Yellow and Caucasian Bluestems

Invasive Threats to Biodiversity in the Southwest

Old World Bluestems

Yellow bluestem (Bothriochloa ischaemum) and Caucasian bluestem (B. bladhii) are perennial warm-season bunchgrasses.  These two species are part of a global complex of bluestem species collectively called Old World bluestems (OWBs) to differentiate them from North American bluestem species. Yellow bluestem’s flowering head (panicle) has a fan or finger-like appearance.  The species is named for the pale yellow stem color below the nodes that transitions into green.  The panicle of Caucasian bluestem looks somewhat like an evergreen tree.  Caucasian bluestem has dense blue-green leaves with a distinctive sweet odor.  In comparison to native bluestems that have relatively stout stems, both yellow and Caucasian bluestems have spindly stems that are 1-3 ft. tall.

Yellow and Caucasian bluestems were originally imported from Eurasia and northern Africa in the early 1900s for erosion control and as forage crops for haying and grazing.  Additional varieties of the two species have since been imported for development of cultivars through selective breeding.  Currently, yellow bluestem is far more widely distributed across the Southwest than Caucasian bluestem, especially along roadsides. 

Uses

The most common use for OWBs is cattle forage, although they are less palatable and less nutritious for cattle than warm-season native grasses.  Cattle typically graze OWBs only if left with no other choice, which gives OWBs a competitive advantage.  OWBs have been used widely in grassland plantings on the Southern Great Plains, including Conservation Reserve Program (CRP) lands.  State and county highway departments have also used OWBs in seeding rights-of-way (ROWs) for erosion control, as they are readily established and can provide quick ground cover.

Spread

Both OWBs reproduce by producing seed as well as by rhizomes (underground stems) and stolons (above-ground stems) under certain conditions.  If unassisted, OWB seed has relatively limited dispersion and will establish seedlings close to parent plants.  However, seeds readily disperse via vehicular traffic, mowing operations in ROWs, flowing water, or wind-swept conditions over open terrain.

Invasive Traits

 

OWBs are highly aggressive and very adaptable, especially in disturbed areas.  Although they are used for agricultural purposes, OWBs share a number of characteristics common to invasive species including:

• Fast growth
• Rapid reproduction
• High dispersal capability
• Tolerance to a wide range of environmental conditions

These extremely persistent plants are drought-resistant and will outperform many native grasses under arid conditions.  The two species can adapt to highly calcareous soils with high pH but do well on any well-drained soil.  Because of the characteristics, OWBs have become invasive in native grasslands and pastures of Midwestern states, southcentral Arizona, and the Southern Great Plains (Oklahoma, Texas and eastern New Mexico).

Impacts

OWBs form monocultures that lower biodiversity of native plant communities by reducing abundance, diversity and richness of native plant species.  OWB infestations can also alter soil carbon:nitrogen ratios and the composition of soil microbial communities, including arbuscular mycorrhizae.  These transformations can inhibit growth of native plants.  In some cases, OWBs may form a complex with other less desirable non-native grass species such as Lehmann lovegrass (Eragrostis lehmanniana), which in combination can act similarly to a biodiversity-inhibiting monoculture.

OWB-infested grasslands can be relatively unsuitable for nesting, brood rearing or year-round habitat for grassland bird species such as the field sparrow (Spizella pusilla).  The lower bird numbers may reflect declines in arthropod abundance and/or biomass that can occur in plant communities dominated by OWBs.

Management

Individual OWB plants may be removed by digging them up.  Tillage (disking, ploughing, etc.) repeated over time may be used to disrupt OWB infestations established in agricultural areas or other tillable landscapes.  However, only non-selective herbicides are available for OWB control if manual removal or tillage is not an option.  Primary herbicides for OWB control are glyphosate and imazapyr.  Glyphosate requires one or two applications during the growing season and it can encourage re-growth of OWBs.  Imazapyr may cause less damage to desirable warm season grasses than glyphosate.  Spot, broadcast, or ropewick treatments can be used to apply herbicide but must be repeated over a span of several years to manage surviving plants and seedlings arising from the seed bank.  Fire, mowing or grazing should not be expected to control either OWB species adequately, but they may be used to increase herbicide efficacy.  If mowing is used, do not mow once seed heads start to form.  The mowing machinery should be cleaned after use.

OWBs are not currently listed on noxious weed lists for Arizona or New Mexico, although listing of yellow bluestem is pending on Arizona’s noxious weed list.  Regardless of their regulatory status, OWBs should be managed proactively as they are practically impossible to eradicate once established.  Control progressively becomes more difficult and expensive the longer OWBs are allowed to grow and spread.  It is therefore necessary to eradicate or contain new populations when possible; otherwise, intensive management measures may need to be implemented.

Photo Credits

• Figure 1: Coutesy of Billy Warrick; Soil, Crop, and More Information
• Figure 2: Forest Service photo by Christine Bishop
• Figure 3: U.S. Fish and Wildlife Service Photo