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Weed & Pest

Extension Education: Bulbous Bluegrass – A Newcomer or a Case of Increased Notoriety?

I have received many requests for information this year about the grass that was turning purple early in the summer, but that was not cheatgrass. Bulbous bluegrass (Poa bulbosa L.), like cheatgrass, is an introduced grass that matures early in the year. Unlike cheatgrass, it is a short-lived perennial species that produces a bulb from which roots extend each year. 

The species is very interesting from a biological standpoint because it is reportedly the only grass to produce a true bulb. It is also known for its unique inflorescence that produces bulblets rather than seeds – essentially giving rise to small live plants where we would normally see seeds. The bulblets, which are small copies of the parent plant, make the plant easy to identify once they are produced.

Bulbous bluegrass was reportedly introduced to the United States in two separate events. It was accidently introduced as a contaminant in alfalfa and clover seed, although this introduction scenario is often speculated for many invasive species. 

It was also introduced intentionally by the USDA Office of Foreign Plant Introduction in the early 1900s to be evaluated as a forage species. The evaluations determined that bulbous bluegrass had no promise as a forage source, so the program was abandoned at that time. Further work to develop suitable cultivars for turf and forage purposes occurred in the 1950s, but the efforts were not continued. 

Bulbous bluegrass matures very early in the season, and palatability decreases rapidly resulting in a narrow window of grazing for the species. This short window and low biomass production make the species undesirable as a grazing resource. Bulbous bluegrass also competes with more desirable grasses for early spring moisture, so high bulbous bluegrass densities may reduce the productivity of other grasses.

The first Wyoming specimen of bulbous bluegrass in the Rocky Mountain Herbarium was collected near Gillette in 1938, so it has been present in the state for quite some time. The species has been collected in most Wyoming counties since that time.

It is nota new species in the state, but it is not an invasive plant that has received much focus in the past. It is generally adapted to areas of Wyoming that receive greater than 12 inches of precipitation annually, but it does occur in drier areas as well. Because of these moisture requirements, it is commonly seen at relatively higher elevations within the state.

Bulbous bluegrass grows well in areas that have been subject to soil disturbance like roadsides, field edges, energy development sites, etc., but it will also spread into pastures and rangelands. It may also be problematic in hay fields, but the establishment of a healthy stand of alfalfa should reduce bulbous bluegrass infestations in those sites. 

Intensive grazing in the early spring for multiple years has been reported to reduce bulbous bluegrass populations – this control method should be planned carefully to minimize potential damage to desirable grasses. 

Unfortunately, there are not many herbicides specifically labeled for bulbous bluegrass control in range and pasture settings. Removing a perennial grass weed from a site where perennial grasses are also the desirable plants, such as a rangeland or pasture, can be difficult using herbicides because of potential non-target damage. For more information on chemical control of bulbous bluegrass, visit wric.ucdavis.edu/information/crop/natural%20areas/wr_P/Poa_bulbosa.pdf

2013 has been an excellent year for bulbous bluegrass in much of Wyoming, so it was highly visible and noticeable as it began to mature. There has not been much information collected about this species in the state and more research is needed to develop clear recommendations for management of this potentially increasing invasive plant. 

Brian A. Mealor is an assistant professor and Weed Extension Specialist at the University of Wyoming and can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it.. More information about ongoing weed science research at the University of Wyoming can be found at weedcontrolfreaks.com.