 cm in length) with the upper branches often ascending while lower branches are reflexed. Spikelets are four- to ten-flowered and terete (1.8-4 cm long and 2.5-7 mm wide). Glumes are subequal in length (5-10 mm long). Lemmas (9-14 mm long) are bifid at the tip and usually awnless or with an awn tip (to 2 mm long). Two subspecies are recognized. The more common Bromus inermis Leyss. subsp. inermis is described above. Bromus inermis Leyss. subsp. pumpellianus (Scribn.) Wagnon can be distinguished by its pubescent nodes and leaf blades, as well as by awns on the lemmas (awns to 6 mm in length). In addition to these two subspecies, over 30 varieties have been described. Two natural strains have been recognized, a more cold tolerant "northern" and a more heat and drought tolerant "southern" strain.

Smooth brome was introduced from Eurasia into the United States in 1884 by the California Experiment Station. Now considered to be naturalized, smooth brome occurs from the northeast, south to Tennessee, west to the Pacific Coast, south to northern and central New Mexico and Arizona, north to Alaska, and throughout Canada. It is widely planted as a forage species, with irrigation often expanding its distribution. It has escaped throughout its range and is often considered to be a highly competitive weed of roadsides, forests, prairies, fields, lawns, and lightly disturbed sites. It grows in a wide range of soils, though growing best on deep, fertile, well-drained silt loam or clay loam soils. Smooth brome is quite drought and temperature tolerant, becoming dormant during the warmer months and, depending upon soil moisture, may regrow in September or October. Smooth brome tolerates flooding for up to 24 days and is considered to be moderately tolerant of saline conditions. It does not appear to tolerate highly organic soils, possibly because of poor soil aeration. Smooth brome is not shade tolerant.

Smooth brome is a cool-season grass and begins growth in mid- to late March. Stem elongation occurs in March or April. It flowers between May and early July. Inflorescence production requires that smooth brome undergo vernalization under short-days. The cold requirement is only one to 14 days, thus allowing some early emerged shoots to be vernalized and then flower in the same year. Flowering occurs with warmer temperatures and when day lengths reach 13 to 18 hours. Seeds ripen between June and August. The number of seeds produced per plant varies from under 50 to over 10,000. Germination is primarily in the early spring, but it will occur in the early fall if soil moisture is adequate. Adequate soil nitrogen is also necessary for seedling establishment.

Smooth brome is a rhizomatous, sod-forming, species that maintains and readily expands its population base vegetatively, often aggressively. Rhizome development begins between 3 weeks and 6 months after germination. Individual rhizomes are reported to have a longevity of 1 year.

Several, widespread, and dense populations of smooth brome occur at Pipestone National Monument (PIPE). If combined, the populations would cover in excess of 50 hectares. Smooth brome is typically found in late successional sites that were disturbed over 50 years ago, but it may spread vegetatively into undisturbed areas. Smooth brome will have a long-term (10+ years) effect by modifying or retarding natural succession. It poses a significant threat to the successional resources, and it has a major visual impact on PIPE.

Research on the control of smooth brome is often combined with control efforts to control Kentucky bluegrass (Poa pratensis L.). Some data suggest that smooth brome may be morphologically about 2 or 3 weeks behind development of Kentucky bluegrass. Cultural, chemical, and mechanical control methods have all been used with varying levels of success. Important considerations in controlling smooth brome are that it is an aggressive plant and vigorously reproduces vegetatively. It produces a high number of seeds that may remain viable in the soil for 2 to 10 years. Once controlled, a concentrated effort will be required to keep it out because of its aggressiveness and the high number of propagules around PIPE.

An important consideration prior to applying control is to determine if enough desirable plants are present to replace the controlled plants. If desired vegetation is scarce or absent control will be of little value. Most control methods harm other plants. The resulting disturbances favor reinvasion by smooth brome or other exotic species.

Cultural control methods for smooth brome include prescribed burning and grazing. Most studies conclude that late spring prescribed burning works well to reduce smooth brome. Timing of prescribed burns is extremely important. This is due to the importance of catching the plant at the correct phenological stage, when the growth point has been elevated above ground level. Fire at this time will kill the tiller. Many burns are conducted too early in the spring (March to April) to negatively influence smooth brome. The literature indicates that the root reserves are at the lowest just prior to or at the boot stage. This should be an optimal time for prescribed burning to control smooth brome, providing the timing of this burn will not adversely effect desirable vegetation. Current research indicates that annually repeated mid- to late spring (May to early June) prescribed burning reduces smooth brome tiller numbers and favors growth and development of native warm-season grasses. Smooth brome is generally tolerant to grazing, but it can be damaged by repeated, heavy, early spring grazing. This is probably a result of the elevated growth point and reduced root reserves.

Clean cultivation for several years is effective, but occasionally prevents smooth brome from becoming sod bound and enhances its growth. Studies have shown that repeated mowing for several years during the boot stage reduces but does not control smooth brome. The boot stage usually occurs when the plants have reached a height of between 45 and 60 cm. The best control has been achieved when the mowing treatment was proceeded by hot, moist weather followed by a dry period. Repeated mowing throughout the summer can keep root reserves at a low level reducing vigor of the stand. It is important to note that one of these mowings should occur when the plants are in the boot stage. Some research has shown that repeated mowings within a year are no more effective in controlling smooth brome than a single cutting while plants are in the boot stage.

Chemical control methods for smooth brome have been widely studied. Most herbicides are not specific for smooth brome or may not be specifically labeled for this use. It is important to read and follow all label directions.

Roundup (glyphosate) applied in April or May is effective on smooth brome. Other chemicals that have controlled smooth brome are AAtrex (atrazine), Kerb (pronamide), Arsenal (imazapyr), Bromax (bromacil), and Princep (simazine).

Itonidid midges, specifically the bromegrass seed midge (Stenodiplosis bromicola), and chalcid flies are predators of maturing seeds. Seedling blight and grasshoppers are known to negatively effect seedling establishment. No research has apparently been done in developing potential biological controls because of the extensive use of smooth brome as a forage.


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