Kentucky Bluegrass Invasion in the Northern Great Plains

Introduction

Kentucky bluegrass (Poa pratensis L.) is a common lawn and turf grass that is innocuous in most native worldwide ecosystems.  It is a cool-season, perennial grass native to Europe and Asia introduced by settlers in the late 1800s.  While it has been an invasive grass in the tall grass prairie, in the past 20 years Kentucky bluegrass has also become an invasive species in the mixed grass regions of the northern Great Plains (NGPs).  In many of the prairies of the NGPs Kentucky bluegrass can form nearly monotypic stands which reduces the abundance of native plant species (Fig. 1).  The loss of native plant diversity can have major ramifications for soil health, wildlife habitat, ecosystem services, grazing nutrition, and water resources.  In order to preserve the diversity of the northern tall and mixed grass prairie land managers need a better understanding of the reasons for this expansion and work on controlling Kentucky bluegrass invasion in the NGPs.  

Figure 1. A private rangeland heavily invaded with Kentucky bluegrass.

Photo credit: Carl Piper

 

How does Kentucky bluegrass affect ecosystems?

Kentucky bluegrass’s root system is different from most native plants.  Kentucky bluegrass only occupies the first few inches of soil, whereas many native species occupy several feet of soil.  Root systems and soil interact with each other.  Roots harbor microorganisms, decompose (which renews the nutrients in the soil), and stabilize soil.  A prairie dominated by Kentucky bluegrass may jeopardize all these specialized root services.  Additionally, Kentucky bluegrass is also known to develop a thick thatch (dead plant material) in only a few years after invasion.  This thick thatch may choke out many native plant species by preventing seedlings access to light resources.  Once Kentucky bluegrass has invaded a prairie it has changed the availability of the landscape for a number of bird, mammal, and insect species, which can be a threat to biodiversity.

Figure 2. Image of a number of native species' root next to Kentucky bluegrass on the far left. Photo credit to Heidi Natura

(http://www.shootingstarnativeseed.com/documents/native-roots.pdf)

 

How do I identify Kentucky bluegrass?

There are a few major characteristics to look for when identifying Kentucky bluegrass.  One major characteristic to look for is a large stand of grass growing no larger than four feet in height connected by underground stems known as rhizomes.  The leaf tip is boat-shaped with a double mid-rib (Fig. 3).  The leaves can be flat to folded.  If you fold back the leaf, you will find a ligule protruding off the base of the leaf (Fig. 4).   The ligule is a membranous protrusion that is very small.  The flower head can be loosely arranged to compact which ranges in size from one to five inches (Fig. 5).  For more information on identifying Kentucky bluegrass, see the websites below.

http://horticulture.oregonstate.edu/content/kentucky-bluegrass-poa-pratensis-l

http://www.library.nd.gov/statedocs/AgDept/Kentuckybluegrass20070703.pdf

Figure 3. The boat shaped leaf tip.

Photo credit: Doug Goldman, hosted by the USDA-NRCS PLANTS Database

 

 

Figure 4. There is a small ligule, which is a membranous protrusion at the base of the leaf right next to the stem.

Photo credit: Doug Goldman, hosted by the USDA-NRCS PLANTS Database

 

Figure 5. The flower head is loosely arranged and spreading.

Photo credit: USDA-NRCS PLANTS Database

Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner'sSons, New York. Vol. 1: 256.

 

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